Medical Entomology (Insects)

Medical entomology is concerned with the impact of insects and related arthropods on the mental and physical health of humans, domestic animals, and wildlife. It is often subdivided into public health entomology and veterinary entomology. These divisions are tenuous since many of the same arthropods cause similar injuries and diseases in both humans and other animals. The history of medical entomology dates from the end of the 19th century, when arthropods were first shown to transmit important human diseases such as filariasis and malaria. Most arthropod-borne diseases are zoonotic infections that occur naturally in nonhuman hosts. Malaria, dengue fever, and most forms of filariasis are important exceptions. Household pests such as cockroaches and filth flies are sometimes included within medical entomology. When synanthropic flies and cockroaches mechanically contaminate food or other media with infectious organisms, they are clearly of medical importance. Nonetheless, these insects are generally treated in greater depth within the scope of urban entomology.

MEDICAL IMPORTANCE OF ARTHROPODS

Arthropods influence animal health in multiple ways. The most significant impact involves their role as primary vectors and alternate hosts of many devastating infectious disease agents. Parasitic agents transmitted by hematophagous arthropods include filariae, protozoa, bacteria, rickettsiae, and viruses. Arthropods also affect the health of vertebrates directly by triggering altered mental states (delusional parasitosis and entomophobia/arachnophobia), contact allergies, feeding annoyance and blood loss, envenomization, and myiasis.
Each year these arthropod relationships collectively cause the death of millions of humans and bring illness to hundreds of millions more. Their impact is greatest in poor tropical countries where they are one of the major factors limiting animal production, agricultural productivity, economic development, and well-being. Several vector-borne diseases (e.g., plague, malaria, leishmaniasis, yellow fever,and dengue) apparently were transported from the Old World to the Western Hemisphere via the slave trade or ship crews. Related species of insects in the New World were able to successfully maintain and transmit these introduced parasites. In recent years, several native but previously unrecognized arthropod-borne infections, including tick-borne Lyme disease and ehrlichiosis, have been discovered in the United States and elsewhere. Well-known diseases such as dengue and malaria have been reemerging or expanding in many regions of the world. Other diseases such as West Nile fever have recently been introduced into new regions with devastating effects. A combination of population growth, rapid movement of people and other animals, and environmental disruption has contributed to the growing threat posed by vector-borne diseases. Control of these diseases is complicated by a lack of investment in public health and development of drug-resistant parasites and insecticide-resistant vectors.

ORDERS AND FAMILIES OF MEDICAL CONCERN

Phylum Arthropoda contains several classes of invertebrates that have direct medical importance. In addition to insects, these include arachnids (spiders, mites, ticks, and scorpions), millipedes, and centipedes. Some crustaceans (sowbugs and copepods), molluscs (snails), arachnids (oribatid mites), and insects (mainly beetle larvae) are intermediate hosts for parasitic worms.
Orders and families of blood-feeding species are of major medical significance (Table I). Most but not all of these are involved in the transmission of microparasites. Nonetheless, the simple act of feeding by arthropod ectoparasites can result in blood loss, anemia, stress, discomfort, allergic reactions, and reduced productivity. A few insects are specialized ectoparasites on humans. Bed bugs and kissing bugs are nest parasites and some have permanently invaded human dwellings and feed at night on sleeping people. Humans are also parasitized by three species of sucking lice (i.e., pubic louse, head louse, and body louse), each of which specializes in a different region of the body. Biting flies such as mosquitoes, black flies, deer flies, and biting midges can reach annoyance levels that make outdoor activities nearly impossible, but these hordes normally feed on a wide variety of domestic and wild animals. Only a few biting flies such as the yellow-fever mosquito (Aedes aegypti), the tropical house mosquito (Culex pipiens quinquefasciatus), and some vector species of Anopheles, Simulium, Phlebotomus, and Lutzomyia feed preferentially on humans.
Both insects and arachnids can cause harm because of the venoms they contain. Venomous insects are found mainly in the order Hymenoptera within the families Formicidae (ants), Vespidae (yellowjackets and hornets), Mutillidae (velvet ants), and Apidae (honey bees and bumble bees). Some Coleoptera (e.g., Meloidae, Staphylinidae, Chrysomelidae, and Dermestidae) and Lepidoptera (e.g., Noctuidae, Saturniidae, Sphingidae, and Nymphalidae) also produce toxic defensive secretions through specialized glands and urticating hairs or have hemolymph that is toxic to vertebrates if the insect is crushed. These secretions are especially toxic when exposed to mucus or lachrymal glands. Spiders in the genera Loxosceles (recluses), Latrodectus (widows), Atrax (Australian funnel-webs), Harpactirella (South Africa), Lycosa (Central and South America), and Phoneutria (Brazil) include some of the most highly venomous species. All scorpions have venomous stings and those within the family Buthidae can be fatal to humans. Desert regions of the Americas, the Mediterranean, and northern Africa are home to most of the highly poisonous scorpions. Centipedes also have venomous

aEctoparasites that feed mainly on skin/feather tissues rather than on blood. bL, larvae; N, nymph.
bites and millipedes have toxic defensive secretions, but these normally are not severe or life threatening.
Arthropod allergens that cause acute asthma in humans are mostly associated with house dust mites (Dermatophagoides spp.) and cockroaches (Periplaneta and Blattella); however, the airborne wing scales or fine setae associated with large populations of other insects such as mayflies, caddisflies, and gypsy moths may invoke allergic reactions in sensitized individuals. Three families of dipterous insects (Calliphoridae, Sarcophagidae, and Oestridae) include species whose larvae are obligate parasites living within the flesh of vertebrates, a condition referred to as myiasis. The unusual parasitic mites that live within the feathers, nasal passages, and lungs of birds are relatively benign.

DIRECT INJURIES CAUSED BY ARTHROPODS

The various direct effects of arthropods on humans are summarized in Table II. The most common of these are the asthma suffered by millions of people, especially children, who are allergic to the fine airborne particulates generated by insects and mites. These allergens are most often associated with feces or decomposing body parts. The advent of air-conditioning and wall-to-wall carpeting seems to haveexacerbated this problem especially among the middle and upper classes. Asthma is one of the fastest growing medical problems, particularly among children. Respiratory failure is not an uncommon outcome. Concern over life-threatening multiple stings and allergic reactions (anaphylaxis) to the venom of insects has increased as a result of the introduction and spread of the hybrid of the African honey bee (Apis mellifera adansonii) and the imported fire ant (Solenopsis invicta) in the Western Hemisphere.
Myiasis is a serious problem in animal production, especially in the neotropics where millions of dollars is lost annually due to these tissue-invading flies. These flies are often found within families that include species that normally feed on the decaying tissues of dead or wounded animals, that is, flesh flies in the family Sarcophagidae and blowflies in the family Calliphoridae. Some species of Calliphoridae (and related families) sometimes will facultatively invade living tissues, while others are so restricted to dead tissues that they are used in maggot therapy to clean deep wounds. All members of the four subfamilies of bot flies (Oestrinae, Gasterophilinae, Hypoderminae, and Cuterebrinae of the family Oestridae) are obligate parasites. The torsalo (Dermatobia hominis) is a neotropical dipteran whose eggs are glued to the abdomen of biting flies, and its larvae emerge during blood feeding by the host fly.

ARTHROPOD TRANSMISSION OF MICROPARASITES

Vector-borne diseases can be transmitted either biologically or mechanically. In mechanical transmission, vector mouthparts serve as contaminated hypodermic needles since there is no replication or development of the microparasite in the vector; the vector does not serve as an alternate host as in the case of biologically transmitted diseases. For infectious organisms to be mechanically transmitted by arthropods efficiently, they must be abundant in circulating blood or cutaneous tissues and able to survive external exposure. Diseases that are mechanically transmitted by arthropods generally have other transmission mechanisms as well.
Biological transmission takes one of three forms: propagative transmission, which involves the replication of eukaryotic parasites and dissemination to the salivary glands prior to transmission; cyclodevelopmental transmission, which occurs among filarial parasites, in which development of the parasite to the infective stage is required prior to transmission but there is no increase in the number of parasites; or cyclopropagative transmission, which involves both development and multiplication by the parasite as occurs with protozoan parasites such as Plasmodium, Leishmania, and Trypanosoma.
After the appropriate extrinsic incubation period for replication and/or development of the parasite, the arthropod vector is said to be infective, that is, able to transmit. Infectious agents are passed to the vertebrate host through a number of different routes: (1) transovari-ally from the female to her offspring or transtadially from one stage to the next, (2) venereally from infected males to uninfected females, (3) through cofeeding when infected and uninfected vectors group feed, or (4) horizontally through (a) infective saliva injected during feeding, (b) regurgitation of parasites blocking the food canal, (c) defecation of infective feces on the skin, or (d) active escape from the mouthparts and invasion of the skin. In some cases, hosts must assist the transmission process by crushing the infective insect, scratching the contaminated area, and rubbing the eyes. Transmission may be promoted by the physiological or behavioral effect of parasites on the vector. Invasion of host cells or tissues by the parasite can be modulated by host immune responses to the salivary secretions of the vector.
The efficiency of transmission is determined primarily by the “competence” of the arthropod species to support development/ replication of the parasite and by the ecology and behavior of the arthropod species. The latter determines the temporal and spatial connection between hosts and potential vectors. Thus, the density, feeding frequency, and host preferences of the vector play critical roles in establishing the vectorial capacity (number of infective bites received daily by a single host) of any given species. Environmental conditions also play an important role.

DISEASES TRANSMITTED BY ARTHROPODS

The major diseases transmitted by arthropods are listed in Table III. Protozoan parasites dominate this list in terms of worldwide importance. Malaria is the single most significant vector-borne disease, with an estimated 300 million people infected annually and over 1 million deaths among young children in Africa alone. It is endemic in most tropical and subtropical regions of the world, where it has been resurging since eradication attempts ended some 30 years ago. It is transmitted by Anopheles mosquitoes and can be successfully treated with drugs if promptly available. Drug resistance is a growing problem. Tsetse-transmitted African trypanosomiasis remains a serious human disease in parts of tropical Africa, but its impact on domestic cattle production is even more severe. Wild bovines are the reservoir of the acute Rhodesian form, and humans and porcines are the reservoir of the chronic Gambian form. American trypanosomiasis is restricted to the mountain regions of tropical America and is often a silent disease that leads to early death. Great efforts are under way to control this zoonotic disease by spraying residual pesticides and by constructing houses with materials that prevent invasion by domesticated kissing bugs. Visceral and cutaneous forms of leishmaniasis affect millions of people and cause significant mortality in Africa, Asia, and South America. The reservoirs for these parasites are dogs, rodents, and a variety of other wild mammals. The cutaneous form also exists in southern Europe and Central America, but disfigurement rather than mortality is usually associated with this form.
Lymphatic filariasis (elephantiasis), transmitted by Culex quinque-fasciatus and other human-biting mosquitoes, infects many millions of residents throughout the tropics. The long-lived nematode parasites cause debilitation, especially of the lower limbs, but seldom result in death. Onchocerciasis (river blindness), also a filarial infection, is limited to sub-Saharan Africa and some coffee-growing regions in Central and South America. Both of these nematode infections can be prevented by treatment with a new drug, ivermectin. River blindness has been effectively controlled in much of West Africa in recent years through pesticidal elimination of the black fly vectors breeding in streams and the use of antihelminthics to treat the human population. There are a large number of bacterial and rickettsial infections transmitted by arthropods (mainly ticks, fleas, lice, and mites) annually but none compare to the impact of plague and typhus epidemics in earlier times. Some of these, such as tick-borne Lyme disease and ehrlichiosis, have just been recognized within the past 30 years. Lyme

aWN, West Nile; SLE, St. Louis encephalitis; JE, Japanese encephalitis; MVE, Murray Valley encephalitis; WSL, Wesselsbron; RSSE, Russian spring-summer encephalitis; OMSK, Omsk hemorrhagic fever; KFD, Kyasanur Forest disease; LI, Louping ill; POW, Powassan; CE, California encephalitis; LAC, La Crosse encephalitis; RVF, Rift Valley fever; ORO, Racio; CCHF, Crimean-Congo hemorrhagic fever; NSD, Nairobi sheep disease; SFF, sand fly fever; EEE, Eastern equine encephalitis; WEE, Western equine encephalitis; VEE, Venezuelan equine encephalitis; RR, Ross River; VS, vesicular stomatitis; BEF, bovine ephemeral fever; AHS, African horse sickness; EHD, epizootic hemorrhagic disease.
disease is the most prevalent vector-borne disease in the United States. Protective vaccines exist for few bacterial and rickettsial diseases, but all respond to timely antibiotic therapy.
Arthropod-borne viral diseases (arboviruses) are transmitted mainly by mosquitoes and biting midges, but Russian spring-summer encephalitis and several other zoonotic infections (especially of livestock) are transmitted by ticks. Historically, yellow fever was the most important human arboviral disease, but today it has been replaced by dengue fever. Dengue viruses infect over a million people annually and can produce a fatal hemorrhagic disease, especially among children. This disease has reinvaded the Western Hemisphere in recent decades and now causes hundreds of thousands of cases annually. Vaccines exist for a few arboviral diseases (yellow fever and Japanese encephalitis), but vector control is often the only preventative measure.

DISEASE AND VECTOR MANAGEMENT

Efforts to develop vaccines for a wide range of vector-borne diseases, including malaria, have been vigorously supported but success has been slow. Antigenic variation in parasites has been a major deterrent. New drugs for treatment of parasitic diseases also are under development, and several important new drugs for treatment of helminth and protozoan parasites have been marketed in recent years. Nonetheless, vector control is often the first line of defense against the transmission of these diseases and, during active epidemics, this is the only option outside of public education. Control of vertebrate reservoir animals has occasionally been practiced for diseases that are maintained by rodents. An early example was control of wild bovine reservoirs in parts of Africa to control trypanosomiasis in humans (sleeping sickness) and cattle (Nagana). Vector control programs are generally based on surveillance systems that monitor and report cases of disease or vector population levels. A variety of tools are available to manage vector populations, including chemical pesticides, biological controls, habitat alteration, and personal protection strategies (e.g., screens, bed nets, and repellents). Currently, genetic control strategies are receiving much attention. Optimal vector control programs utilize integrated vector management strategies and strive to maintain vector populations below the threshold densities required for transmission. Targeting the immature stages of vectors that blood feed and transmit disease only as adults normally is more efficient and cost effective. Lack of public health funds is the major limitation on surveillance and vector control programs, especially in developing countries where these diseases have the greatest impact.