Medical Entomology

Many insects, spiders, mites, myriapods, and other arthropods are medically important, acting either as agents of harm to humans or as vectors of pathogenic microorganisms. This is such an important aspect of our existence that several textbooks treat the subject from an overall perspective in considerable detail (Faust et al. 1962; Flechtmann 1973; Horsfall 1962; James and Harwood 1969; Kettle 1984; Smith 1973). Regional discussions are also available for Argentina (del Ponte 1958), Brazil (Pinto 1930), Central America (Baerg 1929), Panama (Méndez and Chaniotis 1987), and South America (Bücherl 1969).

Because they inject or dispense venoms, members of many groups (Bücherl and Buckley 1971) are serious agents of medical problems throughout Latin America. The most important offenders are scorpions (Tityus, Centruroides), spiders (Latrodectus, Loxosceles, and Phoneutria), and stinging Hy-menoptera (Apis, ants, and wasps) (Akre and Davis 1978). Poisons that act topically (vesicants) are produced by millipedes, blister beetles, fire beetles (Paederus), and others (Hoffman 1927). Nettling hairs or spines, such as adorn many caterpillars (Saturniidae, Limacodidae, Megalopygi-dae, etc.), also implant toxins (urtication, erucism).

Reactions to toxic substances (Tu 1984) may be slight to severe, even fatal in rare cases. Such effects occur either through direct toxification or by eliciting allergic responses through antigens (Frazier 1969), superficially (Orkin and Maibach 1985) or systemically. Any protein derived from the insect's body may cause a harmful reaction if it comes in contact with tissue topically or by injection. A common means of injection is through the bite of blood-feeding forms, that is, mosquitoes, ticks, mites, and sand flies (Feingold et al. 1968). The antigen is contained in the saliva and enters the bloodstream directly or via the lymphatics. Hypersensitive individuals exhibit varied symptoms, ranging from mild dermatosis to anaphylactic shock, which can be fatal.

Many people have extreme fears or phobias of insects and similar creatures, leading to psychoneuroses. A common form is delusory parasitosis (Waldron 1963), the unshakable belief that one's skin and orifices are infested with minute, barely visible insects, mites, or other vermin. This condition appears to be a symptom of a variety of organic and mental disorders. Extreme phobias also affect many persons, especially against large, very hairy, dark, or noisy species. These emotional complaints are not as well documented in Latin America as in other parts of the world but are surely as widespread.

Human myiasis is another major problem caused directly by larval Diptera, especially of blowflies and a few special types like the human botfly (Dermatobia). The body may be invaded by maggots, leading to a variety of symptoms, many highly repugnant psychologically as well as physically (Beesley 1974; and see Myiasis, chap. 11).

Insects and their relatives are highly efficient and diverse as transmitters of other pathogenic organisms. Disease microorganisms may be carried by the insect passively (mechanically) or may pass through certain of its developmental stages in the arthropod host, which is then considered an obligatory or "biological" vector. Several major groups are vectors of human and animal pathogens, including biting flies, fleas (Bibikova 1977), kissing bugs, blood-feeding mites, and ticks.

Many viruses, bacteria, and amoebic or worm cysts are mechanically transmitted. They are carried on the bodies, on the mouthparts, and in the intestines of filth flies, cockroaches, and other insects that frequent contaminated matter and food eaten later by humans. Many dysenteries, tapeworm, and nematode diseases are spread in this way, particularly under very unsanitary conditions when poverty or social disruption, such as war or natural disasters, prevails in a human population. Poliomyelitis, typhoid fevers, cholera, leprosy, and other diseases may also find new human hosts in this manner. Research indicates that the AIDS virus is not transmitted by insects, blood feeding or otherwise.

Biological vectors are found entirely among blood-feeding types, especially mosquitoes and other biting midges and flies, although ticks and mites are also significant. Their ecology is a principal factor determining the effectiveness of these insects as vectors (Muirhead-Thomson 1968). The organisms of over a dozen major types of human diseases are transmitted by arthropods in Latin America. The most notorious and widespread of these is malaria, which is caused by four species of plasmodial protozoans. These organisms invade various organs and destroy red blood cells, releasing toxins into the circulation which cause racking chills and fever. Vectors are several species of mosquitoes in the genus Anopheles.

Leishmaniasis is an affliction also caused by protozoans, at least three species of flagellates in the genus Leishmania. Sand flies (Lutzomyia, Psychodidae) carry these agents that invade and chemically destroy both dermal and internal tissues of vital organs. Another flagellate protozoan, Trypanosoma cruzi, that develops in kissing bugs (Reduviidae, Triatominae) brings on a serious ailment called Chagas's disease in many parts of Latin America. Visceral organs suffer chronic damage, which leads ultimately to death in many untreated cases.

Various parasitic nematode worms introduced from the Old World have become established in certain areas and cause a variety of filarial infections. These include Wuchereria bancrofti (Bancroftian filariasis), Onchocerca volvulus (onchocerciasis), and Dirofilaria imitis (dog heartworm). While seldom fatal, they wreak considerable dam age by invading the tissues, producing inflammation, enlargement, and destruction. When essential organs such as the eye or brain are involved, critical functions of the senses may be impaired. Vectors are mostly mosquitoes, but blackflies, punkies, and tabanids also serve as carriers.

A large and growing number of viruses ("arboviruses") are being discovered which require biting fly, mite, and tick vectors. The worst of these historically has been the yellow fever virus, transmitted by mosquitoes, especially the yellow fever mosquito, Aedes aegypti. Several kinds of encephali-tides, hemorrhagic fever, and dengue fever are also in this category. The viral genus Phlebovirus, transmitted by phleboto-mine sand flies (Lutzomyia) and mosquitoes, contains many species of human pathogens causing intense flulike diseases (Tesh 1988).

Epidemic (Rickettsia prowazekii) and endemic (R. mooseri) typhus organisms pass to humans from the bodies of lice and fleas. A third rickettsia (R. rickettsii), that of Rocky Mountain spotted fever, is borne by hard ticks. These microbes invade and destroy the inner lining of small blood vessels. High fevers, often followed by death, occur. Similar symptoms follow infection by the spirochetes of relapsing fevers (Borrelia) transmitted through the bite or body secretions of ticks and lice.

The infamous plague bacillus (Yersinia pestis) still resides in animal reservoirs in parts of Latin America and sometimes makes its way to the human population via flea bites. Fortunately, epidemics like those of the past in Europe and elsewhere have not occurred in recent times.

Most of these diseases have been controlled by modern insecticides applied against the carriers and by drugs that kill the pathogens. However, as a result of relaxation of abatement campaigns and development of chemical resistance by both insects and microorganisms, some diseases are experiencing a resurgence and are again causing major problems in areas formerly thought free of them.

References

Akre, R. D., and H. G. Davis. 1978. Biology and pest status of venomous wasps. Ann. Rev. Entomol. 23: 215-238.

Baerg, W.J. 1929. Some poisonous arthropods of North and Central America. 4th Int. Cong. Entomol. (Ithaca, 1928) Trans. 2: 418-438.

Bef.sley, W. N. 1974. Arthropods—Oestridae, myiases and acariñes. In E. J. L. Southby, Parasitic zoonoses. Academic, New York. Pp. 349-368.

Bibikova, V. A. 1977. Contemporary views on the interrelationships between fleas and the pathogens of human and animal diseases. Ann. Rev. Entomol. 22: 23-32.

Bücherl, W. 1969. Giftige arthropoden. In E.J. Eittkau, J. lilies, H. Klinge, G. H. Schwabe, and H. Sioli, Biogeography and ecology in South America. 2: 764-793. Junk, The Hague.

Bücherl, W., and E. E. Buckley. 1971. Venomous animals and their venoms. 3. Venomous invertebrates. Academic, New York.

del Ponte, E. 1958. Manual de entomología médica y veterinaria Argentinas. Ed. Lib. Colegio, Buenos Aires.

Faust, E. C., P. C. Beaver, and R. C. Jung. 1962. Animal agents and vectors of human disease. Lea and Febiger, Philadelphia.

Feingold, B. F., E. Benjamini, and D. Michaeli. 1968. The allergic responses to insect bites. Ann. Rev. Entomol. 13: 137-158.

Flechtmann, C. H. W. 1973. Acaros de importancia médico veterinária. Liv. Nobel, Säo Paulo.

Frazier, C. A. 1969. Insect allergy, allergic and toxic reactions to insects and other arthropods. Green, St. Louis.

Hoffman, W. A. 1927. Irritation due to insect secretion. Amer. Med. Assoc. J. 88: 145 — 146.

Horsfall, W. R. 1962. Medical entomology, arthropods and human disease. Ronald, New York.

James, M. T., and R. F. Harwood. 1969. Herm's medical entomology. 6th ed. Mac-Millan, London.

Kettle, D. S. 1984. Medical and veterinary entomology. Wiley, Somerset, N.J.

Méndez, E., and B. Chaniotis. 1987. Reseña de las principales enfermedades transmitidas por garrapatas en Panamá. Rev. Med. Panamá 12: 217-223.

Méndez, E., and B. Chaniotis. 1987. Reseña de las principales enfermedades transmitadas por insectos en Panamá. Rev. Med. Panamá 12: 205-216.

Muirhead-Thomson, R. C. 1968. Ecology of insect vector populations. Academic, London.

Orkin, M., and H. I. Maibach. 1985. Cutaneous infestations and insect bites. Marcel Dek-ker, New York.

Pinto, C. 1930. Arthropodes parasitos e transmissores de doen^as. Vols. 1—2. Pimento de Mello, Rio de Janeiro.

Smith, K. G. V., ed. 1973. Insects and other arthropods of medical importance. Trus. Brit. Mus. Nat. Hist., London.

Tesh, R. B. 1988. The genus Phl.ebovi.us and its vectors. Ann. Rev. Entomol. 33: 169-181.

Tu, A. T. 1984. Handbook of natural toxins: Insect poisons, allergens, and other invertebrate venoms. Vol. 2. Marcel Dekker, New York.

Waldron, W. 1963. Psychiatric and entomological aspects of delusory parasitosis. Arner. Med. Assoc. j. 186: 213-214.

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