Cochliomyia spp (New World Screw worm) are a zoonotic fly endemic to some tropical countries that may result in myiasis that can be lethal in dogs.
Myiasis caused by the larvae of these dipteran flies is a result of larval deposition in both necrosed and living tissues, often associated with open or chronic wounds such as is seen with dog fight wounds. The larvae feed on wound exudates and serum and illness associated with myiasis usually occurs in warm humid climates, frequent in developing countries.
Pathogenic species include:
- Cochliomyia hominivorax
- Cochliomyia macellaria
Cochliomyiasis has been reported in dogs, as well as cattle, cats and other animals, across Central and South America and the Caribbean Islands, but sporadic outbreaks have been reported across the northern hemisphere from the USA to northern Africa and Europe.
Prior to its eradication from the United States in 1982, C. hominivorax caused significant economic losses to the livestock industry. Detection of Cochliomyia hominivorax on livestock in the United States is reportable to the State Veterinarian USDA Animal and Plant Health Inspection Service.
Sporadic outbreaks in humans are less common and occur in patients with the following risk factors: open wounds, poor hygiene, low educational level, alcoholism, immobility and physical or mental disability.
Adult female flies lay batches of 200 - 400 eggs in rows that overlap like shingles in a mass on the edge of a fresh wound. After 12-21 hr, larvae hatch, crawl into the wound, and burrow into the flesh. The larvae feed on wound fluids and live tissue, resulting in penetrating wounds that become secondarily infected with bacteria, leading to open, fistulous wounds. Flies regularly deposit onto the dog's coat and ears.
In urban dogs, deaths can result from large wound becoming repeatedly infected with screw worms.
Diagnosis is based on laboratory identification of larvae, which are characteristically tapered and have mouth hooks at the narrow end and breathing spiracles at the wide end. Body segments are ringed with spines. Fully grown larvae can be as long as 1.5 cm. Larvae are often identified by their “wood screw” shape.
Screwworm infestation must be reported to both state and federal authorities. C. hominivorax has been eradicated from the USA but occasionally enters the country surreptitiously on imported animals.
Control involves use of topical lindane, coumaphos, lindane, or ronnel.
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