Shoo, Fly!

Posted on Tue May 20 2008

By: Hilary Feldman in Environment, Science & Technology

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A deadly fly outbreak is threatening livestock in Yemen.

The screw-worm fly lays eggs on mammals, targeting open wounds. The eggs hatch and emerging larvae feed on surrounding living tissue. Larvae are obligatory parasites, leaving lesions known as myiasis. The infestation can be lethal if left untreated. There are two species - the Old World screw-worm (Chrysomia bezziana) and New World screw-worm (Cochliomyia hominivorax). Livestock such as cattle, goats, and sheep are often the most financially costly victims, but any mammal can be affected.

This is the first official outbreak of Old World screw-worm in Yemen, according to officials. Already, 20,000 sheep and goats have been affected, along with a few cases in humans. In Yemen, where livestock rearing is widespread, the fly threatens traditional pastoral lifestyles.

Even the most minor wound can offer an entry for flies. These opportunistic sites can include the unhealed navels of newborn animals, stumps left from dehorning, castration wounds, insect bites, and scratches. Since screw-worms are not well-known locally, information transmission is one of the most crucial aspects to combat the flies. Livestock farmers need to know how to recognize the flies (although they are seldom seen) and their larvae, as well as how to treat infected animals. In fact, in the initial stages, the affected area can be scraped and treated with pesticides (such as Ivermectin and coumaphos) to destroy remaining eggs.

Screw-worm probably entered Yemen with infected livestock from other countries. This is the most common form of transmission, although some flies have been known to travel on air currents or carried on unusual hosts (e.g. dogs, humans). The flies are not migratory, but females can travel up to 50 kilometres to find suitable hosts. In tropical areas, the fly’s life cycle can be rapid, with up to 15 generations in a year - meaning rapid population growth under favourable conditions.

One approach has worked against the flies in other countries. North America, Central America, and Libya have all successfully eliminated New World screw-worm through the sterile insect technique. In the laboratory, screw-worm flies are raised and then rendered sterile with radiation. These flies go on to mate but no offspring result, reducing the pest population. In Libya, it took four years from the initial introduction of screw-worm to its eradication. Certainly, using the sterile-insect technique offers a pesticide-free option for fighting this potentially deadly parasite.