National Cattlemen May 2026 | Page 14

As New World Screwworm( NWS) treks north through Mexico, USDA and NCBA, in partnership with state affiliates, are taking action before it reaches the U. S. southern border. While the pest was eradicated in the U. S. in 1966 using the sterile fly technique( more on this in the next issue of National Cattlemen), there have been numerous cases identified less than 200 miles south of the Texas border. 1

Screwworms are a fly larvae, or maggots, that burrow or hook into the flesh of living animals causing serious, and even deadly, damage to the animal. Not only can the parasite lead to animal suffering, but also economic loss for operations, as many producers can attest to during the 1960s when the NWS was prevalent in the U. S. 2

It is critical that cattle producers are vigilant in searching for signs of NWS, especially as temperatures rise heading into summer, as the NWS thrives in warmer temperatures. 3 The adult screwworm fly resembles a common household fly but is slightly larger and has distinctive orange eyes, a metallic blue or green body and three dark stripes along the back. 2

Signs to look for on cattle include:

• Bloody or light-colored drainage from a cut or wound

• A cut or wound that rapidly enlarges for seemingly no reason

• White or cream-colored runny substance( the eggs) in and around a wound

• Presence of fly larvae( maggots) in wounds

• Irritated behavior and signs of pain

• Head shaking

• Reduced appetite

• Fever and other signs of secondary infection

• Foul odor or smell of decay

Infestations begin when a female fly lays eggs on an open wound or orifice of a warm-blooded animal. Females can lay 200 – 300 eggs at a time and up to 3,000 eggs during her lifespan of 10 to 30 days. Within hours, the eggs hatch into larvae and burrow into the wound, causing pain and tissue damage. After about a week of feeding, the larvae drop to the ground, pupate in the soil, and emerge as adult flies within 7 to 54 days, depending on environmental conditions. 4

If you believe you’ ve found NWS, contact your veterinarian immediately. Find resources for what to look for at ncba. org / education-resources / industry-updates-resources / new-worldscrewworm-resources. References: 1. Current Status of New World Screwworm. https:// www. aphis. usda. gov / livestock-poultry-disease / stop-screwworm / current-status? page = 1

2. New World Screwworm Prevention for Animals. https:// www. aphis. usda. gov / livestock-poultry-disease / cattle / ticks / screwworm

3. CDFA / AHFSS New World Screwworm. https:// www. cdfa. ca. gov / ahfss / Animal _ Health / screwworm /#:~: text = Climate, Parasite % 20of % 3

4. NCBA NWS Resources. https:// www. ncba. org / educationresources / industry-updates-resources / new-worldscrewworm-resources

L3

L2

L1

14 MAY 2026 www. NCBA. org