New World Screwworm: Five facts about a growing animal health threat

24/07/2026

Despite its name, the New World Screwworm (NWS) isn’t a worm.  It is a parasitic fly. Its name comes from the screw-like motion its larvae make when burrowing into tissue. It is painful and potentially deadly if left untreated. It can also, though less commonly, affect people. NWS is, in short, something to take seriously.

As with the spread of other transboundary animal diseases (TADs), NWS has resurged across the Americas and spread across borders, threatening livestock production, disrupting trade and placing added pressure on farmers and veterinary services. Its impacts can extend well beyond animal health—reducing productivity, affecting food production and putting at risk the livelihoods of people who depend on livestock.

The current outbreak is driven by a combination of factors, including increased movement of animals, people and products; informal or illegal trade; limited awareness among producers about early prevention and control; climate and environmental pressures; and gaps in veterinary, surveillance and response capacity.

Here are five facts you need to know about NWS and what FAO is doing to help countries prevent and control its spread:

1. It mostly affects animals, but humans are also at risk.

The NWS is a parasitic fly whose larvae feed on living flesh. Any warm-blooded animal can be affected, including cattle, horses, pets, wildlife and humans. Young animals are especially vulnerable, particularly right after birth.

Once eggs are laid in a wound, the hatching maggots rapidly destroy tissue, causing a condition called myiasis. Without treatment, infestations can be fatal.

2. It has resurged across countries in the Americas.

The fly is endemic to much of South America. Over the past five years, it has resurged across Central America and spread into Mexico, re-establishing itself across much of its historical range. Eight countries — Belize, Costa Rica, El Salvador, Guatemala, Honduras, Mexico, Panama and, most recently, the United States of America — have reported active cases.

3. It spreads through movement not direct contact

NWS does not spread through direct contact between animals or from animals to people. It spreads through the movement of infested animals, adult flies crossing borders, wildlife migration and accidental transport. This is why international coordination and border surveillance are essential to containing outbreaks.

4. The Sterile Insect Technique is a proven tool in the fight against NWS

Using the sterile insect technique (SIT), NWS was eradicated from the United States in 1966, from Mexico in 1984 and from several other countries in Central America thereafter.

SIT involves using radiation to produce sterile male flies which are then released into the wild. This mating does not produce offspring, reducing the wild population over time. It has proven to be highly effective and cost-efficient while also being pesticide-free and environmentally friendly.

In a coordinated effort involving 22 donors, eight countries and other United Nations’ agencies, FAO and the International Atomic Energy Agency (IAEA) used the SIT to eradicate NWS in Libya in 1991, when an outbreak there caused devastation and fear that the fly would spread further across Africa and into Europe. The success of this intensive international cooperation demonstrated the effectiveness of a rapid, coordinated response. 

Today, through the Joint Centre of Nuclear Techniques in Food and Agriculture, FAO and IAEA continue to provide research and scientific support for SIT operations.

5. There are solutions to NWS, but countries must invest and act together

NWS can be prevented, controlled and even eradicated. But doing so requires sustained international collaboration and shared investment in the systems, expertise and technologies needed to stop its spread. Like other TADs, NWS does not respect borders and neither a single country nor a single agency can stop its spread alone. Countries must invest together because the protection of one country depends on the preparedness and response capacity of its neighbours. That is why FAO coordinates global and regional response efforts.

As a United Nations specialised technical agency, FAO is uniquely placed to help countries strengthen veterinary services, surveillance and cross-border coordination. FAO provides policy guidance and technical support to governments, helping countries strengthen NWS surveillance, prevention and biosecurity measures. It also provides on-the-ground, emergency technical assistance, trains national veterinary services and supports diagnostic research to help affected countries strengthen their long-term prevention and control strategies.

In early 2026, FAO conducted Good Emergency Management Practices (GEMP) workshops in El Salvador and Honduras.

Across the Americas, FAO is also working with the Panama-United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG) and national veterinary services across Central America, FAO is strengthening prevention and control efforts in countries where NWS has re-emerged.

Under the Global Framework for Transboundary Animal Diseases (GF-TADs), jointly with the World Organisation for Animal Health (WOAH), FAO supports coordination with other regional partners.

Above all, early detection is key. Through its Emergency Prevention System for Animal Health (EMPRES-AH), FAO tracks cases in animals and provides readily accessible data on where the disease has occurred, giving national authorities an improved understanding of NWS spread patterns and potential border threats. These early warning systems help centralise information and ensure coordinated, rapid response across borders.

Building on more than 80 years of experience, FAO is working with Members and partners to strengthen global capacity for prevention, preparedness and response to transboundary animal diseases.

No country can address this threat alone. Protecting animal health in one country depends on strong prevention, surveillance and rapid response in neighbouring countries. Sustained investment and coordinated action are therefore shared responsibilities and essential to preventing further spread.