Prevention of Tropical Diseases

 

Introduction

By prevention we understand the establishment of measures intended to avoid the occurrence and spread of tropical diseases. The nature of these measures depends, in the first place, on the results of observation and scientific investigation of the conditions under which such diseases arise and spread. They come mainly under the heads: physiology, pathology and hygiene, and are obtained with the help of parasitology, pathologic anatomy (both macroscopic and microscopic), historical and geographical pathology, etc. There are many points of contact with physics, especially meteorology, with chemistry (in its application to physiology and pathology), and in general with most branches of natural history. For practical purposes prevention requires the assistance of legislation and administration.

The knowledge of tropical diseases is far from being complete and its problems vary according to the different regions. Therefore one of the first tasks of social organizations with interests in the warmer countries consists in providing institutions for the required scientific and practical work. Private initiative may prove very useful and deserves encouragement, but it would be neither fair nor safe to rely on private enterprise alone.

 

General Considerations of Tropical Diseases

Not every disease, observed in the tropics, may be called a tropical disease. This term applies in the first instance directly to diseases connected with tropical climate; secondly, and indirectly, to those dependent on tropical flora and fauna. These have a threefold importance, firstly in regard to the alimentation, secondly because of injurious or poisonous effects, and thirdly by including intermediate hosts or carriers of numerous parasites and disease germs.

Tropical countries are only those situated in the tropics where the sun at noon is high and sometimes at the zenith. When living near the level of the sea they are distinguished by high temperatures, the medium being not less than 22°C, while the minimum is never below 10°C and generally a few degrees above. In the equatorial belt the seasonal variations may be very slight. Vertical elevations of varying degrees often diminish the average temperature to such extent that the climatic conditions resemble those of sub-tropical or temperate climates. On the other hand, in sub-tropical or even temperate regions there may be long periods of really tropical heat, while the winter is not cold enough to destroy even a tropical vegetation. Therefore many cases of tropical diseases occur and even persist, though the appearance of new infections may be limited to the warmer season. Many diseases, generally classified as tropical, exist at least sporadically in temperate climates. Chronic infections acquired in a tropical climate will persist in any other, the temperature of the human body being everywhere the same. The optimistic expectation that an infection may be suppressed by a mere change of climate is theoretically unsound and practically disproved. In non-infectious diseases, like beriberi, the improvement observed is not due to the change of air but to the change of diet and other conditions.

From a practical point of view, the following definition may prove satisfactory: Tropical diseases are those which find their best conditions in the tropics, where they may occur all the year round, while in other countries they appear only at the time when temperature and other conditions are similar to those prevalent in the tropics.

On account of the position of the sun, all tropical climates have some general attributes in common; for the rest, they show the greatest variations in the distribution of sunshine and rain, moisture of the air, prevalent winds, etc. Due to geographical position and local conditions, the principal products, as well as flora and fauna, show many differences which are related to the alimentation and ways of living. The human races and their peculiar habits are of the greatest importance in explaining the prevalence of certain diseases and directing the preventive measures. All these factors tend to complicate the task which varies from one place to another.

 

Diseases due to Meteorological Conditions

The first, but by no means most important, group consists of the morbid processes depending directly on the meteorological conditions, such as insolation and heat. While the former may be avoided to a great extent by individual prophylaxis, this is hardly possible for the second.

Effects of the Actinic Rays

The less oblique incidence of the sun's rays makes them more active in hot climates, and this action may be intensified by reflection, as on a white beach. These reflected light rays may be strong enough to produce sunburn without the help of direct insolation. Neither is a clear sky a necessary condition, as a sensitive skin may react even on hazy or slightly clouded days. Individuals and races with little pigment are especially sensitive, as are also white animals. Certain morbid conditions, as pellagra in man and lupinosis in the horse, determine a peculiar sensitiveness of the skin to solar rays. A pigmentation of exposed parts of the skin is extremely common in people leading an outdoor life in the tropics, and might be due as well to repeated slight, as to a single intense solar erythema.

A prolonged exposure to the rays of the sun may cause inflammation of the eye, as observed in people who watch eclipses of the sun without the necessary caution or in the so-called snow-blindness. Strong artificial light may produce similar symptoms. However, under natural conditions the reaction is neither very intense, nor of long duration. Repeated exposures may induce catarrhal conjunctivitis and changes in the inner eye. It also seems to have a prominent part in the production of pterygium.

When the head, neck, and back are exposed to intense sun-rays, discomfort followed by chills and elevation of temperature may ensue; meningitic symptoms also are sometimes observed. It is, however, not certain that they represent a direct action of actinic light on the nervous system independent of overheating and infection.

Effects of the Heat

Morbid symptoms due to the action of heat on the organism may follow either prolonged insolation or a high temperature of the air in general. It is well known that when the air is dry, as for instance in the desert, very high temperatures may be endured, while even temperatures in the neighbourhood of 29° C become almost intolerable when the atmosphere is saturated with moisture. For this reason, in many places the nights are more oppressive than the days, even though their temperature is lower. In dry air the body perspires freely, and the evaporation lowers the temperature of the skin, so that it may even feel cool, while in moist air there is hardly any evaporation, and the skin is clammy and warm.

A high air-temperature produces unpleasant effects on the skin and on the nervous system. When the natural temperature regulation is overstrained the body temperature rises, and general symptoms appear which resemble those observed in pyrexia from infectious diseases.

The effect on the skin is mainly due to increased transpiration, which varies individually but is always favored by inappropriate clothing. Woolen underwear, theoretically recommendable, is practically quite inappropriate, and has been found to bear a marked relation to the occurrence of prickly heat.

Prickly heat, which is known by various names, does not seem to be different from the miliaria rubra observed in febrile diseases and especially in miliaria epidemics, where it is constant. It is practically an inflammation of the sweat pores, due to prolonged maceration by perspiration, which also causes intertrigo. It is very irksome in itself, but even more so in the complications due to the accompanying pruritus. This leads to scratching and is followed by results similar to those observed in scabies and pediculosis. Purulent folliculitis impetigo, and ecthyma are common, and various types of superficial staphylococcus infections such as follicutilis, furunculosis and panaritium are favored by prolonged high temperature, and sometimes occur as the results of inoculation from one person to another. Little known, but not uncommon, sequelæ of profuse sweating are inflammation and abscesses of the subaxillary sweat glands. Intertrigo and dyshidrosis of hands and feet are occasional effects. Maceration of the skin between the toes may become troublesome.

Several forms of dermatomycosis, as for instance, tinea versicolor and eczema marginatum, are favored by frequent perspiration and show even uncommon localizations, as on the face. Tokelau ringworm and some of the forms described by Castellani are evidently limited to warmer climates and unknown in cooler zones.

The effects on the digestive system are not very pronounced. The appetite is generally diminished with increased thirst and reduction of muscular exercise. Chronic intestinal troubles are not due to heat alone, and there is no proof of the existence of liver disease due to atmospheric conditions. Congestion of the liver is a hypothetical condition which often covers false or imperfect diagnoses.

The respiratory and circulatory systems show only functional alterations and adaptations, as long as the temperature is not excessively high.

More or less prolonged high temperatures, aggravated by itching, thirst, want of appetite, and insomnia, produce a nervous irritability which manifests itself in uneven temper and different forms of neurasthenia. There is no doubt that the difference in character between the inhabitants of hot and cold climates is due in great part to the effects of the continued heat. A very high temperature may also induce a kind of giddiness which almost prevents mental work.

 

Measures for Ameliorating the Effects of Heat

The heat regulation of the body is quite able to withstand the temperature conditions generally met with in the tropics, but may be overcome by abnormal conditions such as the combination of natural and artificial heat near fires or boilers, saturation of the air with moisture, prevention of the evaporation of perspiration, overcrowding in rooms with stagnant air, warm clothing, sometimes combined with physical exertion, so-called heat-waves leading to a series of abnormally warm days and nights, etc. Heat-waves may cause heatstroke or siriasis in numerous cases giving the impression of an epidemic of some new pyrexia. When the direct effects of high temperature on the human body are understood, rules for their prevention are easily deduced. The meteorological conditions of tropical countries cannot be changed but their effects may be lessened in many ways.

Housing

The establishment of summer or permanent residences at higher altitudes should be rendered practicable by making nearby mountains accessible to hot seaports and other lowland settlements. The seat of government may also be transferred to the higher altitudes. If there are no mountains nearby, a cooler climate should be sought at a greater distance during the hottest months.

Settlements, as well as individual houses, should be placed so as to give free access to the prevalent winds, and provide shelter from the sun. Where the streets are wide enough, shade trees are an improvement. Asphalt, concrete, and stone flags exposed to the rays of the sun retain the heat and reflect it upon the houses.

The best construction for houses is still a matter of debate, but a few general hints may not come amiss. The rooms should be very high, especially where there is only one story. Ventilation under the roof and under the floor is desirable. Windows should be sufficiently protected by blinds or shutters. For isolated houses the cottage style, with large verandas on all sides, gives much satisfaction. Electric fans are useful and should be installed more generally. Artificial refrigeration of rooms has not reached general practicability, but any improvements in this direction are very desirable.

Clothing

In a really hot climate, clothing ought to be as light and loose as possible, but it should be borne in mind that very light textures let the sun's rays through, and that these passing through gauze or lace, may even reproduce a copy of the pattern on the human skin. For soldiers and those who are obliged to walk and work and out-of-doors, a pith helmet with neck flap may be most useful; for the civilian with sedentary occupations it is not so necessary.

Work

Out-of-door work should be interrupted, as much as possible, during the hottest hours of the day, which in many places are used for a siesta. Early morning and even night hours are preferable for journeys through arid zones or along the beach.

Troops and workmen ought always to have an abundant supply of water when long exposed to heat. Heavy clothing must be avoided and the weight of the outfit should be reduced as much as possible. Firemen, stockers and other workmen exposed to additional heat ought to be relieved at short intervals.

Bathing

Bathing facilities are of the greatest importance and ought to exist in every house. Where water is scarce, wet towels may be used with advantage for cooling and fanning the body.

 

Water-borne Diseases and Alimentary Poisoning The use of unboiled water for drinking and bathing is naturally more common in hot countries, and higher temperatures favor the development of germs of many kinds. Heavy rainfalls and the unhygienic conditions commonly prevailing also favor contamination. For these reasons water-borne diseases are very frequent in tropical countries without being peculiar to them. Cholera morbus, typhoid and dysentery (amebic and bacillary), in endemic or epidemic form, are often prevalent, as well as enteritis in children, sometimes assuming the form of cholera infantum. Many of these diseases are due also to contamination of food.

Lead poisoning may be due to the passage of drinking water through lead-pipes and this form of poisoning is intensified by the higher temperature of the water and has even been described as “colique endémique des pays chauds.” Formerly common, this form has now become rare and is easily avoided when recognized.

Polluted water may contain Schistosome cercariæ or nematode larvæ which require only contact with the skin to produce infection. Water, free of deposit and stored for a day or two, may be considered safe in this respect.

A supply of good drinking water is one of the first sanitary necessities and should be obtained, regardless of expense, wherever it is possible. Local conditions, however, may often render this only a pium desiderium. The use of common filters, though improving the water, is hardly sufficient, unless combined with boiling. As boiled water is rarely palatable, it may be used for lemonade or impregnated with carbonic acid gas, or may be taken in the form of weak tea, hot or cold. Coffee or other infusions may be substituted. Porcelain filters and the use of sand filters in the water-supply may often prove necessary.

For preserving food, ice-chests and fly-proof larders give good results when used with ordinary precautions. They are in general use and the latter are almost indispensable.

Poisoning with decayed meat, fish and other products of the sea is rather common and ought to be checked by sanitary supervision. It often takes the form of cholera nostras.

Fly prophylaxis is an urgent necessity and should be supervised everywhere by the authorities just as much as measures directed against mosquitoes and rats.

 

Relation of Tropical Diseases to the Vegetable

Kingdom

The local flora of tropical climates varies according to the country but is widely different from the flora of cooler climates. Only among the cultivated plants and the weeds do we often meet the same species. In the hot countries, a considerable proportion of plants have toxic properties. Some may cause accidental poisoning, others may be used for criminal purposes, and a few are employed for poisoning arrows. Some more or less toxic plants are used in various ways for their supposed agreeable effects and may thus lead to acute or chronic intoxication. While such accidents properly constitute a chapter of toxicology and while their prevention is simple enough when the cause of the symptoms is recognized, it is not superfluous to point them out in this connection, as some of the more obscure sporadic or epidemic diseases may prove to be of toxic rather than of infections nature, as has been found true of lathyrism and ergotism.

It is now generally admitted that certain forms of diseases are caused by an unbalanced alimentation, in which certain substances, not well known but commonly designated as vitamins, are deficient or altogether wanting; as in certain chronic intoxications, the symptoms may appear only after a considerable period and not always at the same time in individuals, living under the same conditions.

The classical example of a pathologic condition due to the use of food lacking certain vitamins is beriberi, which, though not limited to tropical and warmer climates, has been extensively prevalent in these regions. The reason for this prevalence is evident since the disease is caused by an alimentation based mainly on polished rice. This cereal requires comparatively high temperatures to be grown successfully, and only where it is abundant and cheap is it likely to become the staple diet of the population. Even then it is only dangerous when polished and unmixed with food containing enough vitamins to make up for the deficiency. The cooking process destroys the vitamins in proportion to the heat employed. For this reason alimentation with tinned food may produce symptoms similar to those of beriberi. Other foodstuffs deprived of vitamins may, as proved by experience, induce scurvy, indicating the danger of practically exclusive alimentation with only one kind of food in cooked condition. Fresh vegetables are generally rich in vitamins and some, like fruit, may be used in the uncooked state. Their usefulness in scurvy has been known for ages. Though the pellagra question has not yet been solved, its relation to a limited diet, even when such diets do not include Indian corn (maize), can hardly be denied and the use of that product as a staple food ought to be avoided, particularly where the diet fails to provide any fresh meat, milk, and vegetables.

It is worthy of note that all the cereals suffer by long storage, especially in warm climates. Generally the outer coatings of the grains (which are first attacked and destroyed by animal and vegetable parasites) contain most of the vitamins.

Under normal conditions the diseases caused by inappropriate alimentation are limited to the poorer part of the population or to those whose food is selected for them, generally with a view to economy. For this reason they are observed principally among sailors, soldiers, prisoners, the inmates of poor houses, lunatic asylums and schools. It is part of the duty of public authorities to control the selection of food for these classes, and to see that this does not consist solely of one staple article, nor of foodstuffs, bought more cheaply on account of poor quality. Beans, peas, fresh vegetables and fruit, all of which are rich in vitamins, should form a proportionate part of the diet. Preserved meat and tinned preserves should not be used exclusively over a long period. Rice should be used in the unpolished state, and the polished rice should be prohibited or heavily taxed where there is a tendency to use it as the main article of food.

 

Relation of Tropical Diseases to Animals

The food derived from animals is not known to convey any essentially tropical disease. Tænia saginata is common in some of the warmer countries, as are many other forms of helminthiasis, while Tænia solium and Cysticercus cellulosæ are less frequent. Trichinosis seems to be rare and is unknown in many places. Botulism is little known, but poisoning by shrimps and other crustaceans or shellfish is in many places not uncommon. Fish may become poisonous through deterioration, but some of the species found in warmer seas have toxic qualities, principally Diodon and Tetrodon, which are easily recognized by two or four large teeth in front of the mouth. The toxicity must be considerably diminished by cooking, as fresh specimens may be very poisonous for animals. Raw mollusks, crustaceans and fish may transmit some parasitical worms or produce poisoning, and are better avoided on general principles.

None of the milk-borne diseases are characteristic of the tropics, nor are they specially prevalent. Where goat milk is used the possible occurrence of undulant fever should be remembered.

Different forms of ringworm are frequent in domestic animals and some of them may be transmitted to man by direct contact. Children very often become infected by cats and dogs.

Epizoòtics, like anthrax, glanders and hydrophobia, occurring in domesticated animals, are often found in the tropics. Glanders is not easily transmitted; the other two are not rare in man. Echinococcus is associated with sheep-breeding and is, therefore, rare in very hot climates. Preventive measures need not be discussed, as they are generally well known.

In the tropics many animals invade human habitations for shelter and food. Among these, rats and mice are of special importance because of their role in plague, which, of late, has invaded most of the tropical countries. They may also transmit infectious jaundice, sporotrichosis and rat-bite fever.

The rat problem is most important in tropical countries, where rats are generally very numerous. Rats and mice may be caught in traps or destroyed by asphyxiating gases, poisons, natural enemies, etc., but relief thus secured is only temporary. The most important measure is the construction of rat-proof dwellings. The floors of the houses (where cellars are generally unknown) must be raised high above the level of the ground with a cement floor underneath. Food should be stored in places inaccessible to rats and mice, and no refuse containing scraps of food should be kept overnight, except in metallic tight-shutting receptacles. Where there is no accessible food, the rodents rapidly disappear, while abundance of nourishment causes them to increase at an alarming rate. These measures must be regulated by the authorities.

While the larger and most dangerous wild animals are rapidly decreasing in the tropics as civilization advances, animals with venomous bites and stings such as snakes, scorpions, centipedes and spiders are not so easily exterminated. Some of them penetrate human dwellings so that houses, as well as the surroundings, must be kept very clean. Going barefoot should be avoided and plantation work and the collecting of firewood call for special precautions. The payment of a premium for poisonous snakes and scorpions (living or dead) is a good measure, especially as it may be combined with the preparation of antivenom serum. Some instruction is very desirable, as the public always destroys every snake, unaware of the fact that some are the natural enemies of the poisonous varieties. The use of birds and mammals for destroying snakes has not proved of much practical use except under very special conditions.

While some ants prove useful in exterminating domestic pests such as cockroaches and bugs, others, like the leaf cutters of the genus Atta, must not be permitted near human dwellings. They not only devastate the gardens but may cause serious danger by making tunnels between wells and cesspools and deposits of manure. The destruction of these ants should be undertaken by the authorities, or at least by the individuals on whose premises the nests are located.

Rodents and other burrowing animals may occasionally cause similar damages.

 

Diseases due to Parasites

Macroscopic Parasites

In tropical countries a considerable number of pathologic conditions are due to macroscopic parasites invading either the skin, the intestinal tract, or other organs and tissues of the body. Some are even localized in the lymph and blood-vessels. These macroscopic parasites generally belong to the vermes, only a small number to the arthropods.

Helminthiasis

Helminthiasis plays an important role among the tropical diseases, being in many ways favored by a warm climate. Development of ova and larvæ, and infestation of intermediary hosts take place during the entire year, while human infestation is fostered by the habit of going barefoot and the use of contaminated water for drinking and bathing. Absence of even the most primitive latrines is quite general; excrement is deposited anywhere and in certain seasons the heavy rains carry ova and larvæ to streams and pools where they may infect man directly or through intermediary hosts among the aquatic mollusks.

Many of the parasitic worms are confined to warm climates. This may, to a certain extent, depend on the fact that their intermediate host is found only there, but as a rule it is largely a question of temperature. Proof of this is shown in uncinariasis and the infestation with Schistosomun mansoni. Although species of Planorbis, which might constitute intermediary hosts, are found all over the larger continents, the disease is limited to warmer climates because the evolution is stopped by lower temperatures. It seems rather logical that many parasites, living in an adult state at a constantly high temperature, should require for their evolution thermal conditions which are not too different. This is not only true for worms, but also for protozoa, which undergo a change of hosts, as seen in malaria.

Blood parasites find especially good conditions in the tropics, since bloodsuckers are numerous and active, and the high temperature allows a speedy evolution in the intermediate host.

Prevention of Helminthiasis

The problem of preventing infestation varies according to the nature and evolution of the parasite. Those living in the intestinal tract and urinary system are propagated by ova and larvæ voided with the excrement and the urine, while the larvæ circulating in the blood must be extracted by bloodsucking arthropods. The larvæ of a few filariæ escape through the ulcerated skin and the presence of water may be necessary for the development of the next stage of evolution.

The larvæ in the blood belong to filariæ living in lymph or blood-vessels or serous cavities. Preventive measures consist in protecting the patients against the bites of blood-suckers such as mosquitoes, simuliidæ, tabanidæ and ticks (Ornithodoros moubata), which may constitute the intermediate hosts.

Cyclopidæ may become infested by the larvæ of Dracunculus medinensis, voided in the water, and thus convey the infection through drinking unfiltered water which often contains them. This should be avoided, as well as any collections of water in which mollusks, especially Planorbis and Physopsis, are found.

Many parasites, as the Trichinella, some flukes and several tapeworms are introduced into the human body by eating raw meat of mammals or fish and crustaceans. These may be easily avoided by sufficient cooking of the food.

In the case of intestinal worms without intermediate hosts (Uncinaria and Necator), prophylaxis consists in preventing the scattering of fecal matter, the use of unpolluted water and the protection of the skin from contact with mud, as in going barefoot. One of the most important measures consists in anthelmintic treatment of the carriers, recognized as such by microscopic examination of their feces, urine, blood, etc. Heavy infestations call for special supervision.

Though treatment is successful in many forms of helminthiasis, in others it is much less so, or not at all promising. It does not always act cito and tute and hardly ever jucunde, so that it is difficult to use as a general measure and can not be relied upon as the only means of prophylaxis.

All preventive measures imply a certain understanding of their importance which is difficult to develop, even in the more educated part of the population. As for the others who are mainly responsible for the existence of intestinal helminthiasis, it takes generations to accustom them to more cleanly habits and to educate them to understand their necessity. Coercion may be used but is often rather difficult. It is desirable to take advantage of the existing public and private schools for instructing the rising generation and improving their habits.

Arthropods

The Linguatulidæ or Pentastomatidæ belong zoõlogically to the Arachnoidea, but, implying a change of hosts in consequence of their stationary parasitism, they show greater affinities to the parasitic worms with which they were formerly confounded. Larvæ of a few species may be found encysted in the human body and one species at least may be considered to belong to warm countries, because the definite host is found there. They are not of much importance, as it is almost impossible to diagnose them. Prevention comes under the general head of keeping water and food free from contamination with the ova of animal parasites.

Myiasis

The parasitism of fly larvæ in the human body is generally called myiasis and may be considered as a tropical affection, as it is more common in tropical and warm countries. Some of the larvæ burrow into the human skin, leaving it only to pupate. They attain considerable size, but are generally not numerous and often single. Their parasitism is not very destructive and rather an inconvenience than a disease, the more so, as they can be removed without difficulty as soon as they are recognized. None of them are exclusively human parasites and a few species are only exceptionally found in a human host. There is not much to be recommended to prevent such accidents, except to remove the larvæ from infected animals and to avoid sleeping in the open air without protecting netting. In the case of the Dermatobia cyaniventris, which is limited to America, the eggs are laid on flies or mosquitoes who carry them to the skin; for the species or Cordylobia (= Ochromyia), found in Africa, the conditions favoring infestation are not well known.

Far more destructive and serious are the forms of myiasis due to Lucilia (Chrysomyia) macellaria and other flies or allied genera which deposit eggs or maggots in wounds or on the mucosæ of their victims where they develop rapidly and burrow in the ulcerating tissues. The cure depends on early recognition. Prophylaxis is the same as in the other case with the additional precaution of carefully guarding against any exposure of wounds.

In Africa some maggots belonging to flies of the genus Auchmeromyia, known as floor maggots, live in human habitations and attack the sleepers at night, gorging themselves with blood. They may be discovered by removing the mats under which they hide. Sleeping in raised beds is likely to protect sufficiently; the measures used against argasinæ and hemiptera may also be employed.

Sandfleas

The sandflea (Sarcopsylla penetrans) is largely found in warm countries. The fecundated female burrows in the epidermis of any exposed part in contact with or near to the floor or the ground and for this reason usually attacks the bare feet. In the epidermis the abdomen of the female swells, forming a little cystic tumor surrounded by a seropurulent exudation. Unless removed early it produces considerable irritation and is finally eliminated by desquamation and suppuration. The resulting ulcer is liable to contamination with dust and earth, leading to purulent infections or even tetanus.

The best treatment is early removal. The protection of the feet by the use of high shoes, thus avoiding all contact of the bare skin with the ground is practically sufficient to prevent the attacks of this parasite, even in much infested regions. Removal of infested pigs and dogs from the proximity of man, and frequent washing of the floor are additional measures of value. People going barefoot must examine their feet daily in order to remove the jiggers as soon as possible.

Microscopic Parasites

A large number of infectious diseases observed in the warmer countries are due to protozoan parasites. When these are limited to the intestine, the mode of infection is probably the same, as in many intestinal worms (Ascaris and Trichocephalus). Blood parasites belonging to the protozoa and many included among the spirochetes and other not so well known groups are generally transmitted by the bites of insects and arachnoidea (ticks and mites). In such cases the prophylaxis must be directed towards protecting patients and healthy persons from being bitten and reducing the number of possible transmitters in every way.

The following tropical diseases are transmitted by blood-sucking arthropods.

Yellow fever and dengue are carried by mosquitoes of the genus Stegomyia, while malaria is transmitted by mosquitoes belonging to the anophelinæ. The African forms of human trypanosomiasis are transmitted by flies of the genus Glossina, and the American trypanosomiasis is carried by hemiptera of the family Reduviidæ, belonging to the genus Triatoma (syn. Conorhinus and Lamus), and perhaps a few others; spirochetal infections are transmitted in the tropics by argasinæ and in cooler climates by body lice; bubonic plague is mostly transmitted by fleas; pappataci fever is carried by Phlebotomus (Psychodinæ). The same midges are supposed to transmit Verruga and Oroya fever, possibly also various cutaneous and internal forms of leishmaniasis. Blood filariæ of man are transmitted by mosquitoes, simuliidæ, tubanidæ, and perhaps nocturnal ticks (Ornithodorus).

The conditions under which blood-sucking arthropods may become carriers of disease are determined as follows:

(1) The blood-sucking arthropod must bite repeatedly; (2) the interval between the bites must be long enough to permit the evolution and multiplication of the parasitic organism; (3) the blood-sucker must occasionally change its host or transmit the parasite by hereditary infection to another generation; (4) the first-bitten host must have the parasitic organism in his blood in sufficient numbers; (5) the person bitten afterwards must not be immune; (6) temperature conditions must be favorable.

Under such conditions an infected case may rapidly lead to a general epidemic, if the transmitters are present in great numbers. Spontaneous decline and extinction of epidemics are caused by unfavorable meteorological conditions or scarcity of non-immune persons. Meteorological conditions explain why some of the tropical diseases become endemic with seasonal exacerbations and intermissions.

Prophylaxis

Vaccination

The first measure against the spreading of contagious and infections diseases consists in prophylactic vaccination of the population. This has accomplished good results in smallpox, typhoid fever, cholera, and plague. None of these, however, belongs to the tropical diseases. For infections with protozoa and spirochetes satisfactory methods of prophylactic vaccination have not yet been worked out. As a rule, a good result can only be expected where infection leads to immunity. In many of these cases apparent cure is not complete, and the patient continues to harbor the infecting organism, as it happens in malaria and bovine piroplasmosis (Texas fever).

Disinfection and treatment by drugs

In former times prophylaxis was based principally on disinfection of ships, houses, clothes, bedding and excreta of the patients. This is useless in the case of blood parasites and insufficient in other diseases, as convalescents and chronic carriers are often the principal danger. The number of medicines which are effective in protozoan and spirochetal infection is steadily increasing, but the cure is often incomplete and relapses are common, even after the most brilliant results.

Prophylaxis against transmission

Measures directed against transmission by blood-sucking arthropods begin with the protection of the patient; these are most successfully carried out in special hospitals. When the nature of the disease has been recognized too late, as often happens in yellow fever, the infected mosquitoes may yet be destroyed in the rooms of the house where the patient has been living. The separation of the healthy from possible carriers is the next and often the most necessary step. In some cases, as in malaria, prophylactic treatment may be indicated. Mosquito Prophylaxis

(1) Domestic Species. – It is comparatively easy to prevent the breeding of the small number of house mosquitoes which are the principal agents in the spreading of yellow fever, filariasis, dengue, and probably many other diseases, among them leprosy. The most important of these domestic species are Stegomyia egypti (= calopus and fasciata) and Culex fatigans (quinquestriatus). These species, especially the former, generally breed in small vessels containing rain-water or water for drinking, washing, flushing, and gardening, and seem to avoid the breeding places of other mosquitoes. If water is not allowed to collect on the premises there will be no mosquitoes, unless they come from less careful neighbors; but not even one flower vase, jug or other smaller receptacle must be overlooked. It is very desirable that the dwellings be periodically inspected by persons of experience, especially designated by the authorities for that purpose.

The biting of mosquitoes may be avoided to a certain extent by burning insect-powder or fumigating the rooms, by using wire-screens for doors and windows and mosquito-nets for the beds and hammocks; however, on really hot days or nights these render sleeping accommodations and smaller rooms most uncomfortable. Electric fans may then be used with great advantage.

Patients with acute diseases transmissible by mosquitoes must be kept in mosquito-proof buildings. Leper asylums should be constructed where there are no mosquitoes or at least carefully protected against them. The same precautions should be taken for patients infected with Filaria bancrofti and showing the larvæ in the blood.

In order to kill mosquitoes in rooms and houses, they must be made air-tight by cloth or packing paper and then fumigated with pyrethrum, tobacco, sulphur vapors, formaldehyde, etc., in sufficient strength. The Clayton apparatus is very useful.

(2) Outdoor Mosquitoes. – Outdoor mosquitoes may be divided into wood and swamp species. The latter live in ponds and lakes with aquatic vegetation, but principally in the numerous and extensive water collections caused by the periodic overflowing of the rivers, or in puddles and pools caused by heavy rains. Most of the anopheline larvæ are found under such conditions, and where there is a rainy season an increase of malaria is generally observed at its end or shortly afterwards. When the rainy season is over, these collections of water tend to dry up, and, if shallow and unprotected, become so heated by the strong rays of the sun that the larvæ disappear long before the pools are completely dry, and while the adult anophelinæ are still found in shady places.

The wood mosquitoes breed in tree holes or in the water contained in bamboo joints or in bromeliaceous plants. In Brazil an anopheline breeding in Bromeliaceæ has caused many epidemics of malaria, often on steep mountain sides. The other wood species have little importance for human pathology, and the same may be said of the other swamp species, though some of them are most troublesome.

The outdoor mosquitoes only invade the human habitations when they are near to their breeding places. Workmen camping in open ranchos and people living near or traveling on the rivers are most exposed to their attacks.

The prophylaxis directed against malaria and outdoor mosquitoes is much more difficult and in some places impossible, so that the prophylactic use of quinine becomes necessary. However, in many places, drainage or filling up of pools and cutting down of woods have given very good results. Houses and ranchos ought to be placed on high ground far from the breeding places and, if possible, protected by wire netting. Mosquito nets may also be used. Infected people ought to be isolated and the blood of workmen should be examined before employment, excluding all carriers of gametes. This is of the greatest importance, as epidemics arise generally from unsuspected chronic cases.

Most of the smaller fishes, like goldfish and principally diminutive cyprinodonts, some of them known as millions or killifishes, are very useful for destroying mosquito larvæ, and will keep water collections free from them, provided the larvæ do not find hiding places. For this reason algæ and water plants must often be removed. Ponds, tanks, and wells with smooth, cemented sides may be made perfectly safe. In other collections of water (which are not required for domestic purposes and are difficult to drain), the surface may be periodically covered with paraffin oil (petroleum, kerosene), or other preparations recommended. The water may also be mixed with disinfectants such as creolin or some of the more toxic aniline dyes.

Prophylaxis against Nematocerae

The habits of the Phlebotomus larvæ are not sufficiently known to indicate a successful way of destroying them. For the adults the measures directed against mosquitoes are equally useful. Light attracts them and ought to be avoided in the bedrooms when the windows are open. Patients suffering from leishmaniasis should not be permitted to work in places where these blood-suckers are common. Individuals suffering from kala-azar, infantile leishmaniasis and pappataci fever might be isolated by fine wire netting.

Simuliidæ attacking man are often abundant in mountainous regions and are a great nuisance. In Africa they transmit Onchocerca volvulus. It seems quite likely that they may harbor evolutionary stages of other parasites of man and domestic animals. Most of the larvæ can be got rid of by cleaning out the little waterfalls in brooks and streamlets of the neighborhood, as most of them stick to water plants, dead branches and leaves; some of them live on stones where they may be scraped off. The adults are rather sensitive to tobacco smoke. They are semi-crepuscular but do not bite at night.

Punkies are small blood-sucking midges of the sub-family Ceratopogoninæ, family Chironomidæ. Most species come in the genus Culicoides Latr. The larvæ live in mangrove swamps, in crab and tree holes, perhaps also in fresh water pools. The adults (which are not known to carry diseases) are generally limited to certain localities; they are very sensitive to smoke, while mosquito-netting affords little protection in lighted rooms.

Tabanidæ are serious pests for domestic animals by sucking blood and carrying diseases, as trypanosomiasis. Man is much less attacked, and in the tropics they are only known to transmit Filaria loa loa. (Tularæmia is not known in the tropics.) Most of the tabanids are diurnal but several are crepuscular, while apparently there are no strictly nocturnal species. They are common near water and in woods, at least part of the year. The larvæ live mostly in the mud, near or at the bottom of the water, where they suck the blood of worms and mollusks. Their number may be greatly reduced by draining swamps and wet grounds and filling up of unnecessary pools.

(1) Stomoxydidæ and Common Flies. – In the Old World there are many genera and species of blood-sucking flies, closely allied to the Stomoxys calcitrans which, like the housefly, has spread over all the continents and most of the larger isles. They probably carry several diseases of domestic animals which are their principal prey, and may possibly also play an occasional role in human pathology. They generally breed in the manure of domestic animals and, if this is not allowed to remain near human habitations, they and also the domestic fly should never become numerous. Manure can be stored in such a way that the larvæ are killed by the heat which develops in the heaps or drowned when they leave them; nor would it be difficult to destroy the escaping adult flies, due to their tendency to seek the light. Stables should be controlled by the sanitary police when in cities or near human dwellings.

Stomoxys breeds very well in the excrement of rabbits and guinea-pigs, and the common fly in most kinds of refuse. Speedy removal and destruction of all offal is a very useful measure. Privies ought not to be accessible to flies, and slaughter-houses, butcher shops, etc., must be kept as clean as possible.

Though the domestic fly and also species of Drosophila, Phora and many other similar kinds do not suck blood, they may be disease-carriers as they visit wounds, discharges of mucous membranes, excrement and dead bodies, alternating with fruit and articles of food. Some of the smallest flies seem responsible for the spread of phagedenic ulcers due to spirillosis, and occasionally may transfer trachoma. They deserve special attention.

(2) Glossinæ. – The genus Glossina contains numerous species and is limited to Africa. Though nearly allied to Stomoxys, it deserves special mention because of its biologic peculiarities and its pathologic importance. All the known species are blood-suckers and larviparous. The larvæ pupate immediately after birth without taking food. They are deposited generally in the shade of shrubs and the small trees near to water, where people are also subject to their attack. Most of the glossinæ are diurnal and may be avoided by traveling at night; some species, however, are said to bite even then.

Glossina has many species which may carry trypanosomiasis of domestic animals and cause in man a febrile disease ending in sleeping sickness; it is due to infection with trypanosomes, apparently of more than one species.

Human trypanosomiasis is now widely spread in Africa and has been thoroughly investigated. Though occasionally communicated by sexual intercourse, it is generally acquired through the bites of infected glossinæ of the species palpalis, fusca, morsitans and perhaps some others. Prophylaxis consists in detecting, isolating, and treating the patients, destroying animals which serve as a virus-reservoir, cutting down the trees which shelter the flies, etc. Immigration from and visits to infected areas must be limited as much as possible, and subjected to every known precaution. As the files do not propagate rapidly, some result may be expected from catching the adults and collecting the pupæ, which may be done under proper protection.

Prophylaxis against Hemiptera

Bedbugs and allied species are found all over the world as occasional human ectoparasites. Though they have been suspected again and again as carriers of human diseases, the actual verdict must be “not proven”. If they transmitted leprosy, the disease ought to spread in Paris, Vienna, and other cities where both patients and bugs are always found. Though they may become infected with Trypanosoma cruzi and transmit it experimentally, the disease does not spread to places where only bedbugs are found. Their role in the transmission of plague and Kala-azar is equally doubtful. However, it is better to err on the side of safety, and consider bedbugs as possible transmitters of various human diseases.

There is, however, one infection, the South American trypanosomiasis, which is undoubtedly transmitted to man by other blood-sucking hemiptera. These are Reduviidæ, and belong to several species of Triatoma (Conorhinus) and closely allied genera. Some of them are found in human habitations, where they attack animals and man. Several species are found in North America and others in the Old World. One of the largest species is Triatoma megista, which shows a predilection for human dwellings. In poor houses built of mud, where there are plenty of chinks and cracks, it multiplies enormously, while in well built houses it is unknown or only rarely met with. One of the principal prophylactic measures consists in having the houses well constructed and white-washed. Some of the Reduviidæ are able to fly in the evening or at night, as they have nocturnal habits. The common bedbug, having no wings, is generally carried from infected houses in boxes or trunks, not rarely also in packages of clothes given out for washing or in second-hand furniture. The Triatoma may be conveyed in the same way.

The prevention of diseases transmitted by Triatoma (Conorhinus) and Argasinæ must be accomplished by laws providing for appropriate buildings and occasional fumigations where the danger exists. Cases of relapsing fever (spirochetal) ought to be subject to notification and the patients isolated in hospitals, where today they may be rapidly cured.

(1) Lice. – Body lice act as transmitters of exanthematic typhus and of a form of relapsing fever. Tropical conditions are not favorable for the development of body lice. This explains why the first disease is hardly known in the tropics and does not spread, while, as a rule, spirillary fevers are transmitted only by other parasites.

(2) Ticks. – Ticks may be divided into Ixodinæ and Argasinæ. The first attach themselves to their hosts for shorter or longer periods, while the Argasinæ mostly live like the bedbug, hiding by day and attacking at night when they gorge themselves with blood. Both subfamilies may transmit human and animal diseases due to blood parasites, but, so far as we know, no human disease found in the tropics is carried by Ixodinæ. However, the possibility of such a transmission cannot be excluded.

Ixodinæ do not include man among their regular hosts, nor do they seem to have a special predilection for human blood. Those attacking man live, as a rule, on other animals or attack many animals indiscriminately, such as Amblyomma cajennense. This tick is extremely troublesome, principally in its larval state, because of its great number. As a prophylactic measure one may keep to the beaten path, principally in the season of larval ticks, wear smooth gaiters, change clothes after being exposed to infestation and sponge the body with spirits after walks.

The Argasinæ are more common in warm countries and exist in all the larger continents. In man and domestic animals they transmit spirochete infections which may easily be acquired while traveling, often without any knowledge of the existing danger. Prophylactic measures are practically the same as those used against bedbugs and other blood-sucking hemiptera. The danger is mainly limited to the sleeping places and may be avoided by a careful selection of such. In some countries it is preferable to use tents and erect them in places distant from human dwellings.

Some of the Argasinæ burrow in dust and sand, others hide in chinks and cracks of the floor, the walls, and the furniture. Probably all of them are independent of human blood and may get their supply from domestic or wild animals. The burrows of such are most likely to be their first natural living place.

(3) Fleas. – It is now generally supposed that only rat fleas transmit the plague to man, but I have reasons to believe that the fleas of man, dog, and cat may occasionally act in the same way, as they are much less specialized than the rat fleas. When hungry, dog fleas will attack man and almost any animal; they are also much more common than Pulex irritans.

To prevent fleas from breeding, houses, kennels, etc., may be washed once a week with hot or plenty of cold water. Creolin may be added but is not necessary. Dogs and cats, when kept in the house, may be freed from fleas by washing with creolin solution or by the use of insect powder. Young animals are especially subject to fleas.

A few tapeworms of cats, dogs and rats, for which the fleas are intermediate hosts, may occasionally infest man, mostly children, for instance, by infected fleas falling into milk or other liquids used for drinking.

* Chapter X of Nelson's Loose Leaf Living Medicine. New York/London, The Thomas Nelson & Cia, p. 365-78. Typewritten copies with handwritten amendments and printed trial copy with corrections can be found in BR. MN. Fundo Adolpho Lutz, box 27, folder 53.