You probably know that New World Screwworm (NWS) is in the United States again. News that a pet dog in New Mexico was infected no doubt stoked some fear in conscientious dog owners everywhere. (This case originally was reported in Texas, the location of the reporting veterinarian. The dog lived in New Mexico leading to reclassification of the case from that state.) Although the risk for the average dog (and cat) is still quite low, it is a good to be aware of developments and know how to detect any signs of infestation (a condition known as myiasis). Research, and prior history, with this parasite shows that it can spread widely and quickly. Animals involved in rescue or who have been transported across regions warrant extra attention. Vacationing with a dog in an endemic area or transporting a dog back to the continental US after time in such an area increases the risk to everyone. Early intervention is crucial for the health of the individual dog and for helping to control the spread of this potentially devastating condition.
NWS (Scientific name Cochliomyia hominivorax) can infect a variety of animals including livestock, wildlife, pets, and rarely, birds and humans. Typically livestock are of greatest concern. Close proximity of cattle to each other, dispersion over a grazing range, and the difficulty of catching an infestation early because of the diligent observation required enable New World Screwworm to do significant damage fairly quickly. For our beloved pets, living conditions are far different making it easier to catch any problem early, provided we know what to look for and what to do.
The screwworm damage is done by the larvae of the adult Screwworm fly, a type of blowfly. The adult looks very much like a common house fly but has some distinct features. The screwworm fly is larger than a house fly. The eyes of the screwworm fly are orange or red in appearance and there are three dark stripes on the back. The body is a noticeable metallic green or blue. While the common housefly brings its own set of threats, those larvae (maggots) feed on dead or decaying tissue. The NWS larvae, in contrast, feed on living tissue, burrowing (or screwing) their way into the tissue where they can do substantial damage, even to the point of causing death if undetected. NWS gains access typically through open wounds but also can enter through other openings in the body such as the eyes, nose, and mouth. The female screwworm fly will lay 100 or more eggs which hatch within 10-12 hours. This fly reproduces quickly and starts feeding on live tissue quickly as well. Even small openings in the skin and surgical incisions are vulnerable as the fly will lay eggs on any open area. Careful observation of your dog is important for early detection.
Symptoms to watch for include skin wounds that become worse over a few days time, a foul-smelling discharge from a wound or from another body opening, and wounds that seem particularly painful. If any worrisome wound develops or other symptoms are observed, the dog should be seen promptly by a veterinarian. Not only can the veterinarian determine the cause of the symptoms, if it is NWS the condition needs to be reported for tracking and surveillance purposes. The veterinarian also can ensure that any larvae present are properly removed and also submitted for testing. Attempts to remove at home could cause more damage to the affected tissue with high risk of spreading infection.
There are prescription medications available for treatment to ensure eradication. Some of the commonly used parasite preventatives offer some protection. Overall, one of the most important things a dog parent can do is be vigilant and observant, and do not hesitate to get care whenever there is anything of concern. When doing a daily check for lumps, bumps, fleas, and ticks, also be on the lookout for any wounds or drainage. Observe also for any behavior change such as licking, shaking or head or legs, or agitation. Catching developments early offers the best chance for a good outcome. Currently dogs are considered to be at low risk, and proper observation and care will help to keep the risk low.
For further reading:
Cornell Feline Health Center, New World Screwworm. https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/new-world-screwworm
Digging Deeper: How to Protect Pets from the New World Screwworm. Texas A&M University, https://vetmed.tamu.edu/news/pet-talk/https-vetmed-tamu-edu-news-pet-talk-new-world-screwworm-pet-protection-prevention/
US Centers for Disease Control, About New World Screwworm. https://www.cdc.gov/new-world-screwworm/about/index.html
USDA, Animal and Plant Health Inspection Service (APHIS). https://www.aphis.usda.gov/livestock-poultry-disease/stop-screwworm/surveillance
USDA, APHIS. https://www.aphis.usda.gov/news/agency-announcements/usda-confirms-first-case-new-world-screwworm-dog-lea-county-new-mexico
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