Cochliomyia hominivorax

Synopsis

CAPC Recommends

• Monitor wounds and any suspect fly larvae found on wounds of companion animals should be identified to rule out screwworm infestation.
• Monitor animals if traveling to areas where screwworm outbreaks have been reported or endemic areas.
• Screwworm infestation is a reportable, zoonotic disease. Contact the state veterinarian to report suspect or confirmed cases of screwworm.

Species

Feline

Canine

Cochliomyia hominivorax - New World Screwworm

Cochliomyia hominivorax 3rd instar larva

Cochliomyia hominivorax 3rd lnstar larva - anterior end

Cochliomyia hominivorax 3rd instar larva - tracheal trunks

Overview of Life Cycle

• Cochliomyia hominivorax under-go obligatory myiasis. Adults mate once and are oviparous. Females lay large numbers of eggs on the host near open wounds or body orifices (sites of castration or the umbilicus are common locations).
• The eggs hatch within 24 hours and the larvae feed on living tissue.
• The larvae mature in 5-7 days, drop off the host, and pupate in the environment.
• The adult screwworm lives for approximately 7-10 days.

Stages

Adult fly

• Metallic blue-green body with a reddish face and 3 prominent longitudinal stripes on the dorsal thorax.

Larvae

• Mature 3^rd instar larvae are approximately 15 mm in length. They are pointed anteriorly and truncated posteriorly. They are identified based on morphological characteristics including the species specific spiracular plates on the posterior end and dark tracheal trunks.
• The larval stage consumes healthy flesh in the affected animal.

Prevalence

• Cochliomyia hominivorax was historically found throughout the southern United States, Central America, and the northern countries of South America. Today, screwworm has been eradicated in the United States, Mexico, Central America, and several Caribbean islands. Screwworm eradication programs have not been successful in Jamaica.
• Screwworm exists in South America, extending to as far south as Uruguay, northern Argentina and Chile. Imported cases of C. hominivorax have occurred sporadically in the United States, Mexico, and the United Kingdom. A recent outbreak of screw worm in the Florida Keys is described below.

Diagnosis

• Screwworm infestation is associated with the presence of a pre-existing wound, such as the naval of newborn animals. Wounds may be malodorous and produce suppurative or serosanguinous discharge. The wounds may have secondary bacterial infection. Depending on the severity of the infection, animals may show signs of systemic illness.
• Differential diagnoses for screwworm myiasis should include any blowfly which can infest wounds, including Cochliomyia macellaria (secondary screwworm) and Phormia regina.
• Screwworm infestation should be considered in any case of myiasis. Larvae should be collected from affected animals using forceps. It is recommended to collect larvae from the deepest area of the wound to avoid collecting secondary agents of myiasis. If found, eggs can also be collected for evaluation. Samples can be saved in 80% ethanol or isopropyl alcohol; do not use formalin.
• Screwworm myiasis is a reportable disease and the state veterinarian must be notified. The state veterinarian can direct samples to the appropriate diagnostic lab. Even suspect cases should be reported.

Treatment

• Treatment of screwworm infection involves removing visible eggs and larvae from the wound. The wound should then be thoroughly cleaned.
• Depending on severity, the animal may need extensive wound care, systemic antimicrobials, and analgesia.
• Nitenpyram is larvacidal and has shown 100% efficacy in affected animals (Correia, et al. 2010).

Control and Prevention

• Screwworm was eradicated in the United States in the 1950’s and 60’s, with only sporadic cases since. The mainstay of preventing screwworm re-entry to the US is sanitary and medical prophylaxis. Any animals that are imported from endemic areas should be inspected thoroughly and dewormed with approved insecticides.
• Prevention of screwworm infestation has traditionally focused on livestock due to the massive economic impact screwworm had on the industry. Studies show that the use of macrocyclic lactones, such as ivermectin and doramectin, are effective at preventing and treating screwworm infestation.
• Ivermectin has also been used in the treatment and prevention of screwworm in dogs.
• There is increasing interest in the possible efficacy of topical products that possess claims against mosquitoes and other biting flies.

Public Health and Eradication

• Screwworm is important to both human and veterinary medicine due to its zoonotic potential. Before the eradication program, C. hominivorax caused significant production losses in livestock in the United States and Mexico. The eradication program launched in 1957 by releasing sterile flies to the areas with screwworm populations. Since then, screwworm has been eliminated in most areas north of Panama, including the United States and Mexico. Since eradication, the causes of screwworm myiasis in the United States have been related to the importation of improperly inspected animals. In September 2016, the USDA confirmed the presence of C. hominivorax in Key deer in Big Pine Key, Florida. Further investigation revealed that veterinarians diagnosed several domestic animals with unusual myiasis in the months prior. None of the cases of myiasis in domestic animals were reported. Authorities are not certain how screwworm was re-introduced to this area. During this time, all animals entering and exiting the Florida Keys were inspected for the presence of myiasis. Badly affected Key deer were euthanized, and the remaining population treated through the efforts of authorities and volunteers.
• During the outbreak, 130 deer died from screwworm infestation. Currently, sterile flies are being released from numerous sites throughout the Florida Keys. There have been no new cases of screwworm myiasis since November 2016.

Selected References

• Anziani, O.S., et al. "Persistent activity of doramectin and ivermectin in the prevention of cutaneous myiasis in cattle experimentally infested with Cochliomyia hominivorax." Veterinary Parasitology, vol. 87, nos. 2-3, Jan. 2000, pp. 243-47.
• Correia, T. R., et al. "Larvicidal efficacy of nitenpyram on the treatment of myiasis caused by Cochliomyia hominivorax (Diptera: Calliphoridae) in dogs." Veterinary Parasitology, vol. 173, nos. 1-2, 2010, pp. 169-72.
• Orcutt, C., DVM. (2017). Avian and Exotic News. Journal of Exotic Pet Medicine, 26(2), 93-95.
• "Screwworm (Old World and New World)." World Organisation for Animal Health, www.oie.int/fileadmin/Home/eng/Animal_Health_in_the_World/docs/pdf/Disease_cards/SCREWWORM.pdf. Accessed 3 May 2017.
• Spradberry, J. P., et al. "The efficacy of ivermectin against larvae of the screw-worm fly(Chrysomya bezziana)." Australian Veterinary Journal, vol. 62, no. 9, Sept. 1985, pp. 311-14.
• Wall, Richard, and David Schearer. "Myiasis." Veterinary Entomology, Chapman & Hall, 1992, pp.197-232.
• Wyss, J. H. (2000), Screwworm Eradication in the Americas. Annals of the New York Academy of Sciences, 916: 186–193. doi:10.1111/j.1749-6632.2000.tb05289.