MYIASIS.

Definition-

Myiasis is the infestation of tissue with fly larvae, commonly referred to asmaggots.It is widespread in the tropics and subtropics of Africa and the Americas, and occurs with significantly less frequency in most other areas of the world.The infestation is most often subcutaneous and produces a furunculoid or boil-like lesion, but it is also known to occur in wounds and certain body cavities.As travel to endemic regions becomes more common, physicians, particularly dermatologists, in non-endemic areas are increasingly confronted with cases of myiasis.Physicians in the northern developed countries may be unfamiliar with this parasitic infection; thus, misdiagnosis and inappropriate treatment regimens are not infrequent.Greater awareness on the part of physicians about clinical symptoms and relevant exposure histories would improve the expediency and efficacy of treatment for patients with myiasis.II.Agent(and Synonyms)Myiasis may be caused in human hosts by several species of arthropods of theorder Diptera, the two-winged true fly.Two approaches to classification are possible, one entomological and the other etiological, derived from differences in the behavior of the various species of fly and the nature of the parasitic relationship (Noutsis and Millikan).Only those agents with public health significance will be explored in greater detail, but the taxonomy provides a notion of the breadth of sources and manifestations of myiasis.A.Taxonomic Classification of Myiasis-Causing FliesGenus Cordylobia:Cordylobia anthropophaga(tumbufly) is found throughout sub-Saharan tropical Africa.Cordylobia (Stassisia) rhodaini(Lund’s fly) is found primarily in rain forests.Adult females deposit eggs on sand, soil, bedding, or clothing, particularly if soiled with feces or urine.Larvae hatch anduse oral hooks to infect the unbroken skin of a human host.Genus Chrysomia:Flies of this genus are distributed across Africa, Australia, and Asia.Infestation with larvae of the Old World screw worm,Chrysomyia bezziana, can be disfiguring.Like the New World screw worm,C. bezzianacan invade bone.Infestation of the eye is particularly dangerous.Genus Auchmeromyia:The Congo floor maggot,Auchmeromyia luteola, does not infest human tissue but must feed on human blood.The larvae inhabit soils in tropical Africa, and feed on persons sleeping on dirt floors.Genus Wohlfahrtia:Wohlfahrtia magnificalarvae infest the ear, eye, and nose, damaging living tissues.W. magnificais found in south-eastern Europe, southern and AsiaticRussia, the Middle East and North Africa.The larvae of the North American species,Wohlfahrtia vigilandWohlfahrtia opaca, are incapable of penetrating adult skin; infestation occurs only in infants.Genus Dermatobia:Dermatobia hominis, the human botfly, has a distribution ranging from northern Argentina to southern Mexico, and isusually found in warm, humid lowland forests.D. hominiscauses cutaneous myiasis in humans and other mammals.Infestation of cattle has important economic consequences.Adult females deposit eggs on blood-sucking arthropods, which transmit the infectious larvae to the host.Genus Hypoderma:The warble flies,Hypoderma spp., are cattle parasites.The uncommoninfestations in human hosts producemigratory subcutaneous swellings orharmful invasion of the eye.B.Etiological Classification of Myiasis-Causing FliesObligatory (Specific) AgentsBothD. hominisandC. anthropophagaare obligatory parasites, whose larval stages can occur only in the living tissue of animal or human hosts.Myiases caused by larvae of the human botflyand the tumbu fly predominate, but several other species that generally parasitize animals occasionally cause infections in humans as well.Other obligatory flies include the generaOestrus,Rhinoestrus,Gasterophilus,Hypoderma,Chrysomyia, andWohlfahrtia.Transmission and the type of tissue affected varies with the species of the parasite.Facultative (Semi-specific) AgentsFlies of the familySarcophagidaenormally develop in decomposing tissue and are considered facultativeparasites.Larvae of these flies parasitize wounds and other damaged tissues, and some species further invade living tissues adjacentto the wound.Important genera in this category includeMusca,Calliphora, andLucilia.Accidental (Non-specific) AgentsAccidental myiasis occurs when egg-stage flies are ingested on contaminated food or come in contact with the genitourinary tract.Flies of the familiesMuscidae,Calliphoridae, andArcophagidaemaybe involved (Powers and Yorgensen).III.History of DiscoveryMaggots have an important role in the history of biology, as they were central to experiments that rejected the theory of spontaneous generation.Well into the 17thcentury, European scientists believed that rotten meat itself gave rise to maggots and flies.In 1668, the Italian poet and physician Francisco Redi (1626-1697) conducted the following experiment, one of the first ever to utilize appropriate controls: he placed samples of meat in two series of jars, half of them lidded and the other half open, and watched the meat for signs of rotting and myiasis.The meat in the lidded jars began to rot but, having had no contact with adult flies, did not produce maggots, while the meat in the open jars was visited by adult flies, became infested with fly larvae,and eventually produced more adult flies.Redi correctly judged that the maggots developed from eggs, too small to be seen, that were deposited on the meat samples by the adult flies (Britannica).IV.Clinical Presentation in HumansFuruncular Cutaneous MyiasisFuruncular myiasis is caused by both the human botfly and the tumbufly.The location of lesions varies because of the different means of transmission.Myiasis caused by the tumbu fly,C. anthropophaga, usuallyappears on the trunk, buttocks, and thighs, while myiasis caused by the human botfly,D. hominis, commonlyoccurs on exposed sites such as thescalp, face, forearms, and legs (Lucchina et al).While abrasions and wounds are commonly inhabited by several larvae, perhaps even by several species of larvae, furuncular and migratory myiatic lesions usually contain one or very few larvae (Bapat).Larval infestation of cutaneous tissue results in the development of a pruritic papule, of aproximately 2 to3 mm in diameter, within 24 hours of initial contact with the larvae (Purych-Alberta, Swetter et al).This papule gradually extends, forming a lesion with diameter ranging from 1.0 to 3.5 cm, and a height between 0.5 and 1.0 cm (Purych).The respiratory sinuses and occasionallythe posterior end of the larva itself may be seen through a central punctum of 2 to 3 mm in diameter (Purych, Noutsis and Millikan, Swetter et al, Tsuda et al).The patient may describe a sensation of pain “caused by the tearing of host tissues as the larva feeds or from theouter spines irritating the surrounding skin as the larvae moves” (Purych).No systemic symptoms are observed (Purych).In addition to the symptoms described above, which apply to boththe human botfly and the tumbu fly, furuncular infestations caused byC. anthropophagamay become “crusted, odoriferous, purulent, or [may exude] serosanguinous discharge” (Lucchina et al).Patients infested with tumbu fly larvae often exhibit a greater number of lesions; Jelinek notes that this frequently results in faster referral to a dermatologist than in cases of infestation with the human botfly.Analysis of tissues exhibiting an inflammatory response to maggot infestation reveals a high concentration of lymphocytes, giant cells, neutrophils, eosinophils, and plasma cells.There is no inflammatory response to infestationwith larvae ofHypoderma spp(Noutsis and Millikan).Secondary infection by bacteria is uncommon, because “bacteriostatic activity in the gut of the larva seems to prevent undesirable overgrowth ofpyogenic bacteria” (MacNamara and Durham).Furuncular myiasis caused by rodentor rabbit botflies,Cuterebra spp., may present as “subcutaneous abscesses on the face, scalp, neck, shoulders or chest” (Shorter et al).These cases are acquired in North American, generally during the summer months, and involve a single lesion with only one larva.