7 common myths about New World Screwworm and the real facts
By MWI Animal Health
This article is intended to communicate MWI Animal Health's capabilities which are backed by the author’s expertise. However, MWI Animal Health and its parent, Cencora, strongly encourage readers to review the references provided with this article and all available information related to the topics mentioned herein and to rely on their own experience and expertise in making decisions related thereto as the article may contain certain marketing statements and does not constitute legal advice.
These days, producers like you may be hearing a lot about New World screwworm. As your partner in animal health, MWI wants to provide you with accurate, unbiased information about what it means for your cattle.
Myth 1: All maggot infestations are caused by New World Screwworm (NWS)
Fact: Not all myiasis (maggot infestations) are caused by NWS. Other flies—such as botflies and blowflies—can also infest wounds and body openings and may look very similar at first glance. In some cases, animals may be infested with more than one species at the same time. Because visual signs alone are not enough, species identification by a trained entomologist is required to confirm a true NWS case.
Why this matters: Mistaking a routine maggot infestation for NWS can lead to unnecessary alarm. Accurate identification ensures the right response—without overreacting or delaying treatment for the animal.
Myth 2: Coughing or respiratory illness means an animal has NWS
Fact: NWS does not cause respiratory disease. Signs of infestation are localized to wounds or body openings and may include visible maggots, foul odor, tissue damage, and animal discomfort.
Why this matters: Correctly recognizing symptoms supports faster diagnosis and avoids confusion with unrelated illnesses.
Myth 3: NWS spreads from animals to people or from person to person
Fact: NWS is not contagious. It does not spread through direct contact between animals or people. Infestations only occur when a female screwworm fly lays eggs directly into an open wound or body opening. Once the life cycle is complete, larvae drop to the ground and continue development in the soil—not on other hosts.
Why this matters: Standard wound care and fly control are effective prevention tools, and there is no risk of “catching” NWS from another animal or person.
Myth 4: If one animal has NWS, the entire herd must be culled
Fact: Herd culling is not used to manage NWS. If an animal is suspected of infestation, the premises may be placed under a temporary hold while animals are examined. Infested animals are treated, isolated, and monitored until wounds heal and follow up checks confirm there is no reinfestation. Humane euthanasia is considered only in rare cases where treatment is not possible.
Why this matters: Early detection protects both animal welfare and producer livelihoods. USDA response protocols are designed to control the pest—not destroy herds.
Myth 5: Animals with NWS—or products from affected animals—cannot safely enter the food supply
Fact: Animals that have fully recovered from NWS may enter the food supply if they meet USDA regulatory requirements and pass standard inspection. NWS itself is not transmitted through meat, poultry, or dairy products.
Why this matters: Food safety protections prevent unnecessary economic losses and ensure the health of the food supply chain.
Myth 6: One infested animal crossing the border would cause a nationwide outbreak
Fact: USDA maintains continuous surveillance and response systems—including animal monitoring, fly trapping, and controlled animal movement—to detect and contain suspected cases quickly.
Why this matters: This reinforces confidence in national preparedness and biosecurity systems.
Myth 7: NWS would spread through the U.S. as rapidly as it does in Mexico
Fact: NWS thrives in tropical climates and does not tolerate extreme weather. The United States has proven eradication tools that allow for rapid containment if needed.
Why this matters: History shows that NWS can be controlled—and eliminated—when detected early and managed correctly.
Source: https://www.aphis.usda.gov/sites/default/files/nws-myth-busters.pdf
Disclaimer: Information about NWS is constantly changing. For the most up-to-date information, please visit https://www.aphis.usda.gov/livestock-poultry-disease/stop-screwworm
