United States Warns of New World Screwworm Threat Amid Parasitic Fly Infestation Threat For Mexico, Costa Rica, Guatemala, El Salvador, Honduras and More in September 2025, What Travelers Need To Know

Published on September 17, 2025

In September 2025, the United States issued a comprehensive advisory regarding the New World Screwworm (NWS) (Cochliomyia hominivorax), a parasitic fly whose larvae can infest living tissue of mammals, including humans. The U.S. Department of Agriculture (USDA) and the Centers for Disease Control and Prevention (CDC) have highlighted the increasing threat posed by NWS, particularly in Mexico and Central America. Recent outbreaks have been reported in countries such as Mexico, Costa Rica, Guatemala, El Salvador, Honduras, and Nicaragua, marking a resurgence of this pest in regions where it had previously been eradicated.

What is the New World Screwworm?

The New World Screwworm (scientifically known as Cochliomyia hominivorax) is a parasitic fly that can cause significant harm to warm-blooded animals, including livestock, pets, birds, and humans. The larvae (maggots) of this fly are notorious for burrowing into the flesh of their host, causing painful sores, infections, and in severe cases, death. Though the risk to humans in the U.S. remains relatively low, health officials are still on high alert due to the increasing spread of the infestation in surrounding regions.

The U.S. Department of Agriculture (USDA) has highlighted that infestations of this fly could lead to grave economic consequences, particularly in the agricultural sector. The potential damage to livestock could result in decreased production, loss of animals, and a reduction in the availability of products such as milk, meat, and even manure, affecting the nation’s food supply.

Where Has the New World Screwworm Been Found?

As of early September 2025, the New World Screwworm has spread to several states in southern Mexico, including Chiapas, Oaxaca, Tabasco, Veracruz, and others, with more than 111,000 animals infested and 720 human cases reported. Tragically, six people have died as a result of infestations, underscoring the severity of the threat. The most alarming incident occurred in August 2025, when a traveler from El Salvador arrived in Maryland with visible signs of infestation.

Although the fly has not been detected in the wild within a 20-mile radius of the Maryland case, the risk of further spread remains significant, prompting California officials to issue a health advisory. The parasite’s rapid movement northward is of particular concern to health authorities, especially considering its historical presence in South America, Cuba, and the Caribbean.

Countries Monitoring or Affected by New World Screwworm

• United States: The U.S. has issued travel advisories and health alerts, particularly in states near the southern border, due to the detection of NWS in neighboring countries. CDPH
• Mexico: Confirmed cases of NWS have been reported in several states, including Oaxaca and Veracruz, leading to import restrictions and increased surveillance. Reuters
• Costa Rica: The U.S. Embassy issued a health alert in June 2024 following the first human death attributed to NWS infection in the country. cr.usembassy.gov
• Nicaragua: The U.S. Embassy issued a health alert in July 2025 regarding NWS cases, advising travelers to take precautions. ni.usembassy.gov
• Honduras: Reports indicate the presence of NWS, with authorities monitoring the situation closely. APHIS
• Guatemala: NWS has been detected, prompting increased vigilance and preventive measures. APHIS
• Belize: The Belize Agricultural Health Authority continues to monitor the outbreak, with heightened awareness among local authorities. APHIS
• El Salvador: A travel-associated human case was confirmed in Maryland, USA, in August 2025, linked to a traveler from El Salvador. kdhe.ks.gov
• Panama: Historically, Panama has been a focal point for NWS eradication efforts, with ongoing surveillance and control programs. APHIS
• Colombia: While not currently reporting active cases, Colombia is considered at risk due to its proximity to affected regions.

Travelers to these countries are advised to exercise caution, particularly in rural areas and regions with known outbreaks. Preventive measures include avoiding contact with livestock, using insect repellents, and promptly seeking medical attention if signs of infestation occur.

For the most current information and travel advisories, consult the U.S. Centers for Disease Control and Prevention (CDC) and the U.S. Department of Agriculture (USDA).

Impact on Tourism and Business Travel

For business and leisure travelers, the New World Screwworm poses a potential risk when visiting areas where the fly has been reported. The infestation can spread through the movement of infested animals or even travelers carrying the larvae on their clothing or possessions. While the current threat is not widespread in the U.S., particularly in California, health officials warn that those traveling to or from affected regions should take steps to avoid contact with infested areas.

In addition to the risks posed to individuals, the infestation could have serious ramifications for businesses, especially in agricultural or rural regions. Livestock producers could face significant financial losses, and tourism in affected areas could experience downturns as travelers become more cautious about the threat of parasitic infestations.

Precautions for Travelers

Travelers heading to regions affected by the New World Screwworm are urged to take several important precautions to avoid contact with the fly:

1. Avoid Animal Contact: Refrain from coming into close contact with livestock, pets, or wildlife in areas where the parasite has been reported.
2. Keep Wounds Covered: Anyone with open cuts or wounds should ensure that they are properly cleaned and covered. The fly’s larvae can enter through small openings, such as insect bites, nose, mouth, or genitals.
3. Use Insect Repellent: Travelers are advised to use insect repellent that is registered by the Environmental Protection Agency (EPA). Wearing loose-fitting long-sleeved shirts, pants, and socks can help minimize exposure.
4. Check for Symptoms: If you are in an area with known outbreaks and experience symptoms such as painful sores, non-healing wounds, or a foul odor coming from a wound, seek medical attention immediately. The larvae must be removed promptly to prevent further complications.
5. Avoid Agricultural Areas: If possible, avoid traveling through agricultural zones where livestock are kept, as these areas are the primary source of the fly’s spread.

Treatment for Infestation

In the event of an infestation, medical treatment is required to remove the maggots. Health professionals may use concentrated alcohol to kill the larvae, which are then disposed of as biohazardous waste. Prompt intervention is crucial to prevent severe health complications.

While deaths from the infestation are rare, untreated infestations can lead to infections and long-term health issues, particularly for individuals who are immunocompromised, malnourished, or the very young and old.

Government Efforts to Combat the New World Screwworm

In response to the growing threat of the New World Screwworm, the USDA has invested in new technology to accelerate sterile fly production in a bid to eradicate the parasite. A sterile-fly production facility will soon open in Edinburg, Texas, which will work alongside facilities in Panama and Mexico to produce up to 300 million sterile flies per week. These sterile flies will be released in areas with active infestations to prevent further reproduction and spread.

The USDA’s historical efforts to eliminate the New World Screwworm were successful in the 1960s, and the agency remains committed to preventing the parasite’s return to the U.S.

Key Points:

• New World Screwworm infestations have been reported in southern Mexico, with hundreds of animals and several human cases.
• California health officials have issued a warning about the potential spread of the fly to the U.S.
• Travelers are urged to cover open wounds, avoid animal contact, and use insect repellent in affected areas.
• The USDA has invested in sterile fly production to combat the spread of the parasite.
• Early detection and treatment are crucial to preventing complications from infestations.

Conclusion:

Travelers planning visits to Mexico, Central America, or the southern United States are strongly advised to take preventive measures against NWS. This includes avoiding contact with livestock, using insect repellents, and keeping wounds clean and covered. The USDA has implemented strategies such as the release of sterile flies and enhanced surveillance to combat the spread of NWS USDA. By staying informed and vigilant, travelers can help mitigate the risks associated with this parasitic threat.