Disease Reporting and Surveillance

New World Screwworm

New World Screwworm (NWS) is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals, including livestock, wildlife, pets, and even humans. Left untreated, infestations are often fatal. The pest is not currently present in the United States, but an outbreak was confirmed in Southern Mexico in November 2024. With detections just 70 miles from the U.S. border as of November 2025, time is of the essence to stop the spread.

NWS has been spreading north from South America through Central America. Since the first detection in Mexico, there have now been thousands of cases reported. Experts believe illegal livestock imports from South America into Mexico are fueling the continued spread. In response, APHIS is also raising awareness about the risk of illegal or stray equines, cattle, and infested wildlife crossing into the U.S., which could unknowingly introduce the parasite.

While the land border is closed to livestock, horses may still be flown into U.S. import centers from Mexico. Exports of animals from the United States to Mexico remain open.

To combat screwworm, the U.S. and Panama have long maintained a sterile fly release program to prevent the pest’s northward spread. However, the Panama facility is now at maximum capacity, producing over 100 million sterile flies a week. A second sterile fly production facility—ideally located in the U.S.—is urgently needed, but funding for construction must approved by the USDA or Congress.

APHIS urges all livestock handlers, veterinarians, and equine professionals to stay alert for signs of NWS infestation and report any suspicious cases immediately. Preventing the reintroduction of this devastating parasite is critical to safeguarding U.S. animal agriculture.

You can learn more at https://www.aphis.usda.gov/animal-emergencies/nws

African Horse Sickness

African Horse Sickness is transmitted by biting midges and flies that occurs regularly in most countries of sub-Saharan Africa. In naive populations of horses, which are the most susceptible equids, mortality may reach 90% in epidemics. The disease, which presents as a pulmonary and/or a cardiac form, is characterized by high fever, depression, and respiratory symptoms. The clinically affected animal has trouble breathing, starts coughing frothy fluid from nostril and mouth, and shows signs of pulmonary edema within four days. Serious lung congestion causes respiratory failure and results in death in under 24 hours.

The introduction of this disease into the United States would have catastrophic implications for the health of our national herd and be insurmountable for the continuity of the domestic horse industry.

The USDA asked the American Horse Council to participate in the development of a diagnostic drill for state and federal animal health officials to use in the identification of African Horse Sickness (AHS) cases in the United States. The drill was developed and a test drill was run in October of 2021 and was released for Federal and State Animal Health Official use is in early 2022.

Equine Viral Arteritis

The USDA released guidelines for breeding a mare to an EVA shedding stallion. You can review the guidelines here: EVA Uniform Methods and Rules Effective April 19, 2004 https://www.aphis.usda.gov/vs/nahss/equine/eva/eva-umr.pdf

Contagious Equine Metritis

Contagious equine metritis (CEM) is a venereal disease found in horses caused by bacteria. It is spread during breeding or through contact with contaminated objects. This disease occurs very rarely in the United States and does not affect other livestock or people. However, it is highly contagious among horses and can be difficult to detect and control. Signs of illness in infected mares may not be obvious, and stallions carry the bacteria without showing any signs at all. CEM can have a negative impact on fertility in both mares and stallions. If the disease became widespread in the United States, the horse industry could suffer considerable economic losses.

The USDA’s continued prioritization of policies and practices that have prevented the reintroduction of Contagious Equine Metritis (CEM) are well-recognized and equally appreciated. The extension of the 60-day waiver to 90 days is a welcome step. In light of the overwhelming support this change has within the equine industry and the USDA’s own statements regarding the minimal threat posed by this rule change, the American Horse Council requested the USDA implement this aspect of the proposed rule change as soon as possible.

Vesicular Stomatitis

Vesicular stomatitis is a viral disease that primarily affects horses and cattle. It occasionally affects swine, sheep, goats, llamas, alpacas, and people who handle affected animals. Vesicular stomatitis found in the Western Hemisphere, where it is endemic in warmer climates and occurs sporadically in temperate climates.

In the past decade, the southwestern and western United States experienced a number of vesicular stomatitis outbreaks. Outbreaks usually occur during the warmer months, often along waterways. In some years, only a few premises in a single state were affected. However, in other years, multiple states and many premises were involved.

It is essential that veterinarians and livestock owners in these regions are aware that vesicular stomatitis occurs in their area – and be on the alert for animals displaying clinical signs of the disease.

Learn more at https://www.aphis.usda.gov/livestock-poultry-disease/cattle/vesicular-stomatitis