Very Common
OTHER DIAGNOSES CONSIDERED
Other conditions or ailments that might also need to be ruled out by a vet.
Diagnosis
Conditions or ailments that are the cause of a problem that you see - your observation.
Your vet may diagnose
Screwworm Infestation
Synonyms: New World Screwworm infestation
Screwworm Myiasis
Summary
Screwworms are fly larvae (maggots) that infest and feed on LIVING flesh (called myiasis). There are New World screwworms (Chrysomyia hominivorax) in the Americas, and Old World screwworms (Chrysomyia bezziana) in other parts of the world. Unlike common blowflies whose larvae (maggots) feed on dead tissue, screwworm larvae feed on LIVING flesh — burrowing deep into healthy tissue and enlarging wounds rapidly. This makes screwworm infestation very dangerous. This is in contrast to ordinary maggot infestation, in which maggots actually provide a valuable service in cleaning up the dead tissue of a wound.
New World Screwworm (NWS) is a devastating fly-borne disease caused by the larvae of the blowfly Cochliomyia hominivorax that is currently appearing in the southern United States (June 2026).
What you would probably see in a horse with this condition is a wound that seems to be non-healing or ENLARGING.
A female screwworm fly lays 200-300 eggs at the edge of any wound — even a tiny one like a tick bite. The eggs hatch within 10-12 hours and larvae begin consuming living tissue within 3 days. The NWS fly does NOT require a wound — it can also lay eggs on mucous membranes of the eyes, nose, mouth, ears, genitalia, and the naval (umbilicus) of newborn foals.
NWS was eradicated from the United States in the 1960s through a sterile fly release program, but it has recently been confirmed in south Texas (June 2026) and northern Mexico. It remains endemic in (native to) parts of the Caribbean, Central America, and South America. NWS is a REPORTABLE FOREIGN ANIMAL DISEASE in every US state — if you suspect screwworm infestation, contact a vet right away.
These diseases are reportable, meaning that if a horse has or is suspected of having them, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.
DIAGNOSIS requires identification of the larvae by a trained parasitologist, because screwworm larvae look similar to common domestic blowfly larvae. Your vet will collect larvae from the wound and submit them to a federal or state laboratory.
TREATMENT involves applying approved insecticidal products to the wound FIRST (to kill larvae that may fall to ground during cleaning), then thorough removal of all eggs and larvae, wound management, systemic insecticide treatment of the entire animal, supportive care including antibiotics for secondary infection, and mandatory isolation until all wounds are fully healed. Several products have received FDA emergency use authorization for NWS in horses.
PREVENTION is critical. Inspect every horse daily for any wound. Treat all wounds immediately: clean, apply approved insecticide, and cover. Manage ticks aggressively. Apply insecticide before transporting horses. In affected regions, delay elective procedures that create wounds until the threat has passed.
New World Screwworm (NWS) is a devastating fly-borne disease caused by the larvae of the blowfly Cochliomyia hominivorax that is currently appearing in the southern United States (June 2026).
What you would probably see in a horse with this condition is a wound that seems to be non-healing or ENLARGING.
A female screwworm fly lays 200-300 eggs at the edge of any wound — even a tiny one like a tick bite. The eggs hatch within 10-12 hours and larvae begin consuming living tissue within 3 days. The NWS fly does NOT require a wound — it can also lay eggs on mucous membranes of the eyes, nose, mouth, ears, genitalia, and the naval (umbilicus) of newborn foals.
NWS was eradicated from the United States in the 1960s through a sterile fly release program, but it has recently been confirmed in south Texas (June 2026) and northern Mexico. It remains endemic in (native to) parts of the Caribbean, Central America, and South America. NWS is a REPORTABLE FOREIGN ANIMAL DISEASE in every US state — if you suspect screwworm infestation, contact a vet right away.
These diseases are reportable, meaning that if a horse has or is suspected of having them, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.
DIAGNOSIS requires identification of the larvae by a trained parasitologist, because screwworm larvae look similar to common domestic blowfly larvae. Your vet will collect larvae from the wound and submit them to a federal or state laboratory.
TREATMENT involves applying approved insecticidal products to the wound FIRST (to kill larvae that may fall to ground during cleaning), then thorough removal of all eggs and larvae, wound management, systemic insecticide treatment of the entire animal, supportive care including antibiotics for secondary infection, and mandatory isolation until all wounds are fully healed. Several products have received FDA emergency use authorization for NWS in horses.
PREVENTION is critical. Inspect every horse daily for any wound. Treat all wounds immediately: clean, apply approved insecticide, and cover. Manage ticks aggressively. Apply insecticide before transporting horses. In affected regions, delay elective procedures that create wounds until the threat has passed.
my vet's role
Diagnostics Used
These are tests that might be helpful to make this diagnosis or further characterize the condition.
PROGNOSIS AND RELEVANT FACTORS
Prognosis depends on the extent and location of wounds and the speed of treatment. Horses treated promptly — before larvae have caused extensive tissue destruction — generally have a good prognosis. However, animals not treated within 7-14 days may die from advancing wound severity, septicemia, and systemic debilitation. Deep wounds near vital structures (eyes, ears, genitalia) carry a worse prognosis. The key factor is early detection through daily inspection of all animals.my role
I might observe
You might make these observations when a horse has this condition.
Skills I Might Need
I might need these skills if my horse has this diagnosis.
Questions To Ask Your Vet:
- Are screwworm flies present in my area right now?
What approved insecticide products should I have on hand for prevention?
Should I delay any planned procedures (castration, etc.) until the threat passes?
How should I treat wounds differently now that screwworm is in the region?
• Do I need to report this to the state veterinarian, and how do I contact them?
• Can screwworm infest humans?- What precautions should I take?
Prevention
PREVENTION is the most important defense against NWS, especially in regions where the fly is present or nearby. Keeping fly numbers reduced on a facility is key- manure management, keeping facility clean is key. Inspect every horse daily for any wound, no matter how small. Treat all wounds immediately: clean, apply an approved insecticide, and cover if possible. Manage ticks aggressively — tick bites are common entry points. Apply approved insecticide to animals before transporting them. Inspect all new arrivals thoroughly, including an oral exam. In affected regions, delay elective procedures that create wounds (castration, branding, clipping) until the threat has passed or control measures are in place. In this June 2026 appearance of the disease, the USDA is deploying Sterile Insect Technique (SIT) — releasing sterile male flies to disrupt reproduction — which was the strategy that eradicated NWS from the US in the 1960s.
further reading & resources
Outside Resources
CREDIBLE EQUINE HEALTH INFORMATION ON THE INTERNET
