An Introduction to Medical Entomology
For educational purposes.
MYIASIS-CAUSING ARTHROPODS KEY
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[Also See: Key to Myiasis-causing Flies]
The larvae in several orders of insects can cause myiasis in humans and animals, resulting in the destruction of organs and tissues. Service (2008) has defined myiasis as, "The invasion of organs and tissues of humans and other vertebrate animals by fly larvae, which at least for some time feed upon the living or dead tissues or, in the case of intestinal myiasis, on the host's ingested food."
Service (2008) considered different types of myiasis caused by Diptera as accidental, obligatory or facultative. The accidental type comes about by eating contaminated food containing eggs or larvae, which can lead to discomfort in humans and more serious effects in animals. The obligatory type of myiasis requires that fly maggots live in the host for all or part of their life cycle. Facultative myiasis results when larvae that are generally free-living also infect living hosts. Throughout the literature different terms are used to describe myiasis that affects different parts of the body. Service (2008) gave the following examples: Cutaneous = dermal or subdermal myiasis; Urogenital myiasis; Ophthalmic = ocular myiasis; Nasopharyngeal myiasis; and Intestinal = gastrointestinal or enteric myiasis. Referring to appearance, there is Creeping Myiasis; Furuncular Myiasis = boil-like lesions occur; and Traumatic Myiasis = when wounds are infested with living larvae.
Matheson (1950) noted that different names have been applied to the myiasis caused by insect species in the different orders. The term myiasis is used for the Diptera that are responsible for the majority of cases, whereas for Coleoptera it is canthariasis and for Lepidoptera it is scoleciasis. The latter two types are comparatively rare in humans.
There are accidental cases of myiasis involving Coleoptera, many of which are also of doubtful validity. Matheson (1950) discusses the larvae of Dermestidae and Tenebrionidae where infection might occur through the consumption of cold cereals. Most of these refer to Tenebrio molitor, the mealworm, which is an important host of tapeworm, Hymenolepis diminuta. Other beetle species that have been associated with myiasis are Attagenus piceus , Onthophagus bifasciatus, O. unifasciatus, Caccobius mutans (see Caccobius sp.), and Ptinus tectus.
The larvae of Lepidoptera have been connected with myiasis in a few rare cases, although there is some doubt about their accuracy. Matheson (1950) presented one more reliable case of a child who consumed raw cabbage and later vomited larvae of the cabbage butterfly, Pieris brassicae. Church (1936) recorded a case where larvae of the Corn Borer, Pyrausta nubilalis, had attacked the body tissues of a woman.
MYIASIS -- By Diptera
There are many authenticated cases of myiasis being caused by Diptera (flies). The Posterior Spiracular Plates and Cephalopharyngeal Skeletons of some Diptera larvae are used for identification (See: Key to Myiasis-causing Flies. The following are arranged by separate Diptera families:
Sarcophagidae -- flesh Flies
Sarcophagidae adults are very abundant everywhere around decaying vegetation, animal matter and excrement. Most species lay live larvae and not eggs. Among the many varied habits some species are parasitic on warm-blooded animals, on grasshoppers and snails. Many are scavengers and others are attracted to wounds. Because identification of larvae can be difficult it is best to rear encountered larvae to the adult stage for proper identification.
Larvae of the genus Wohlfahrtia is frequently involved in myiasis. One species, Wohlfahrtia vigil Walker being unique among the Sarcophagidae by attacking healthy skin rather than wounds or body orifices. Wohlfahrtia opaca Coq. of America and W. magnifica Schiner of Europe are other important species in the genus but they characteristically attack open wounds.
The cosmopolitan Sarcophaga haemorrhoidalis Fall and other infecting species, S. fuscicauda Keilin and S. sarraceniae Riley, are occasionally found important in causing myiasis particularly in the intestinal tract..
Callitroga americana (Cushing & Patton) is the screwworm fly is important all over the Americas. It is an obligate parasite of humans and animals that deposits larvae in open wounds. Mortality rates are very high among animals that it attacks, and humans can also perish if not treated promptly.
Callitroga macellaria (Fabr.), or secondary screwworm resembles C. americana, and it is also widely distributed in the Americas. They usually deposit eggs on carrion but will also oviposit in the wool of sheep and on wounds, and possibly on humans although this is doubtful as there is confusion with C. americana.
Chrysomya bezziana is related to screwworm fly, but it is most common in Africa, Asia and the Philippines. It prefers to infest wounds of animals, but occasionally will attack humans also. Other species of Chrysomya that occasionally attack humans are C. marginalis (Wied.) in Africa and C. albiceps (Wied.), while in Europe, India and Africa, C. chloropyga (Wied.) and C. rufifacies (Macq.) are of some concern.
Calliphora vomitoria (L.), C. vicina R.-D. and C. livida Hall are common bottle- or blowflies that cause myiasis by ovipositing in open wounds of animals and occasionally humans. They complete their life cycles from egg to adult in 2-4 weeks.
In the genus Lucilia, which includes the green-bottle flies, there are a number of species of medical importance because of their involvement in myiasis. Included are C. americana (L.), L. sericata Meig., L. illustris (Meig.), L. cuprina (Wied.) and L. silvarum (Meig.). Although myiasis has been attributed to these flies under the species names noted, their identifications may be inaccurate.
Cordylobia anthropophaga (= Tumbu or Mango Fly) and Auchmeromyia senegalensis (= Congo floor maggot) of Africa. Infestation occurs from contaminated clothing that has not been washed or is placed on the ground to dry.
Phormia regina (Meig.), the black blowfly, is a cosmopolitan species that causes myiasis in animals and rarely in humans.
The larvae of Cordylobia anthropophaga Grunberg, the tumbu fly of Africa, that begin their growth in decaying organic matter, will penetrate the skin of animals and occasionally humans to complete their development.
Auchmeromyia luteola (Fab.), the Congo floor maggot, larvae attack humans by feeding on their blood during the night. They leave their hosts in daytime only to return again to feed at night.
Pollenia rudis (Fab.), the cluster fly, does not cause myiasis but annoys people when adults enter dwellings. In this case they are parasitic on earthworms.
Musca domestica L., the common housefly, has caused intestinal myiasis in humans. Infection can occur because of this fly's close association with humans in their dwellings. Matheson (1950) details the many situations where infection can occur, which are usually through a lack of sanitation.
Muscina stabulans (Fallen), the nonbiting stable fly, is sometimes abundant around structures. Its habits are similar to Musca domestica by breeding in organic wastes, and it also has caused intestinal myiasis in humans.
Species of the genus Fannia often occur together with Musca domestica in the same habitats of decaying organic matter. Adults of Fannia canicularis (L.), the lesser housefly, will occur in large numbers hovering in and outside of structures. These flies are particularly abundant where poultry dung provides an ideal breeding habitat. Fannia scalaris Fab., the latrine fly, is smaller than F. canicularis and with similar breeding habits. Matheson (1950) reports the existence of numerous records where the larvae of these flies caused gastric and intestinal myiasis in humans.
This group of flies is usually associated with domestic and wild animals, but there are occasional cases of myiasis in humans. Four subspecies are involved: Oestrinae, Gastrophilinae, Hypodermatinae & Cuterebrinae. Two genera of particular medical importance are Oestrus and Rhinoestrus. When humans come in close contact with sheep and other domestic animals they may become infected (e.g., Oestrus ovis L.).
Adults of the subfamily Cuterebrinae resemble bees, and the larvae of all species are parasitic on mammals including humans. Dermatobia hominus.L, the human warble fly, is common especially in forested regions of tropical America. Females of this species infest other arthropods such as mosquitoes, flies and ticks, with their eggs. Humans become infected when coming into contact with the carriers. The warble fly eggs hatch when the carrier contacts the host, and the larvae burrow into the skin, which is facilitated by wounds caused by the carrier. One Cuterebra sp. has also been found to attack humans.
Hypoderma bovis L. & Hypoderma lineatum (Villers) are the more common cosmopolitan warble flies attacking cattle. However, parasitism of humans is not uncommon, and infection is noticeable when the larvae produce a swelling underneath the skin. Both Matheson (1950) and Service (2008) present detailed cases of human infection and frequent encounters with Hypoderma bovis.
There are many species in this Diptera family, all of which resemble bees. Tubifera tenax (L.), the drone fly, is found hovering around rotting organic matter upon which eggs are deposited. The larvae are distinctive because of a tail-like breathing tube, giving them the name "rat-tailed maggots." There have been numerous cases of intestinal myiasis recorded from humans with this species. Matheson (1950) stated that infection with these larvae would probably have come from drinking fouled water containing young larvae or eggs or ingesting rotting fruit. Other less common species of Tubifera that are suspected to have caused myiasis are T. argustorum and T. dimidiatus.
There are also reported cases of intestinal myiasis caused by Syrphus species even though the group is predacious on other insects. Infection with this group is believed to result from ingesting contaminated vegetables.
On rare occasions other Diptera species have been reported in cases of myiasis. Most often found are Psychoda albipennis and P. bipunctata Curt. (Psychodidae), Megaselia scalaris (Phoridae), Piophila casei (Piophilidae), Rhyphus fenestralis Scop. (Anisopodidae), Hermetia illucens L. (Stratiomyidae). [See Matheson 1950 for more details].
In the 20th Century the larvae of blowflies had been used in the treatment of osteomyelitis by devouring dead and dying tissues and simultaneously destroying any invading bacteria. Thoroughly cleaned wounds hastened the healing process and the larvae's saliva apparently possessed bactericidal properties.
When myiasis involves living larvae occurring in sores, wounds and dermal or sub dermal tissues, their removal under aseptic conditions can be quite simple. However, when the larvae are deeper in the tissues or when they have affected the mucous membranes, frontal sinuses or eyes, removal can be complicated so that surgery may be required. In extreme cases the larvae may cause major damage that cannot be undone.
An accurate diagnosis of the causative agents may be useful in the treatment of myiasis, but infection should be precluded by a thorough knowledge of the environment to avoid contaminated foods and the vector carriers of eggs or larvae involving other arthropods such as mosquitoes and ticks. The use of insect repellants and proper clothing can protect against contact. Also there may be seasonal activity of vectors that can be avoided.
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