Dermatobia hominis - the Human Bot-Fly
Dermatobia hominis (tórsalo or human bot fly) is a dermal/sub-dermal parasite of humans, cattle, dogs and a number of other wild and domestic mammals and birds. It occurs throughout Central and South America, where in many parts it is a very serious pest of cattle. The larvae create boil-like swellings where they enter the skin and the hides of infested cattle may be rendered worthless. Excellent sources of information for all aspects of work on D. hominis are provided by the bibliographies of Guimarães and Papavero (1966, 1999) and of Guimarães et al. (1983). The female adopts a unique method of ensuring transportation of her eggs to the host. Females catch other diurnal, host-visiting flies, including mosquitoes (Culicidae, e.g. species of Aedes, Anopheles, Psorophora), and other flies in the families Simuliidae, Tabanidae, Fanniidae, Anthomyiidae, Muscidae. Sarcophagidae and Calliphoridae. They then glue eggs to these vector or porter flies, typically to their abdomen, and release them so that the vectors transport the eggs to the hosts. This 'hitch-hiking' strategy is known as phoresis.
The selection of host is made by the porter fly and so D. hominis is found in a very wide range of hosts, from mammals to birds. Following incubation on the vector, the warmth of the host induces the larva to hatch from the egg after which, within 5-10 minutes, it penetrates the host's skin. At the site of penetration a small nodule of host tissue develops around each larva, with a central breathing pore. Larvae have a distinctive shape, with attenuation of the posterior end, which renders them difficult to remove by manual pressure. This is most pronounced in the second stage. They are also equipped with rows of backward pointing spins. The best method of removal is generally by surgery.
The larva feeds for six to twelve weeks in man, then when mature emerges and drops to the ground to pupate (Lane et al., 1987). The cutaneous swellings containing the larvae are itchy and can be periodically painful, but unless infected are usually of relatively short duration, though the exudations from the wound may acquire a fetid odour and be troublesome, soiling bedding and clothing. Infestation often occurs on exposed skin surfaces such as the head and limbs, although other parts of the body can be infested. Should larvae enter the eye a serious ophthalmomyiasis can result in the loss of an eye. Rarely, in infants, entry of larvae into the head via the fontanel can cause fatal brain damage.
James (1947) gives the distribution of Dermatobia hominis as follows:¬ widespread, but confined to the Neotropical region, i.e., Mexico (northward to Tamaulipas, about 24-26°N), Guatemala, Belize, Honduras, Costa Rica, Panama, Trinidad, Colombia, Ecuador, Venezuela, Guyana, Surinam, French Guiana, Peru, Paraguay, Chile and Argentina (southward to a line from Tucuman to Santa Fe, about 30-32°S). Other authors mention that Chile is the only country between Mexico and Argentina that is free of the parasite (Roncalli and Usher, 1988; Uribe et al., 1989). Cases of D. hominis myiasis can be detected well beyond the natural range of the species in travelers who have recently visited endemic areas (Hall and Smith, 1993; Mallon et al., 1999).
Martin Hall, Natural History Museum, London, unpublished
- Guimarães, J.H. and Papavero, N. (1966). A tentative annotated bibliography of Dermatobia hominis (Linnaeus jr., 1781) (Diptera, Cuterebridae). Archivos de Zoologia do Estado de Sao Paulo 14: 223-294.
- Guimarães, J.H. and Papavero, N. (1999). Myiasis in man and animals in the neotropical region, bibliographic database. São Paulo, Plêiade/FAPESP, 308 pp.
- Guimarães, J.H., Papavero, N. and Pires-do-Prado, A. (1983). As miises na regiao neotropical (identificacao, biologia, bibliografia). Revista Brasileira de Zoologia 1: 239-416.
- Hall, M.J.R. and Smith, K.G.V. (1993) Diptera causing myiasis in man. Chapter 12, pp. 429 469 in, Lane, R.P. and Crosskey, R.W. (Eds) Medical Insects and Arachnids, Chapman & Hall, London.
- James, M.T. (1947). The flies that cause myiasis in man. United States Department of Agriculture Miscellaneous Publication No. 631, 175pp.
- Lancaster, J.L. & Meisch, M.V. (1986). Arthropods in livestock and poultry production. xvi + 402 pp., Ellis Horwood Ltd, Chichester, UK.
- Lane, R.P., Lovell, C.R., Griffiths, W.A.D. and Sonnex, T.S. (1987). Human cutaneous myiasis - a review and report of three cases due to Dermatobia hominis. Clinical and Experimental Dermatology 12: 40-45.
- Mallon, P.W.G., Evans, M., Hall, M. and Bailey, R. (1999) “Something moving in my head.” The Lancet 354: 1260.
- Roncalli, R.A. & Usher, C.B. (1988). Efficacy of ivermectin against Dermatobia hominis in cattle. Veterinary Parasitology 28: 343-346.
- Uribe, L.F., McMullin, P.F., Cramer, L.G. & Amaral, N.K. (1989). Topically applied ivermectin: efficacy against Torsalo (Diptera: Cuterebridae). Journal of Economic Entomology 82: 847-849.