UC Davis Center for Equine Health

Preventing the spread of infectious diseases in horses starts with understanding how pathogens are transmitted. Disease-causing organisms can reach horses through multiple routes, making biosecurity a crucial part of equine care.

How Respiratory Diseases in Horses Spread

Airborne Transmission - One of the most challenging ways infectious diseases spread is through aerosols—tiny respiratory droplets expelled when an infected horse coughs, sneezes, or snorts. These droplets can remain suspended in the air, and even after evaporation, residue and contaminated dust particles may still carry pathogens. Horses become infected when these microorganisms enter the eyes, nose, or mouth.

The ability of pathogens to survive and spread in the air depends on environmental factors like wind, humidity, and temperature. Equine influenza and equine herpesvirus (EHV) are two highly contagious diseases that spread through aerosol transmission.

Minimizing Airborne Disease Risks - Since airborne infections can spread rapidly, controlling exposure is key:

• Maintain Distance: Limit close contact between horses, especially when illness is suspected.
• Control Dust: Keep the environment clean and minimize dust buildup in barns and riding areas.
• Ensure Proper Ventilation: Good airflow in stables and transport vehicles helps reduce airborne pathogens.

Direct contact - Some diseases spread through close physical contact with infected animals, their body fluids or tissues, and direct touching of wounds, skin or mucous membranes around the nose, eyes, and mouth. Affected animals can infect other animals through nose-to-nose contact or biting.

Equine influenza is an example of a disease spread by nose-to-nose contact.

Limit contact with horses between stalls, in aisleways, and at cross-ties or wash racks. Do not tie horses near each other. Clean and disinfect water troughs regularly.

Dreamstime/Luis Carlos Torres

Related: 8 Ways to Think Like a Parasite

Indirect contact - Objects contaminated with infectious organisms are called fomites. These can include tack, buckets, grooming supplies, hoses, clothing, and even vehicles.

Diseases spread by indirect contact include strangles and equine herpes virus 1.

Avoid sharing equipment between horses. If equipment must be shared, clean and disinfect between uses. Dedicate supplies and equipment for sick horses. Keep vehicles and trailers in designated areas. Limit traffic, people, and other animals in horse areas.

Oral - Horses can consume pathogens in feed or water contaminated by feces or urine from infected animals. Licking or chewing contaminated objects in the environment can also lead to ingestion of pathogens.

Diseases spread through oral transmission include salmonellosis and equine coronavirus.

Control feed sources, ensure feed quality, and handle and store feed properly. Clean stalls regularly, manage fecal piles in fields and turnouts, and ensure proper waste management. Avoid natural water sources such as streams that may be contaminated with feces or urine from wildlife and livestock.

Vectors - Insects, such as mosquitoes and flies, or ticks can transmit infectious organisms between animals.

Vector-borne diseases include West Nile, Eastern equine encephalitis, and Western equine encephalitis.

Proper vector control is an essential part of any biosecurity plan. Use insecticide applications as appropriate. Fence off areas with high insect and tick populations, and keep horses inside when these vectors are active (dusk to dawn). Eliminate insect breeding areas, including standing water and decaying organic matter.

Wildlife - Rodents, skunks, opossums, bats, birds, and other wildlife can also act as disease vectors. Diseases spread by wildlife include rabies and equine protozoal myeloencephalitis (EPM).

Prevent wildlife from accessing barn areas. Sweep regularly, keep barns and tack rooms tidy, and clean up and store feed in containers with securable lids. Patch holes and gaps in buildings to prevent wildlife access. Do not leave pet food or water out overnight. Keep compost piles away from barns.

Related: Vaccination Guidelines for Healthier Horses

Printed with the kind permission of the UC Davis Center for Equine Health. The UC Davis Center for Equine Health is dedicated to advancing the health, welfare, performance and veterinary care of horses through research, education and public service.

Main Photo: Shutterstock/Anetapics