Old World Screwworm is a parasite that is usually active during late summer and early autumn months as the wet, hot conditions are conducive to its development.

However, last month, in a surprising report submitted to OIE (the World Organization for Animal Health), a case of Old World Screwworm (OWS) was identified in a sheep in Ninawa, a northern Iraq province.

The parasite, officially Chrysomya bezziana, is a fly, typically found in Asia, India, sub Saharan Africa, and some Middle East countries. The screwworm fly is an “obligate parasite,” meaning that a host is required for development to complete. The fly typically feeds on living tissue of mammal hosts, as opposed to decaying tissue, and will leave its larvae to burrow into the skin where it will grow. This type of infection is called “myiasis.” According to the CDC, Myiasis cannot be spread from person-to-person or animal-to-animal.

The first case of C. bezziana recorded in Iraq was in 1996 at the Mesopotamia valley. In 2005, a group of researchers (A. Siddig et al.) produced a risk map for OWS distribution in Iraq using Geographic Information Systems (GIS) to survey cattle populations, vegetation, watercourse and geographic features of the region. Interestingly, unlike expected, this group of researchers found that the distribution of C. bezziana was not correlated to cattle distribution. Rather, the distribution of OWS was found to be of high correlation to the amount vegetation. Adult flies favor areas with vegetation to protect them from high temperatures and solar radiation.  

The Worm and its Life Cycle:

Adult female flies lay eggs in an open wound or mucous membranes of body orifices on the body of the host. The eggs mature into larvae or also known as maggots. The larva is the infective stage at which it feeds on the host’s skin for 5-10 weeks. When the larvae reach their mature phase, they drop into soil and stay until they become adults ready to mate and start the cycle again. (CDC)

Treatment and prevention:

To prevent infection, some schedule animal breeding and to avoid birthing when it is the season for the flies. Agriculturists should also be sure to prevent injuries to animals. In endemic and non-endemic areas, precautions are taken with every wounded animal by treating them with insecticides. Inspections are done on animals every few days in endemic areas, and they are often sprayed with treating materials.

Infestation caused by C. bezziana, if not treated, could lead to fatal consequences. For treating infected animals, the larvae should be removed by a medical professional and the resulting wound should be kept as clean as possible to avoid further infection. Necrotic tissue might be removed as necessary and antibiotics may be prescribed for possible secondary bacterial infections. (CFSPH)


Female screwworms are capable of traveling long distances of up to 20km, or 12.5 miles, to find a suitable environment and a suitable host to lay their eggs (the host is, basically, ALL warm-blooded mammals). The question, can we eradicate screwworms? Sterile male flies are released repeatedly in endemic areas to mate with wild female flies which results in producing unfertilized eggs. Therefore, fewer flies are coming to be. This method was successful in the United States in the 1960s, though sometimes infected animals make their way into the country, and has since moved on to other countries (USDA Agricultural Research Service). But is that enough for complete eradication? Even if we do eradicate the fly, there are other more fatal worms roaming the world, like the nearly identical New World Screwworm (NWS). Both of these bugs follow a very similar life cycle, however, there are more recorded deaths from the NWS. Not to forget the constantly evolving genome of these worms which will continue to help them survive longer through generating resistance to currently used insecticides.  Here’s to strong surveillance and reporting; innovative medical and public health research; and a little bit of luck for the defeat this flesh-eating bug. (CFSPH)

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