ARTHROPODS OF MEDICAL IMPORTANCE

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I. ARTHROPOD-ASSOCIATED BACTERIAL INFECTION
Many bacterial, rickettsial, parasitic and viral diseases can be classified as zoonoses because they are acquired by humans either directly or indirectly from arthropod vectors. Examples of pathogens acquired in this manner include the Borrelia sp., Yersinia pestis, R. rickettsii, R. typhi, R. prowazekii, Plasmodium sp., Trypanosoma sp., and the arboviruses. Some of these organisms are primarily animal pathogens, causing disease only rarely in humans and only after unique animal contact. Other arthropods are merely vectors of human pathogens.

A. Borrelia recurrentis and Relapsing Fever

Endemic and epidemic diseases
Relapsing fever is a disease transmitted to humans by ticks in the case of endemic relapsing fever or by body lice in the case of epidemic relapsing fever. The causative spirochetes have remarkable antigenic variability. Disease is characterized by two or more relapses associated with selection of antigenic variants.

B. Borrelia burgdorferi and Lyme disease.
Disease is transmitted to humans by the bite of a small tick of the genus Ixodes and is characterized by the "bulls-eye" lesions that begin at the site of the tick bite. Sequelae may include arthralgia, arthritis and other immune complex deposition manifestations. Mice and deer constitute the main animal reservoir of B. burgdorferi within ticks.

C. Severe louse-borne disease due to Rickettsia prowazekii

Endemic foci vs. Epidemic Disease
Primary louse-borne typhus fever is an acute, infectious disease caused by R. prowazekii that is transmitted to humans by the body louse. Historically, it has appeared during periods of war, famine, and social upheaval (crowding, infrequent bathing, etc.). The last North American epidemic was in Philadelphia in 1893; however, endemic typhus foci persist in Eastern Europe, Asia, Africa, and South America. Although primarily a human disease, identical organisms have been recovered from flying squirrels and their ectoparasites in the southeastern United States, and a few human cases of sylvatic typhus have occurred.

The chain of epidemic typhus infection starts with R. prowazekii circulating in a patient's blood during an acute febrile infection. The human body louse becomes infected during one of its frequent blood meals, and, after 5-10 days of incubation, large numbers of rickettsiae appear in its feces. As the louse defecates while it feeds, the organisms can be rubbed into the louse-bite wounds when the host scratches the site. Dried louse feces are also infectious through the mucous membranes of the eye or respiratory tract. The louse dies of its infection in 1-3 weeks, and the rickettsiae are not transmitted transovarially.

D. Murine typhus: Rickettsia typhi
Murine typhus is transmitted to humans by the bite of rodent fleas; symptoms are similar to those caused by the human body louse-transmitted rickettsial diseases.

E. Rickettsial Spotted Fevers (RMSF)
Rickettsia rickettsii is primarily a parasite of ticks. In the western United States, the wood tick (Dermacentor andersoni) is the primary vector. In the East the dog tick (Dermacentor variabilis) and in the Southwest the Lone Star tick (Amblyomma americanum) are the natural carriers and vectors of the disease. Rickettsiae acquired transovarially are present in the larval, nymph, and adult stages of development, and the organism does not kill its arthropod host. The larval and nymph stages require a blood meal from a small mammal to proceed to the next stage. Adult females require a blood meal to lay eggs. Infected adult ticks have been shown to survive for as long as 4 years without feeding. Disease manifestations include characteristic bulls-eye rash and general malaise with or without sequelae.

F. Other Rickettsial Diseases
Rickettsia tsutsugamushi (Scrub Typhus): the vector is the mite (transovarial transfer) with rodents and other small mammals as the reservoir.

Rickettsia akari (Rickettsialpox): mites are vectors (transovarial transmission) and with rats are the reservoir; the disease is spread to humans through a mite bite. The disease produces a mild fever with a skin rash.

II. ARTHROPOD-ASSOCIATED PARASITIC INFECTION

A. Plasmodia: The Malarial Parasites
Female Anopheles mosquitos ingest Plasmodium gametocytes from infected human hosts and the sexual phase of the parasite occurs within the gut, resulting in thousands of sporozoites being released into the body cavity. The sporozoites find their way to the arthropod's salivary glands where they are transmitted to human while the mosquito takes a blood feeding. Within the host red cells the Plasmodia reproduce asexually and eventually burst from the erythrocyte and invade other uninvolved red cells. This event produces periodic fever and anemia in the host, a disease process known as malaria. Included are the species Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparum.

B. Trypanosomiasis
These are invasive blood and tissue parasites that produce highly morbid, frequently lethal diseases via intermediate insect host transmission. As a result, their associated disease states are limited to the semitropical and tropical niches of these insect hosts.

American trypanosomiasis is a disease produced by T. cruzi and transmitted by the bite of true bugs of the family Reduviidae. Endemic areas include Central and South Amierica. Infected bugs may remain infectious for up to 2 years. The trypomastigotes disseminate from the site of inoculation to circulate in the peripheral blood of their human hosts. Clinically, the infection presents as an acute febrile illness in children and a chronic heart or gastrointestinal malady in adults.

West African sleeping sickness is transmitted by the tsetse fly, and is consequently confined to the central area of Africa by the Sahara Desert in the north and the Kalahari Desert in the south.

C. Leishmaniasis
Localized cutaneous lesions or ulcers are produced by infection with one of the Leishmania sp.: L. tropica in the Old World and L. mexicana in the New World; L. braziliensis is the cause of American mucocutaneous leishmaniasis (espundia), and L. donovani of kala azar, a disseminated visceral disease.

All four species transmitted by nocturnally feeding phlebotomine sandflies. These sandflies are small, delicate, short-lived biting insects that are found in animal burrows and crevices throughout the tropics and subtropics. The blood feast transfers Leishmania to the flies, where the parasites multiply within the gut and mature. The mature promastigotes are injected into the skin of the new mammalian host where they are taken up and multiply within macrophages.

The disease is a zoonotic infection of tropical and subtropical rodents particularly common in areas of China, India, Asia Minor, Africa, around the Mediterranean, and Central America. In the Mediterranean area, southern Russia, and India, human disease involves urban transmission, with the domestic dog serves as the reservoir. Direct human-to-human transmission also occurs.

III. ARTHROPOD-ASSOCIATED VIRAL INFECTION

A. Introduction
The arboviruses (arthropod-borne viruses) comprise more than 400 agents with worldwide distribution. Their name is taken from their mode of transmission, which is primarily by infected bloodsucking insects such as mosquitoes, ticks, and phlebotomus flies (sandflies).

B. Disease
On a worldwide basis the most important arboviruses (in terms of amount and seriousness of disease caused) include yellow fever, dengue, Japanese B encephalitis, St. Louis encephalitis, western equine encephalitis, eastern equine encephalitis, Russian spring-summer encephalitis, West Nile fever, and sandfly fever.

C. U.S. Epidemiology
In the U.S. western equine, eastern equine, St. Louis, and California encephalitis viruses are the most important arboviral disease agents; seasonal incidence peaks in the summer and early fall months, the season of greatest activity of the arthropod vectors (usually mosquitoes or ticks). The arboviruses can produce disease ranging from simple, febrile, influenzalike illness to hemorrhagic disease or encephalitis.

IV. SOCIAL AND ECOLOGICAL DETERMINANTS OF EPIDEMIC DISEASES
The association between social/ecological factors and disease is is multifactorial and includes overcrowding, contaminated food and water, an increase in arthropods that parasitize humans and can carry some epidemic diseases, and the reduced immunity that can accompany severe malnutrition or certain types of chronic stress. Overcrowding and understaffing in day-care centers or institutes for the mentally impaired can similarly be associated with epidemics of infections.
Measures must be adopted to control the spread and development of further infection including blocking the route of transmission if possible (improved arthropod control via broad insecticide application and increased personal hygiene). In endemic areas, arthropod contact can be minimized through the use of house screenings, mosquito netting around beds, and insect repellents.
In American trypanosomiasis (Chagas disease), the reduviid vector can be controlled by applying residual insecticides to rural buildings at 2-month intervals. The addition of latex to the insecticide creates a colorless paint that prolongs activity. Fumigants can be used to prevent reinfection. Patching wall cracks, cementing floors, and moving debris and woodpiles away from human dwellings can reduce the number of bugs within the home.

V. HARMFUL DIRECT EFFECTS OF ARTHROPODS
A. Stings and Bites
Some individuals are hardly affected by the bites and stings of arthropods; others react with allergic manifestations and generalized as well as local effects. Reactions include not only the commonly recognized source of arthropod allergy, such as bee or wasp stings, but also flea, mosquito and spider bites.

The stings of certain species of scorpions, the bites of centipedes and of spiders such as Lactrodectus mactans, the black widow, may be very dangerous and even fatal. While healthy adults will usually recover from a moderate dose of venom, weakened individuals or small children may die if proper care is not given. Hospitalization is usually indicated for poisonous arthropod bites, and muscle relaxants may be required.

Spiders of the genus Loxosceles, commonly known as the brown recluse or fiddle-back spider, may cause the appearance of necrotizing skin lesions. The bite itself is usually painless, but develops into mild to severe pain within a few hours. Erythema, vesicle formation and itching occur at the site of the bite, frequently followed by chills, headache and nausea. Some species can cause kidney damage and death. Initial lesions ulcerate, become necrotic and continue to spread for weeks or even months if not treated. Prompt administration of corticosteroids minimizes inflammation. Surgical excision of the lesion and skin grafting may be required in advanced cases.

B.Scabies
Aside from the consideration of disease transmission as vectors of bacterial infection, human ectoparasites are causative of a great deal of human discomfort. Most are associated with intense itching and burning in the infested individual and scratching can result in secondary infections by normal skin flora such as Staph. aureus.

Sarcoptes scabiei parasitizes both humans and domestic animals, causing scabies in the former, mange in the latter. The size is 0.5 mm in length, and the appearance is "like a catcher's mitt with legs". Adult mites enter the skin, digging burrows in the upper epidermal layers. Eggs are deposited in the burrows and hatch in 3-4 days. Preferred sites of infestation are the interdigital and popliteal folds, the groin area and inframammary folds. Secretions of the arthropods cause intense itching. Spread of the organisms to other parts of the body can occur via scratching as well. In chronic infestations, the number of offending organisms is usually lower, and they are more difficult to find. Definitive diagnosis can be made only by finding the parasites or their eggs. Severe infestations can occur in immunocompromised individuals, resulting in "crusted scabies". The decreased immune responsiveness may result in less intense itching. This in turn allows the mites to reproduce unopposed. Debilitated persons can harbor up to 3 million mites in their thickly crusted skin. This condition can be added to the growing list of diseases that present uniquely in AIDS patients.

C. Human Head and Body Lice
Pediculus humanus corporis and Pediculus humanus capitis are the two species that cause human infestations. They are transmitted from human to human by direct contact or via contaminated clothing, bedding and personal hygiene items (combs, brushes, etc.).

Pediculus humanus capitisis the most common, the human head louse; infestations are common in crowded areas such as schools and prisons. Infestations are extremely common among children, who are particularly uninhibited in the sharing of combs, brushes, hair ornaments and hats. The insects are visible with the naked eye, about 2-3 mm in size. The entire life cycle occurs on the human host. Females deposit eggs on the hair shaft, where they become firmly anchored (nits). Both larvae and adults feed on blood obtained by their piercing mouth parts and a pharyngeal pumping device.

Pediculus humanus corporis is the human body louse; infestations are less frequent than those caused by the head louse. The insects live on the "protected" parts of the body, and are commonly transmitted from infested individuals during casual social contact or sexual contact. Adult lice can survive on clothing for more than a week. Cloth-covered seats in theaters, railway carriages and other public places may be a source of infestation. These insects are involved in the transmission of blood-borne pathogens when they feed first on infected individuals and subsequently on a healthy person. Morbidity is generally associated with the intense itching and discomfort caused by the moving insects and the deposition of secretions on the skin, although significant human disease can be transmitted via this vector. Epidemic relapsing fever is a disease transmitted in this manner by Borrelia recurrentis. Primary louse-borne typhus fever is an acute, infectious disease caused by R. prowazekii that is also body louse transmitted.

Pththirus pubis, the crab louse, is closely related to the other two louse species, but is more difficult to transmit and less common. It is associated almost exclusively with sexual transmission, and is rarely found outside the genital region.

Treatment for each of these insect infestations involves the use of special shampoos and lotions containing permethrin or lindane (Tradenames include Qwell and RID). Ridding the infested persons clothing and surroundings of nits and adult lice is also important in preventing reinfestation. Concurrent treatment of close human contacts is also extremely important.

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