Common Infectious Diseases

Diseases Involving The Respiratory System

As in humans, respiratory diseases caused by viruses are not uncommon. And similar to what occurs in human populations, these viruses can transmit readily between susceptible individuals when they are grouped together for periods of time. This happens at shows, trail rides, horse sales and in boarding barns when new horses are introduced or when horses return from shows.

Common viruses affecting the respiratory tract are influenza virus, herpesvirus and the rhinitis viruses. Equine viral arteritis virus (EVA) can also cause respiratory disease.

Equine Influenza (EI)
"Flu" in horses is very contagious and spreads rapidly among susceptible horses by coughing and nasal secretions. All horses are at risk but, similar to the situation in humans, not all become ill when exposed to the virus. Disease typically is more common in young horses although older horses with declining immunity also face an increased risk.

Fever, serous nasal discharge and coughing are common signs. Some horses will become depressed and have weaker appetites. Rest is critical to permit the respiratory tract to heal as the horse recovers. One week's rest per day of fever is the common recommendation, but one that is often not adhered to. Coughing may persist for weeks.

Good nursing care and rest are the recommended treatments. Antibiotics are not typically given unless there is concern that the horse also has a secondary bacterial infection. Nonsteroidal anti-inflammatory drugs may be prescribed to reduce a very high fever. Vaccines are available to help prevent the disease.

Click here to learn more about Equine Influenza.

Equine rhinopneumonitis
The respiratory form of "rhino" is usually caused by type 4 herpesvirus, and less commonly by type 1. Herpesviruses are common in horse populations worldwide and most horses become infected at a young age.  Infection generally is considered to be life-long however, in most horses the virus lies dormant. Under certain conditions (e.g. stress or immunosuppression), the virus may become active again and cause illness. Sick horses once again shed virus in nasal secretions. Infection does not always result in illness. Like influenza virus, bringing susceptible horses together in groups is frequently associated with outbreaks of disease.

Clinical signs are similar to horses ill with influenza as is treatment. Therefore, diagnostic testing early in the onset of the disease is needed to differentiate the two diseases.

Herpesvirus (usually type 1) can also cause abortion in pregnant mares or the birth of weak, sick foals. Type 1 viruses are also known to cause neurological disease, with affected horses showing hind-end weakness and incoordination and in some cases, paralysis. There are vaccines licensed to help prevent the respiratory and abortion forms of disease but none make the claim to prevent neurological disease.

Equine Strangles
Strangles is an infection with a bacterium called Streptococcus equi equi, also referred to as S. equi. It typically causes swelling and abscesses of lymph nodes in the head/throat area that, if large enough, can partially obstruct the horse's breathing and swallowing (hence, the name "Strangles"). Horses will also have fevers, nasal discharge and a cough. The nasal discharge and the material that drains from nodes that break open contain bacteria which contaminate the environment and infect other horses. Quarantine of sick horses combined with strict infectious disease control practices are key in breaking the cycle of this disease in an infected barn. Taking daily temperatures of horses that have been in contact with sick ones is useful because fevers typically precede nasal shedding. Horses with fevers can then quickly be quarantined.

Treatment is largely symptomatic and includes good nursing care. Antimicrobial drugs are often not considered necessary; however, your veterinarian will advise you based on the horse's clinical appearance. Getting the nodes to open, drain and then heal will hasten the horse's recovery and to this end, applying warm compresses may be recommended by your veterinarian. Some horses continue to carry the bacteria within their guttural pouches, and are thought to serve as a source of infection for later outbreaks. [The "guttural pouches" are large sac-like structures unique to the equine species within the domesticated animal population.  Their function is largely unknown although it has been hypothesized that they may be involved with cooling the blood supply to the head and brain. The pouches are located below the floor of the auditory (Eustachian) tubes with which they communicate. The auditory tubes connect the pharynx or throat area with the middle ears.]  

A modified live vaccine to help protect against Strangles is available; it is administered directly into the horse's nasal cavity.

Diseases Involving The Neurological System

In this section, we consider infections that target the horse's brain and/or spinal cord - the Central Nervous System (CNS).

Eastern Equine encephalomyelitis (EEE)
EEE is just one of the mosquito-borne diseases that infect horses. Mosquitoes carry the virus and transmit it to horses (and to humans!) during the process of biting them. Infected horses however do not transmit the disease to other horses. In susceptible horses, the infection targets the central nervous system resulting in clinical signs such as altered mentation, ataxia (staggering/incoordination), head pressing, circling, weakness, paralysis and recumbency, convulsions and death. While the infection is not fatal in all horses, the mortality rate in those animals showing clinical signs is reported to range from 50-90% (Ref: Merck Veterinary Manual, 9th ed). Euthanasia of affected animals when they become unable to stand is a consideration.

There is no specific treatment. Appropriate supportive care and good nursing care are critical but made even more difficult should the horse become recumbent.

Vaccines are available and key in preventing this disease.

West Nile Disease
Similar to EEE, WN disease is transmitted by mosquitoes to both horses and humans. Infected horses do not spread the infection to other horses. Signs can mimic other "CNS" infections. However, horses with the WN infection often exhibit muscle twitching especially involving the head and neck.

The mortality rate of horses exhibiting clinical signs is reported to be 20-40% (Ref: Merck Veterinary Manual, 9th ed). Some recovered horses may show residual signs for some time.

Nursing care and supportive therapy are important.

Vaccines are available and key in preventing this disease.

Click here to learn more about West Nile Disease.

Tetanus is caused by a bacterium called Clostridium tetani. Unfortunately, the bacteria are common in the horse's environment as they live in soil. Wounds, particularly deep wounds are common portals for the bacteria to enter the body, wherein they grow and produce the toxin or 'poison' that ultimately affects the horse's nerves and central nervous system.

Susceptibility to this infection varies among species - horses are very susceptible. Typical clinical signs include localized or generalized muscle stiffness, rigidity of some muscles, difficulties in swallowing, protrusion of the third eyelid (that membrane present at the inner corner of the eye). The horse may become recumbent. Death follows primarily due to paralysis of the muscles used in breathing.

Early diagnosis and treatment with antibiotics and tetanus antitoxin can be successful, but nursing care is often intense. It is far easier to prevent the disease with vaccination. Several vaccines are available for this purpose.