Common Offenders: Equine Pathogens to Keep on Your Radar
Equine herpesvirus: Of the nine equine herpesviruses (EHV), 1, 3 and 4 pose the most serious risks. Of these, EHV-3 causes a venereal disease and EHV-4 causes a respiratory disease in foals. Equine herpesvirus 1 can cause respiratory infection, neonatal infection, abortion, or neurological signs. The neurologic form of EHV-1, equine herpesvirus myeloencephalopathy (EHM), attacks the spinal cord and brain and can be fatal.
Equine influenza: This highly contagious respiratory disease spreads easily through contact with infected horses or infected clothing, equipment, brushes, tack, etc. Outbreaks result in significant economic losses to the equine industry, and are more likely to occur where large groups of horses gather, such as shows, races and events. Increased transportation of horses facilitates the spread of the virus.
Equine coronavirus: Coronaviruses cause illness in many species and attack specific organ systems such as the respiratory or gastrointestinal tracts. There is currently no evidence that equine coronavirus (ECoV) is infectious to humans. Historically, ECoV was thought to occur only in foals, but it has been increasingly reported in adult horses. It is highly contagious and transmitted between horses through exposure to contaminated feces. There are no specific treatments or licensed vaccines. Most horses recover fully, but some develop severe complications.
Salmonella enterica: This bacterium causes diarrhea in humans and horses (Salmonellosis). In horses, transmission occurs through contaminated feed, water, or surfaces, or by contact with infected individuals. Mice, chickens, pigeons, and wildlife can be sources of Salmonella. Some horses are carriers that do not show any signs of disease but can actively transmit the organism. Stress plays an important role in the progression of the disease, and severe cases can be fatal.
Streptococcus equi subspecies equi: This bacterium causes the highly contagious respiratory disease strangles, which results in swollen lymph nodes that can compress the upper respiratory tract. It is the most common infectious agent identified in horses 6 to 10 years of age. Infection occurs through horse-to-horse contact, drinking contaminated water, or contact with contaminated material or equipment. Some horses become carriers that intermittently shed the bacteria, creating a reservoir of chronically infected animals that perpetuate the disease in the population.
Corynebacterium pseudotuberculosis: This bacterium can cause large abscesses on the horse’s chest resembling a pigeon’s breast, hence the name “pigeon fever.” The bacterium can survive for long periods in the ground and shorter periods in hay and shavings. It gains access to the body through small scrapes or wounds in the skin, either by contact with contaminated soil or objects, or by insects that transfer the bacteria to broken skin. Other forms of the disease include internal abscesses, most commonly seen in the liver, spleen, kidneys, and lungs, and ulcerative lymphangitis, a painful infection of the lymphatics, most often involving the hind limbs and causing swelling and oozing sores.
West Nile virus: Mosquito-borne West Nile virus (WNV) can affect horses, humans, and other species. Some infected horses do not show signs of illness, but others develop neurologic disease, which can be fatal. There is no specific treatment for WNV. Many horses recover fully, but some exhibit long-term effects, including gait and behavioral abnormalities. The American Association of Equine Practitioners recommends that horses be vaccinated for WNV annually.
Rhinitis viruses: Equine rhinitis viruses A and B (ERAV and ERBV, respectively) are highly prevalent in horses worldwide and can cause disease that is indistinguishable from other respiratory pathogens such as equine influenza virus. Although not well studied, rhinitis viruses are known to cause mild to severe respiratory disease affecting both the upper and lower airways. They may also contribute to, or exacerbate, inflammatory airway disease and recurrent airway obstruction (heaves).
Foal-specific Pathogens
Rotavirus – Equine rotavirus damages the lining of the intestines, inhibiting digestion and absorption of food. It is one of the most common causes of diarrhea in foals less than six months of age. Infection occurs when foals ingest materials or lick surfaces contaminated with infected feces. Pregnant mares can be vaccinated to provide foals with antibodies to the virus.
Cryptosporidium parvum – These highly contagious parasites infect the intestine and cause diarrhea and weight loss. Cryptosporidiosis is most commonly seen in foals one to four weeks of age, especially those that are immunocompromised or stressed. They become infected by ingesting the parasite in contaminated food or water. There are no specific treatments or available vaccines for Cryptosporidiosis.
Clostridium perfringens – This bacterium causes inflammation of the small and large intestine, resulting in severe abdominal pain, colic, diarrhea, and failure to nurse within the first week of life. The disease progresses rapidly and has a high mortality rate. Intensive medical care is required, but many affected foals do not survive. Mares may be vaccinated prior to foaling to limit foals’ susceptibility to infection.
Rhodococcus equi – This bacterium lives in the soil and causes pneumonia in foals that are one to four months old. Foals inhale airborne bacteria or ingest contaminated soil. The disease progresses slowly and can be life threatening in severe cases. Treatment includes antibiotic therapy. Early detection and treatment
result in the best outcomes. There are currently no vaccines.