Background
NWS can infest cattle, sheep, goats, horses, swine, pets, wildlife, and, in rare cases, humans. Adult female flies lay eggs in open wounds or natural body openings such as the nose, ears, or navel of newborn animals. After hatching, larvae burrow into living tissue in a screw-like pattern, causing severe injury, secondary infections, and potentially death if untreated. The United States eradicated NWS in 1966 using the sterile insect technique (SIT), which involves releasing large numbers of sterilized male flies to disrupt reproduction. A localized outbreak in the Florida Keys in 2016 was successfully contained using this same approach along with quarantine and surveillance measures. In June 2026, NWS was detected again in Texas, prompting renewed concern and control efforts across the southern United States. While there have been no reported cases in Maryland to date, livestock and horse owners should remain vigilant due to animal movement and changing climate conditions that could influence the fly’s range.
Biology and Environmental Preferences
NWS is native to tropical and subtropical regions of the Americas, where warm temperatures and high humidity support its life cycle. The fly is not well adapted to prolonged cold conditions, which historically limited its spread into northern regions such as Maryland. Optimal temperatures for NWS survival and reproduction generally range between 25°C and 30°C (approx. 77°F to 86°F), with high humidity further enhancing egg hatch and larval development. At these temperatures, the life cycle can be completed in as little as 2–3 weeks, allowing populations to grow rapidly under favorable conditions. Cold temperatures significantly reduce survival; frost and extended periods below 10°C (50°F) can kill larvae and suppress fly activity. However, seasonal warming and milder winters associated with climate variability may increase the potential for temporary establishment in more northern regions.
Lifecycle
Adult female NWS flies are attracted to fresh wounds and moist body openings, where they lay eggs. These eggs hatch within 12–24 hours, and the tiny larvae (maggots) quickly burrow head-down into living tissue. They feed for several days, enlarging the wound and causing severe tissue damage. NWS is not typically spread through direct contact between animals. Each case begins when a female fly locates a suitable wound for egg-laying. As larvae feed, they produce odors and fluids that attract additional female screwworm flies, leading to further infestation. After feeding for about a week, mature larvae drop to the ground, burrow into the soil, pupate, and later emerge as adult flies to continue the cycle. Untreated cuts, dehorning, castration wounds, tick bites, or infected navels in newborns put animals at an increased risk of infestation. Horses are particularly vulnerable when they have skin wounds, surgical sites, or irritated tissue around the sheath, vulva, nostrils, ears, or eyes.
Signs to Watch For
Inspect animals closely for the following warning signs:
- Bloody drainage from a wound or cut.
- White or cream-colored discharge in or around a wound.
- Visible maggots in a wound. Wounds that enlarge, deepen, or develop a foul odor instead of healing.
- Pain, irritation, restlessness, fever, or reduced appetite.
Source : umd.edu