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Common Equine Diseases

We often hear about outbreaks of equine diseases. Since Covid-19, we seem to be even more in tune about what it means when you hear the word "outbreak". Today we want to talk about some of the different types of equine diseases that we might see here in Canada, along with what the signs and symptoms could be; and what kind of preventative measures you can take to limit your horses risk of exposure.


West Nile is a mosquito born virus that can cause inflammation or swelling of the brain and spinal cord. WNV has been around for quite some time and thankfully, there is no evidence that it can be spread through horse-to-horse or horse-to-human contact.

Most horses that are bitten by a mosquito carrying WNV will not even develop a clinical disease. They most often will develop an asymptomatic infection that will eliminate the virus and your horse will be none the worse for it. However, if a horse does develop clinical signs, there is no specific treatment for them, and just over 1/3 of horses who show clinical signs may die or be euthanized because of complications. Horses that do recover may still exhibit neurological symptoms.

Signs and Symptoms of West Nile Virus include;


Weakness of Limbs

Inability to Swallow

Loss of Appetite


Muscle Twitching

Inability to Stand


Lack of Coordination



Because WNV is transmitted by mosquitoes, insect control is extremely important. The species of mosquito that carries WNV breeds in small, warm, still puddles of water.

There are vaccinations that can protect horses against infection, and because here in Canada, we have lots of "wet" seasons, we highly recommend WNV as one of your core annual vaccinations.

Eastern Equine Encephalitis (EEE)

This is also known as sleeping sickness, and is a viral disease that causes inflammation of the brain and spinal cord. Wild birds are a natural reservoir for the EEE virus, and mosquitoes who feed on these birds can then transmit this disease to other mammals, including both humans and horses. This disease has an extremely high mortality rate in horses and humans, however it cannot spread from horse to horse or horse to human (nor can it spread human to human).

EEE can be diagnosed through bloodwork however there is no cure for EEE. Treatment mainly consists of supportive care for horses with clinical signs, including IV fluids and anti inflammatory medications. You mainly see EEE cases from late spring through to early fall with the peak season being the summer months.

If a horse contracts EEE, the prognosis is extremely poor, as up to 90% of horses who get EEE do die from the disease. Horses with neurological signs that make them unable to stand will most often be euthanized, and horses that do survive may often have permanent brain damage.

Clinical signs of EEE begin fairly abruptly. These signs include:










Decreased Awareness in Surroundings


We highly recommend EEE as a core annual vaccine that your horse receives. Besides the vaccine, it is important that you practice proactive maintenance to protect your horse from EEE, which includes insect repellent, and eliminating mosquito breeding grounds such as standing water, brush piles and old tires. Remember to keep your water troughs clean as well.


Also known as lockjaw, this is an often fatal disease cause by anaerobic bacteria that grows in low oxygen conditions. The spores of CI. tetani are present in the soil and can contaminate puncture wounds, open lacerations, surgical incisions and the umbilici of foals. Once it gains access to the body, it produces a powerful neurotoxin that blocks neurotransmission, resulting in unopposed muscle contractions and spams. Horses will often have a "sawhorse" posture if they have tetanus. The incubation period for tetanus is approximately 8 days but can go up to at least 21 days. Spores can also remain dormant within the pores of a horse and begin to grow when a physical trauma occurs, making tissue oxygen level low.

This is not a contagious disease. The number of horses annually affected by tetanus is low, however you should ensure that your horse is vaccinated annually and we consider this a core vaccine for your horse. Humans working with horses should also ensure that they are up to date on their own tetanus shots. Proper immunization and wound management should prevent tetanus from ever occurring.


Rabies is a neurological disease that causes inflammation of the brain. This is a disease that can affect all mammals, including humans. Although it rarely occurs in horses, it is inevitably fatal and there is no cure.

Rabies is transmitted through saliva from the bite of an infected (rabid) animal. The virus will travel from the location of the bite to the brain via the nerves. There are high instances of rabies in animals such as raccoons, skunks, foxes and bats. Because of their curious nature, horses are known to investigate something interesting (such as an animal behaving strangely) and can put themselves at risk of being bitten by a rabid animal.

Rabies is preventable by annual vaccination and we highly recommend it as a core vaccine. You can also minimize risk of potential contact with a rabid animal by avoiding attracting, approaching, and/or handling wild animals. It is also highly recommend that all other pets and livestock are vaccinated against rabies.

If you suspect that an animal or human has come into contact with rabies, thoroughly wash the wound area immediately and remove any potentially contaminated equipment or clothing.

Equine Influenza Virus (EIV)

EIV is a highly contagious disease and is one of the most common equine infectious respiratory diseases. The range of time between exposure and when symptoms arise is between 24 hours to 5 days. It is most commonly transmitted from horse to horse contact in respiratory secretions (droplets from coughing and nasal discharge). It can also be transmitted via contaminated equipment or other objects, or from humans on hands or clothing. The virus can live for up to 48 hours on hard surfaces and in the environment, and up to 3 days in water. Viral shedding typically lasts 7-10 days although this period may be shorter in horses that have been vaccinated.

Symptoms of EIV include:


Behavioural Changes


Muscle Stiffness

Lack of Appetite

Nasal Discharge


EIV will typically resolve on its own without treatment other than symptom management. Fatalities from EIV are rare. A horse who shows symptoms of EIV should be isolated immediately even before a confirmed diagnosis, for safety. Biosecurity measures should be followed if a horse shows symptoms of EIV. These include following isolation protocols for 21 days past the last case of EIV.

Vaccination is the most effective way to reduce risk of a horse developing EIV, however it does not prevent EIV. An annual vaccination is recommended if you do a lot of off property riding.

Equine HerpesViruses 1 & 4 (EHV)

These are highly contagious pathogens responsible for a variety of symptoms in horses, including mild to moderate respiratory disease, fever, poor appetite and depression. More serious manifestations include neurological symptoms and spontaneous abortion in pregnant mares (EHV-1). The infectious agents of the disease are more commonly called "Rhino"; true equine rhinovirus which is closely related to human rhinovirus that causes the common cold, inflects a similar yet less progressive upper respiratory disease than EHV.

Horses contracting Rhino will usually develop a transparent or milky nasal discharge as the disease establishes itself 2 to 10 days post exposure. A horse will have an increased temperature (>102F) and will sometimes have a cough. Lymph nodes beneath and at the back of the jaw will become enlarged and interest in food will decrease. Infections that include EHV may then become more complicated.

Neurological symptoms caused by EHV-1 can include mild to significant hind limb ataxis, or incoordination, and may come with urinary or fecal retention. Horses infected with this manifestation of the disease may begin to sit on their haunches and become unable to stand. Secondary respiratory bacteria infections are common and can further complicate the disease, which can increase recovery time or decrease the overall prognosis.

This disease is easily spread by direct contact with infected horses, inhaling aerosolized respiratory secretions and using contaminated equipment, feed or water. Immunity after exposure only lasts a few months at best, therefore thorough disinfection and quarantine procedures are critical in stopping the disease from spreading. The virus can survive without a host for up to 14 days in the right environment. This virus may also lie dormant in a horse and only cause the illness after a period of stress or coincidental disease; which is called a latent infection.

Currently there is no drug known to treat EHV 1 or 4 infections. Mainly it is supportive treatments to help alleviate symptoms and reduce stress on a horses immune system. NSAIDs may be prescribed to help reduce fever and respiratory tract swelling.

There are vaccinations for EHV 1 and 4 available, unfortunately they offer only short immunity from the abortive form (2-3 months), respiratory form (6 months), and no protection from the neurological form of the disease. It is highly recommended that horses in group settings where a case of EHV may be present, get vaccinated to try to slow the spread of the virus.

Thorough sanitation procedures of stalls and equipment, and quarantine of EHV infected or suspected horses is the best method of prevention for the spread of EHV. Any food, water, bedding, etc. exposed to an infected horse should be disposed of, and all contaminated surfaces be cleaned with disinfectant and rinsed with clean water.

Potomac (PHV)

This is an acute enterocolitis syndrome that produces mild colic, fever and diarrhea in horses of all ages and can cause abortion in pregnant mares. Horses with PHV will develop gastrointestinal symptoms, and in some serious cases, laminitis. This is serious and potentially fatal disease that is seasonal and occurs between late spring and early fall in areas. If PHV has ever been confirmed on a farm or particular geographic area, then it is likely more cases will occur in future years.

Horses will only get PHV if there are infected snails and insects where they are pastured. If one horse develops PHV, it means other horses are at risk of contracting it however it is not spread through horse-to-horse contact. It occurs when horses ingest a specific bacterium that lives in flatworms known as "flukes". These flukes then infect other aquatic insects. Horses can become infected by drinking out of water sources infested with these aquatic insects, or by eating infected insects in their feed or pasture. PHV is fatal in up to 25% of horses who contract the disease and more will be euthanized due to laminitis, which is a severe complication of PHV. The earlier a horse is treated, the greater its chance of survival.

Symptoms of PHV include:


Decreased appetite


Stocking up





Adoption in pregnant mares

PHV is typically treated with antibiotics and often horses are administered IV fluids or electrolytes to treat dehydration, as well as non-steroidal anti-inflammatories to help with pain relief. Vaccination should be timed to precede the anticipated peak challenge during the summer or fall months. Vaccines may not be fully protective against infection in an area where PHV commonly occurs.


This is a highly contagious disease that is named for the strangled breathing sounds that an ill horse makes due to extreme nasal discharge and the swellings that form in the head and neck region.

Horses fully recover from strangles in two to four weeks. Immunity against re infection is variable, however in some equines it can last for years. Not all horses develop a protective immunity upon recovery. Some horses, although appear healthy, also shed the bacteria in nasal secretions for a prolonged period of time and can infect nearby horses. Strangles can be fatal for horses due to asphyxiation or "strangling", as well as other complications.

This disease can spread rapidly from horse to horse and is one of the most common bacterial infections for horses. It is spread through nasal discharge and pus from draining abscesses. It can spread through contaminated clothing, hands or equipment, and can also contaminate water or food sources.

Clinical signs of strangles include:

Mild Lethargy

Reduced feed intake

Slight cough

Nasal Discharge


A few days after the onset of fever, the lymph nodes will swell and form abscesses around the throat, as well as some cases under or around the base of the ear. The nasal discharge will begin to change from clear to cloudy and whitish. Once the abscesses have ruptured and drained into the nasal passages, then the discharge usually becomes thick white or yellow. Horses often will put their heads down low to alleviate throat and lymph node pain. It is important to remember that not all cases develop the class abscess formation. Some horses may just have a fever and be off feed for a few days. Without complications, recovery will begin after the abscesses drain and infection starts to be resolved by the body.

There are several types of vaccines to help protect horses from strangles however they do not fully protect every horse from infection. Be sure to make the decisions with your veterinarian based on risk factor.

As with all health related information, ensure that you discuss your vaccination procedures and schedule with your local veterinarian to best protect and limit your horses risk to disease. Preventative measures and cleaning protocols help to alleviate risk.

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