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Ticks are eight-legged arachnids which feed on blood, causing nuisance biting and disease transmission in people and animals worldwide. There are approximately 900 species of ticks comprised of soft tick, hard ticks, and a group that has characteristics of both groups.

 

Rocky Mountain spotted fever

Ticks may transmit a wide variety of bacterial organisms, most notably rickettsiae, classified by scientists into several distinct groups. The spotted fever group (SFG) contains rickettsial species related to the agent of Rocky Mountain spotted fever (RMSF), Rickettsia rickettsii. But there are many other rickettsial species in the spotted fever group; it contains at least ten disease agents and 15 others with low or no pathogenicity to humans. RMSF is the most frequently reported rickettsial disease in the U.S. with about 1800 cases reported each year.  Probably many more cases occur but go unreported. If an unusual febrile illness is treated successfully with doxycycline, there may be little interest in follow-up and reporting. At the time of initial presentation, there is often the classic triad of RMSF: fever, rash, and history of tick bite. Other characteristics are malaise, severe headache, chills, and myalgias. Sometimes, gastrointestinal symptoms such as abdominal pain and diarrhea are reported. More than one member of the family may be infected. The rash, appearing on the third day or after, usually begins on the extremities and then spreads to the rest of the body. However, there have been confirmed cases without rash. Mental confusion, coma, and death may occur in severe cases. Untreated, the mortality rate is about 20%; even with treatment, the rate is approximately 5%.

 

Ehrlichiosis and anaplasmosis

Ehrlichia and Anaplasma are organisms in the family Anaplasmataceae that primarily infect circulating leukocytes. Much of the knowledge gained concerning ehrlichiae has come from veterinary studies on Anaplasma marginale (cattle disease agent), Ehrlichia (Cowdria) ruminantium (cattle, sheep, goats), and Ehrlichia equi (horses). Canine ehrlichiosis, caused by Ehrlichia canis, wiped out 200 to 300 military working dogs during the Vietnam War.

Ehrlichiosis in the U.S. is usually caused by 1-3 closely related Ehrlichia organisms as follows. One, Ehrlichia chaffeensis, the most frequently reported, is the causative agent of human monocytic ehrlichiosis (HME) which occurs mostly in the southern and southcentral U.S. (sporadic cases of HME have also been reported in Europe), and infects mononuclear phagocytes in blood and tissues. There were 944 cases of HME reported to the CDC in 2009. The second, E. ewingii, mostly a dog and deer pathogen, infects granulocytes and causes a clinical illness similar to HME. The third ehrlichial agent, E. muris-like (sometimes called EML), causes fever, malaise, headache, lymphopenia, and elevated liver enzymes in humans. Thus far, it has been reported from the upper midwestern U.S. and the agent is transmitted by the deer tick, Ixodes scapularis.

 

Babesiosis

Human babesiosis is a malaria-like disease primarily associated with two protozoa of the family Piroplasmordia: Babesia microti and Babesia divergens, although other newly recognized species may also cause infection. The disease is a malaria-like syndrome characterized by fever, fatigue, and hemolytic anemia lasting from several days to a few months. In terms of clinical manifestations, babesiosis may vary widely, from asymptomatic infection to a severe, rapidly fatal disease.

Babesial parasites, along with members of the genus Theileria, are called piroplasms because of their pear-shaped intraerythrocytic stages. There are at least 100 species of tick-transmitted Babesia parasitizing a wide variety of vertebrate animals. Some notorious ones are as follows: Babesia bigemina, the causative agent of Texas cattle fever; B. canis and B. gibsoni, canine pathogens; B. equi, a horse pathogen that occasionally infects humans; B. divergens, a cattle parasite that infects humans; and B. microti, a rodent parasite that may infect humans.

 

Tularemia

Tularemia, sometimes called rabbit fever or deer fly fever, is a bacterial zoonosis occurring throughout temperate climates of the Northern Hemisphere. Approximately 150 to 300 human cases occur in the U.S. each year, but most cases occur in Arkansas, Missouri, and Oklahoma. The causative organism, Francisella tularensis, is a small, Gram-negative, nonmotile coccobacillus named after Sir Edward Francis (who did the classical early studies on the organism) and Tulare, CA (where it was first isolated). The disease may be contracted in a variety of ways: food, water, mud, articles of clothing, and (particularly) arthropod bites.

Arthropods involved in transmission of tularemia include ticks, biting flies, and possibly even mosquitoes.

 

Colorado tick fever

Colorado tick fever (CTF) is a generally moderate, acute, self-limited, febrile illness occurring out West which is caused by a Coltivirus in the Reoviridae. Typically, onset of CTF is sudden, with chilly sensations, high fever, headache, photophobia, mild conjunctivitis, lethargy, myalgias, and arthralgias. At least in humans. the body temperature pattern may be biphasic, with a 2- to 3-d febrile period, a remission lasting 1- to 2-d, then another 2- to 3-d of fever, sometimes with more severe symptoms. Peak incidence is during April and May at lower elevations and during June and July at higher elevations. The virus is maintained in nature by cycles of infection among various small mammals and the ticks that parasitize them. Infection in humans is by the bite of an infected tick. Several tick species have been found infected with the virus, but Dermacentor andersoni is by far the most common. This tick is especially prevalent where there is brushy vegetation to provide good protection for small mammalian hosts of immature ticks and yet with sufficient forage to attract large hosts required for the adults.

 

Relapsing fever

Tick-borne (endemic) relapsing fever (TBRF) is a systemic spirochetal disease characterized by periods of fever lasting 2 to 9 d alternating with afebrile periods of 2 to 4 d. The total number of relapses can vary from 1 to 10 or more, lasting 2 or 3 weeks. Transitory petechial rashes are common during the initial febrile period. Untreated, the mortality rate is between 2 and 10%.

 

TBRF is caused by a variety of tick-adapted Borrelia species (some researchers say that all of the tick-adapted strains are really just one species). The spirochetes are transmitted to humans and animals by several species of soft ticks in the genus Ornithodoros. Several hundred cases of TBRF are reported worldwide each year, with approximately 30 to 50 of those being diagnosed in the U.S. (primarily in Washington, Oregon, and northern California).

 

Tick-borne encephalitis

The term tick-borne encephalitis (TBE) generally describes disease entities caused by several subtypes of a flavivirus: European tick-borne encephalitis (TBEV-Eur), Siberian (TBEV-Sib), and Far Eastern (TBEV-FE). However, there are a couple of these agents occurring in the Western Hemisphere such as Powassan encephalitis and deer tick encephalitis.  Powassan encephalitis (POW) is a rare infection of humans that mostly occurs in the northeastern U.S., adjacent regions of Canada, and parts of Russia. POW causes sudden onset of fever with temperature up to 40°C along with convulsions. Also, accompanying encephalitis is usually severe, characterized by vomiting, respiratory distress, and prolonged, sustained fever. Only about 35 cases of POW have been reported in North America, although its reported incidence may be increasing. Recognized cases have occurred in children and adults, with a case fatality rate of approximately 50%. POW is transmitted in an enzootic cycle among ticks (primarily Ixodes cookei) and rodents and carnivores. Ixodes cookei only occasionally bites people; this may explain the low case numbers. Antibody prevalence to POW in residents of affected areas is less than 1%, indicating that human exposure to the virus life cycle is a rare event.

Deer tick encephalitis, closely related to POW, is another clinical entity in the TBE complex which was first discovered in North America in the late 1990's. Few clinical cases have ever been described, although at least one death has been attributed to this virus. The agent has been found along the Atlantic Coast and in Wisconsin and is primarily associated with the deer tick, Ixodes scapularis.

 

Tick paralysis

Tick paralysis can occur in people or animals and is characterized by an acute, ascending, flaccid motor paralysis that may terminate fatally if the tick is not located and removed. The causative agent is believed to be a salivary toxin produced by ticks when they feed. In the strictest sense, tick paralysis is not a zoonosis; however, many contend that zoonoses should include not only infections that humans acquire from animals, but also diseases induced by noninfective agents such as toxins and poisons. The disease is more common than one might think. In North America, hundreds of cases have been documented from the Montana–British Columbia region. It occurs in the southeastern U.S. as well. Tick paralysis is also especially common in Australia. Sporadic cases may occur in Europe, Africa, and South America.

 

Mosquitoes

 

Mosquitoes are second only to ticks as disease transmitters to animals. These blood-sucking insects cause significant annoyance and disease in animals worldwide. About 3,500 species of mosquitoes have been described worldwide. Relatively few of them are significant vectors of animal diseases; however, the mosquito-transmitted disease problem among animals worldwide is quite severe.

 

Filarial worms

Many different types of filarial worms are transmitted to humans and other mammals by mosquitoes and black flies. Examples include the causative agents of Bancroftian and Malayan filariasis, loiasis, onchocerciasis, and dirofilariasis (dog heartworm). Other filarial worms may or may not cause symptomatic disease and are less well known (and thus have no common name), such as Mansonella ozzardi, M. streptocerca, M. perstans, Dirofilaria tenuis, D. ursi, D. repens, and others.

The dog heartworm, Dirofilaria immitis, occurs mainly in the tropics and subtropics but also extends into southern Europe and North America. This worm infects several canid species, sometimes cats, and, rarely, humans. Numerous mosquito species are capable of transmitting dog heartworm, especially those in the genera Aedes, Ochlerotatus, Anopheles, and Culex.

Mosquitoes pick up the microfilariae with their blood meal when feeding on infected dogs. In endemic areas, a fairly high infection rate may occur in local mosquitoes. The closely related D. tenuis is commonly found in the subcutaneous tissues of raccoons (again, mosquito-transmitted) and may accidentally infest humans as nodules in subcutaneous tissues.

 

Viruses transmitted by mosquitoes

In temperate North America, the most common mosquito-borne diseases are probably the encephalitides. Certainly not all cases of encephalitis are mosquito-caused (enteroviruses and other agents are often involved), but mosquito-borne encephalitis has the potential to become a serious cause of morbidity and mortality covering widespread geographic areas each year.

Eastern equine encephalomyelitis (EEE) is generally the most virulent, being severe and frequently fatal (mortality rate 30 to 60%) in horses and humans. Fortunately, large and widespread outbreaks are not common; between 1961 and 1985 only 99 human cases were reported. EEE occurs in late summer and early fall in the central and north central U.S., parts of Canada, southward along the coastal margins of the eastern U.S. and the Gulf of Mexico, and sparsely throughout Central and South America. Horses are especially susceptible to EEE infection and may serve as sentinel animals to indicate virus activity in an area. However, widespread use of the eastern-western-tetanus vaccine may bias surveys of horse cases of EEE. Recently, human cases of EEE seem to be occurring more northeasterly into Maine and Vermont. The ecology of EEE is complex. The virus circulates in wild bird populations, and the exact mosquito vectors responsible for spread to humans are not well known. Some species likely involved include Ochlerotatus sollicitans, Coquillettidia perturbans, Culex salinarius, and Ae. vexans, although certain Anopheles species may serve as bridge vectors during epizootics.

The West Nile virus (WNV) was identified for the first time in the Western Hemisphere in New York in 1999. By the end of the year, it had caused encephalitis in 62 people and numerous horses in and around New York City, resulting in 7 human and 10 equine deaths. The virus has continued to spread in subsequent years to all the continental United States, at least 7 Canadian provinces, Mexico, the Caribbean, and portions of South America. As far as severity of the disease, WNV is similar to SLE (one of our “native” encephalitis viruses). Approximately 80% of all WNV infections are asymptomatic, approximately 20% cause West Nile fever, and less than 1% cause West Nile neuroinvasive disease. From 1999-2005, more than 8,000 cases of neuroinvasive WNV disease were reported in the U.S., resulting in over 780 deaths.  However, in recent years, there has been a decline in WNV case numbers. In 2009, there were only 720 cases nationwide reported to the CDC. As with SLE, WNV is more dangerous to older patients; people in the 60-89 age range are particularly at risk. Ecologically, WNV is a bird disease transmitted from bird-to-bird by various species of mosquitoes. The house sparrow has been found to be one of the best amplifying hosts in nature, producing highest viremias for the longest period of time. Although the virus has been isolated from many mosquito species, the main vectors to humans are believed to be Culex pipiens, Cx. quinquefasciatus, Cx. salinarius, Cx. restuans, and Cx. tarsalis.

 

Mites

Mites are tiny arachnids, closely related to ticks, which are mostly free-living, but a few species are parasitic.  Mites are fairly notorious in the veterinary sciences due to their association with poultry and also as causes of mange.

 

Northern fowl mite

Ornithonyssus sylviarum also is similar in appearance to the tropical rat mite but has a much shorter sternal plate. This plate has only four setae; the setae on the dorsal plate are quite short. The northern fowl mite is a pest of domestic fowl, pigeons, sparrows, and starlings. The species overwinters in bird nests or cracks and crevices of buildings. Unlike the chicken mite, D. gallinae, the northern fowl mite spends its entire life on the host. In poultry houses the mites are usually only found on the birds, but they have been found on eggs and cage litter. Northern fowl mites cannot survive more than a month or so in the absence of their poultry hosts. The northern fowl mite occurs in temperate regions worldwide.

 

Chicken mite

Dermanyssus gallinae, the chicken mite, also known as the red mite of poultry, is commonly found on domestic fowl, pigeons, English sparrows, starlings, and other birds. This mite is one of the most common species causing human dermatitis in poultry houses, farms, ranches, and markets where chickens are traded or sold. Poultry workers are often bitten on the backs of the hands and on the forearms. D. gallinae is nocturnal; during the day the mites hide in cracks and crevices in chicken houses or buildings where infested birds nest. Eggs are deposited in these hiding places. The chicken mite occurs worldwide.

 

Cheyletid mite

The mite family Cheyletidae includes species in the genera Cheyleta and Cheyletiella. Cheyletiella yasguri, C. blakei, and C. parasitivorax have fused cheliceral bases further fused with the subcapitulum forming a capsular gnathosoma. This makes it look as though they are wearing a helmet. They have free and highly developed palpi with strong curved claws that look like an extra pair of legs near the mouthparts. Cheyletiella mites are parasites of birds and various species of small mammals. The Cheyletiella spp. mentioned in this section are obligate parasites of small- or medium-sized mammals (including pet dogs, cats, and pet rabbits), living on the keratin layer of the epidermis; they do not burrow. These mites may cause a mangelike condition on pets and a transient itching dermatitis on humans who handle these pets.

 

Scabies mites

Scabies, caused by Sarcoptes scabei, is probably the most important disease caused by mites, with at least 300 million cases annually. It occurs worldwide, affecting all races and socioeconomic classes in all climates. The tiny mites burrow under the skin, leaving small open sores and linear burrows that contain the mites and their eggs. When a person is infested with scabies mites for the first time, there is little pathology for about a month, until sensitization develops. When that happens, there is severe itching, especially at night and frequently over much of the body. Large patches of erythema or rash may occur on the body. The patient’s tissues apparently become sensitized to various proteins liberated by the mites. Interestingly, the generalized rash may not correspond to the sites where the mites are burrowing. In humans, the burrows are usually located on the hands, wrists, and elbows, especially in the webbing between the fingers and the folds of the wrists.

It should be noted here that animal forms of scabies such as canine or equine are also caused by “races” of S. scabei (sometimes with devastating results), but these mites cannot propagate in human skin. Canine scabies can be temporarily transferred to humans from dogs, causing itching and papular or vesicular lesions primarily on the waist, chest, or forearms. However, treatment or removal of the infested dog will result in a gradual resolution of this type of scabies. Sarcoptes scabei are very tiny (0.2 to 0.4 mm long), oval, saclike, eyeless mites. Their legs are rudimentary; the anterior two pairs have bell-shaped suckers on their tips. The body is covered with striations and has several stout blunt spines and a few long setae. Scabies mite mouthparts are composed of toothed chelicerae and one-segmented palps fused to the central hypostome.

Nymphs look almost identical to the adults, except that they are smaller.

Although there are numerous species of Demodex (family Demodecidae) infesting wild and domestic animals, only two species of the mites are specific human-associated mites and are called follicle mites. The minute, wormlike mites live exclusively in hair follicles or sebaceous glands. They have no proven detrimental effect on humans, although some authors have attributed various pathological conditions of the skin to Demodex. For humans, Demodex is a harmless saprophyte. It is only exceptionally that it appears to exercise a pathogenic influence, as, for example, when excessive amounts of cosmetics prepare the ground for its proliferation or when it escapes into the dermis.” Various estimates of the incidence of human Demodex infestation range from about 25 to 100%, and clinicians should be aware of mite appearance, as they may be seen during skin-scraping examination.

 

Follicle mites

Demodex folliculorum lives in the hair follicles and D. brevis in the sebaceous glands. Both species are similar in appearance (with the exception that D. brevis is a shortened form) and are elongated, wormlike mites with only rudimentary legs. They are pproximately 0.1 to 0.4 mm long and have transverse striations over much of the body. These mites most commonly occur on the forehead, malar areas of the cheeks, nose and nasolabial fold, but they can occur anywhere on the face, around the ears, and occasionally elsewhere. Most people acquire Demodex mites early in life from household contacts — primarily maternal.

 

Fleas

Fleas are small, laterally flattened, wingless insects that are of great importance as vectors of disease in many parts of the world. Public health workers are most concerned with fleas that carry the agents of bubonic plague and murine typhus from rats to people and fleas that transmit plague among wild rodents and secondarily to humans. However, there are other fleaborne diseases. Rickettsia felis, a member of a transitional group of rickettsiae, has been found worldwide in cat fleas and can apparently infect humans, producing a murine typhuslike illness. Certain rodent fleas are efficient vectors of Bartonella organisms. Also, fleas may serve as intermediate hosts for helminths like the dog tapeworm. Despite these disease threats, for many people (especially the lay public), the insidious attacks by fleas on people and domestic animals causing irritation, blood loss, and severe discomfort are equal in importance to disease transmission. Two species of fleas are often seen in veterinary practice.

 

Cat and Dog Fleas, Ctenocephalides felis and C. canis

Cat fleas are the fleas most often encountered by people in the U.S. (the dog flea is relatively rare in North America). Contrary to their name designation, dog fleas may feed on cats and cat fleas on dogs. In fact, in many areas the predominant flea species infesting dogs is the cat flea. Both species are mainly just pest species, although there is some evidence that cat fleas may transmit Rickettsia felis to humans. In addition, cat fleas are intermediate hosts of the dog tapeworm, Dipylidium caninum, and their bites may produce papular urticaria. Children sometimes become infected via close contact with a flea-infested dog.

 

Sticktight Flea, Echidnophaga gallinacea

This flea, sometimes called the hen flea, is primarily a pest of poultry, but humans are often attacked. As with the chigoe flea, E. gallinacea attaches firmly to its host and engorges with blood. It may remain embedded in the integument of the host for some time. Chickens frequently have dark flea-covered patches around the eyes, comb, or wattles.

 

Flies

Flies are two-winged insects in the insect order Diptera. Most flies are non-biting, but some are notorious veterinary pests causing millions of dollars in damage to agricultural operations.

 

Black flies

Black flies (also called buffalo gnats, turkey gnats, and Kolumbtz flies) are small, humpbacked flies that are important as vectors of disease and as nuisance pests. In the tropics, black flies are vectors of the parasite, Onchocerca volvulus, which causes a chronic nonfatal disease with fibrous nodules in subcutaneous tissues and sometimes visual disturbances and blindness (river blindness). The World Health Organization estimates that about 17.7 million people have onchocerciasis in Africa and Latin America, with approximately 270,000 cases of microfilarial- induced blindness and another 500,000 people with severe visual impairment. Since 1987 onchocerciasis control has been greatly aided by donations of Mectizan® brand of ivermectin (Merck and Co.) which has been a gold-standard anti-parasitic drug.  However, resistance to ivermectin has recently been reported, forcing reconsideration of other drugs and strategies in a more integrated approach.

 

Deer flies

Deer flies belong to the family Tabanidae (the same one as horse flies) but are usually much smaller. Deer flies are extremely annoying to people in the outdoors during summer months, often circling persistently around the head. Like horse flies, deer flies have scissorlike mouthparts and can inflict painful bites. Deer fly bites often become secondarily infected; in hypersensitive individuals they have been known to produce systemic reactions characterized by generalized urticaria and wheezing. In the U.S., the deer fly Chrysops discalis mechanically transmits tularemia organisms from rabbits to people by its bites, a condition sometimes called deer-fly fever. In a recent outbreak, 64% of human cases of tularemia in Wyoming were attributed to deer fly bites. In the African equatorial rain forest, deer flies (particularly C. silacea and C. dimidiata) transmit the filarial parasite Loa loa. Loiasis affects an estimated 2 to 13 million individuals and is characterized by Calabar swellings (localized nonpitting edema mainly on the wrists or ankles, 5 to 20 cm in diameter, lasting from a few hours to a few days), generalized pruritus, arthralgia, fatigue, hypereosinophilia, and sometimes serious central nervous system (CNS) involvement.

The pathognomonic symptom of loiasis, subconjunctival migration of the worm In the eye, is uncommon but still reported.

 

Horse flies

Horse flies (also family Tabanidae) are large, robust bloodsucking flies that are notorious pests of horses, cattle, deer, and other mammals. Several species of horse flies will also attack people. Horse fly bites have been known to produce systemic reactions in humans characterized by generalized urticaria and wheezing. Horse flies look like giant robust house flies. They are often 20 to 25 mm long and have large prominent eyes. Some species are called green heads because of their big green eyes. The antennae have only three sections. Their proboscis projects forward, and the female’s mouthparts are bladelike for a slashing/lapping feeding method. Horse fly larvae are spindle-shaped and generally white, tan, brown, or even greenish in color.

Horse fly bites are generally few and self-limiting. If secondary infection/cellulitis develops, appropriate systemic antibiotics are indicated. Antiseptic and soothing lotions may relieve pain and itching. Allergic reactions characterized by hives, wheezing, and widespread urticaria require intensive evaluation and treatment. If the reaction is mild, oral antihistamine therapy may suffice, but severe reactions involving shock will probably require epinephrine.

 

Biting gnats

The biting midges are very tiny slender gnats in the family Ceratopogonidae; they are sometimes called punkies, no-see-ums, gnats, or flying teeth. Adult biting midges are vicious and persistent biters, and some persons have strong reactions to their bites. Their small size allows them to pass through ordinary screen wire used to cover windows and doors. These tiny insects are generally not involved in the transmission of disease agents to humans in the U.S. However, in Africa and South America certain species may be able to transmit filariae, protozoa, and viruses (particularly Shuni and Oropouche viruses in the Simbu serogroup of arboviruses).

Biting midges are typically gray in color (although some species may be yellowish), extremely small, 0.6 to 1.5 mm, and delicate with narrow wings that have few veins and no scales. The wings may be clear or hairy, sometimes distinctly spotted (with pigment, not scales as in mosquitoes), and folded scissorlike over the abdomen at rest. The eyes on each side of the head are black and the proboscis protrudes forward and downward. Biting midges somewhat resemble other small species of nonbiting gnats in the family Chironomidae, but they are not as large and mosquito-like as chironomids. Swarms of biting midges are small and inconspicuous. People attacked by this midge will often comment when outdoors, “Something is biting me but I can’t see what it is.”

 

Sand flies

Sand flies are tiny bloodsucking flies in the family Psychodidae that transmit the causative agents of bartonellosis (Carrión’s disease), sand fly fever, and leishmaniasis. Sand fly fever, a viral disease, occurs in those parts of southern Europe, the Mediterranean, the Near and Middle East, Asia, and Central and South America where the Phlebotomus vectors exist. Bartonellosis

caused by the bacillus, Bartonella bacilliformis, occurs in the mountain valleys of Peru, Ecuador, and southwest Colombia. Leishmaniasis occurs in tropical and subtropical areas over much of the world. There has been a recent resurgence of leishmaniasis. Clinically, leishmaniasis manifests itself in four main forms: (1) cutaneous, (2) mucocutaneous, (3) diffuse cutaneous, and

(4) visceral. The cutaneous form may appear as small and self-limiting ulcers that are slow to heal. When there is destruction of nasal and oral mucosa, the disease is labeled mucocutaneous leishmaniasis. Sometimes there are widespread cutaneous papules or nodules all over the body, a condition termed diffuse cutaneous leishmaniasis. Finally, the condition in which the parasites invade cells of the spleen, bone marrow, and liver — causing widespread visceral involvement

— is termed visceral leishmaniasis or kala azar. There have been reports of visceral leishmaniasis in foxhounds in the U.S., but the only human sand fly-transmitted disease in the

U.S. is probably the few cases of cutaneous leishmaniasis diagnosed each year in south Texas.

 

Stable flies

The stable fly, Stomoxys calcitrans, is a significant medical and veterinary pest. People who say “a house fly bit me” are usually mistakenly referring to the stable fly. The flies (sometimes also called dog flies) are fierce biters of people, pets, and livestock, and are a major pest in some seacoast areas, impeding development. Because of their bloodsucking habits, the flies have been suspected of transmitting a number of human diseases by mechanical action, but proof is lacking.

Stable flies are 5 to 6 mm long, have a dull gray thorax with four dark longitudinal stripes, and have a dull gray abdomen with dark spots. They look very similar to house flies, but they are slightly larger and have a rigid proboscis projecting forward in a bayonet-like fashion. In contrast, house flies have sponging mouthparts that project downward.

 

Tsetse flies

There are over 20 species of flies in the genus Glossina that are called tsetse flies. Tsetse flies are 7 to 13 mm long and yellow, brown, or black. They fold their wings scissorlike over their back at rest, and this, along with other body features, makes them appear wasp- or honey bee-like. Most of the species are vectors of trypanosomes of people and animals; however, at least six species are of primary importance as vectors of African trypanosomiasis, caused by subspecies of the protozoan, Trypanosoma brucei. The disease is called sleeping sickness because meningoencephalitis associated with the disease causes apathy, fatigability, confusion, and somnolence. The patient may gradually become more and more difficult to arouse and finally becomes comatose. In 1998, the WHO estimated that at least 300,000 cases of African trypanosmiasis were undiagnosed and untreated. However, intensive tsetse fly trapping and control, as well as case surveillance/treatment, brought the number of cases in 2009 to below 10,000 for the first time in 50 years.

 

Fly larvae that may cause problems in people and animals Bot flies

The condition of fly larvae occurring in human tissues is called myiasis. Some fly larvae develop in living flesh; the human bot fly is one of them. This fly, Dermatobia hominis, is a parasite of humans, cattle, swine, cats, dogs, horses, sheep, other mammals, and a few birds in Mexico and Central and South America.  The larvae burrow into the host’s tissues, feeding and eventually emerging to drop to the ground and pupate. In people, the larvae have been recovered from the head, arms, back, abdomen, buttocks, thighs, and axilla. Human infestation is often characterized by painful discharging cutaneous swellings on the body. The condition is rarely fatal, except possibly in very young children (less than 5 years old); the larvae infesting the scalp penetrate into the incompletely ossified skull and enter the brain. Although the parasite does not occur in the U.S., cases are occasionally seen in travelers to endemic areas. One such case was reported from Ohio in which a local physician submitted a second-stage larva to the Ohio Department of Health for identification. The larva had been removed from a patient who had recently returned from Brazil.

D. hominis does not occur in the U.S., but is common in parts of Mexico and Central and South America. Sancho reports its distribution from the northern provinces of Mexico (Taumalipas, bordering southern Texas) to the northern Argentine provinces of Misiones, Tres Rios, Corrientes, and Formosa — roughly between latitudes 25° N and 32° S. Vacationers may acquire this parasite in tropical America, and return home before completion of maggot development.

 

Screwworm flies

Two important species of screwworm flies that feed in living tissues are Chrysomya bezziana, the Old World screwworm, and Cochliomyia hominivorax, the New World screwworm. Larvae of these calliphorid flies, called screwworms, are obligate parasites of living flesh (humans and domestic and wild mammals), feeding during their entire larval period inside a host.

Screwworms feed by the hundreds close together, making pockets in the live flesh, eating downward with their pointed ends (the head end), and leaving their rear ends exposed for breathing. Infested wounds give off a sickening odor and ooze blood continually.

Female flies most often oviposit on or near a wound. However, human infestations have resulted from the flies ovipositing just inside the nostril while a person sleeps during the day, especially if there is a nasal discharge. Upon hatching, the larvae begin feeding, causing extensive destruction of tissue and a bloody discharge. Tissues around the lesion become swollen and pockets may be eaten out beneath the skin. The frontal and ethmoid sinuses may be entered and the cartilage and even the bone attacked. Infested persons may die from tissue destruction.

Human cases are uncommon, but have occurred in areas where screwworm infestations occur in livestock. In 1935 there were 55 reported cases during a large outbreak among livestock in Texas. Because the New World screwworm fly has been eradicated from the U.S., human cases are rare and mostly due to foreign travel (there has been a recent outbreak of the New World screwworm in the Florida Keys).

Females of both screwworm species are attracted to wounds in mammals and lay eggs at the edge of the wounds. Eggs are deposited in batches of 150 to 400 and hatch approximately 15 h later. The larvae feed while embedded inside living tissue; sometimes, however, the peritremes (the plate surrounding the breathing tubes) are visible. The larvae emerge from the host as prepupae 4 to 7 d later and fall to the ground, where they pupate for a week or more. The entire life cycle from egg to egg takes about 24 d under optimum conditions. Adult screwworm flies are active all year round but only fly during daylight.

 

Flesh flies (Family Sarcophagidae)

 

Two important sarcophagid flies that cause myiasis are Wohlfahrtia magnifica and W. vigil. The Wohlfahrtia spp. look very similar to flesh fly adults (Sarcophaga), but instead of the checkerboard pattern on their abdomen they have clearly defined spots.

W. magnifica larvae produce traumatic myiasis throughout their host’s tissues, much like a screwworm fly. Tissues usually affected most commonly include the nose, ears, and eyes, and may result in deafness, blindness, or facial disfiguration. Human fatalities due to this species have been reported. W. vigil larvae are not as invasive; they are usually limited to dermal tissues, producing a furuncular or boil-like lesion.

 

Other flies that might cause myiasis

 

Larvae of flies in the genus Cuterebra, often found in squirrels (called wolves) and rabbits, may rarely parasitize humans, forming a warblelike dermal tumor. In a case the author consulted on, a 3- year-old boy had two Cuterebra larvae — on his side and neck — forming boil-like lesions. Several health care providers examined the boy and either diagnosed the lesions as boils or larval migrans (from dog hookworm), because there were short migration trails visible in the skin. To everyone’s surprise, one physician finally recognized the myiasis and expressed a larva from the neck lesion.

Other flies may occasionally cause myiasis in humans. This behavior is termed facultative myiasis. In some of these cases the larvae enter living tissues after feeding in neglected, - malodorous wounds. In other cases, ports of entry include natural orifices such as the ears, urinary opening, or anus. Sometimes these fly larvae can be identified by looking at the shape and pattern of their posterior spiracles.

One of the most commonly implicated fly groups is the Calliphoridae (blow flies). Several species of blow flies, and especially Lucilia sericata and Phormia regina, have been reported to cause facultative myiasis in humans. Lucilia sericata is a typical-looking blow fly — shiny green or coppery green. Phormia regina, also called the black blow fly, is more slender and is olive-colored or nearly black.

Lucilia sericata is nearly cosmopolitan in distribution and is probably the most abundant species of Lucilia in North America. Phormia regina is Holarctic in distribution   One study found Phormia regina to be the most common blow fly in Mississippi, with specimens encountered throughout the year (peak numbers occurring in April and September).

Other specific comments on these two species are presented here. Lucilia sericata is a species that is quick to appear at fresh carrion, although it is also attracted to a wide range of decaying substances. In many cases, facultative myiasis cases seem to be the result of Lucilia sericata being attracted to festering and malodorous wounds with subsequent invasion of healthy human tissue. Phormia regina is very common throughout the U.S. during the warmer months (April to September). It is also abundant near a wide variety of decaying substances, especially carrion.

 

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