What is New World Screwworm?
New World Screwworm (NWS) infestation is caused by NWS fly larvae (Cochliomyia hominivorax). These maggots consume living tissue or flesh of warm-blooded animals (including horses) by burrowing into and tearing tissue by hooks on the larva. The screwworm fly lays eggs in open wounds. Eggs are creamy white found at the edges of the wound. Larvae invade the wound and subsequently develop into adult flies which transmit the parasite to new wounds. Screwworms can be transmitted to people who can be at risk if traveling in endemic areas, are around livestock and have an open wound. Currently NWS is not present in the United States and is considered to be a foreign animal disease. Suspected cases must be reported to state and federal animal health officials who complete the investigation and establish the appropriate diagnostic testing, treatment and biosecurity protocols. This parasite is common in tropical and subtropical regions in Central and South America. The parasite was identified in Mexico in late 2024.
Clinical Signs:
- Open wound
- Evidence of pain
- Irritation
- Head shaking
- Presence of maggots in wounds
- Foul odor
Diagnosis: Visual inspection of wounds with identification of maggots. There are other flies that can be associated with wounds; for official identification, the larvae should be sent to a specialist.
Treatment: Animals infested with NWS should be treated according to the recommendations of their veterinarian and state and federal animal health officials. Typically, cleaning of wound and surgical removal of maggots is needed with subsequent treatment with approved topical and systemic insecticides. Larvae removed from the wound must be destroyed as directed by federal or state animal health officials and not allowed to enter into the environment. Systemic administration of antibiotics is recommended if the wound becomes infected. Treatment of the environment may also be necessary.
Prevention: Regular inspection of wounds, use of insect repellents and maintaining clean environments. Anyone suspecting a screw worm infection should contact their state veterinarian immediately.