Most Common Viral Horse Diseases, Their Symptoms, And Treatment

Published on Dec 9, 2022 12:00 AM
Most Common Viral Horse Diseases, Their Symptoms, And Treatment

Over the past few decades, viral infections have become a significant threat to the equine family. Horse owners suffer greatly when their four-legged companions face health issues that are often unknown.

Equine diseases can have terrible consequences if the owners or caretakers lack a basic understanding of common horse diseases and their symptoms. In order to ensure the good health of your equine friend, you should be familiar with the viruses in horses.

Several preventable diseases in horses are viral and contagious. A parasite, host, or even the infected horse itself can transmit the disease to your healthy horse. 

So you must be familiar with the common viral disease in horses before you own one. This research-based article will help you learn about the most prevalent viral diseases in the equine family. 

Viral Horse Diseases and Symptoms

There are several viral diseases in horses that can prove fatal if not taken care of timely. Let’s discuss the most common equine diseases, with regard to some important factors, such as causes, symptoms, and treatment. 

•    Equine Influenza

Equine influenza is a respiratory illness that affects horses and other Equidae. Usually, contact with affected horses and contaminated clothes, equipment, brushes, gear, etc. facilitates the EI virus transmission.

Although some horses may not exhibit any overt symptoms of illness, they can still infect nearby susceptibles. At equine exhibitions, races, and events, outbreaks are more likely to happen. In addition, the virus can spread easily when horses move more often across the states.

Equine influenza viruses A and B are the two kinds that often circulate and cause seasonal flu epidemics in horses. 


Equine influenza affects horses' upper respiratory systems, causing symptoms comparable to those of influenza in humans:

•    Dry cough
•    Lethargy
•    Lack of appetite
•    Enlarged lymph nodes
•    Nasal discharge

Prevention and Treatment 

For the prevention of equine influenza, many vaccinations are available. Additionally, hygienic management techniques and immunization are necessary for influenza prevention

Owners can decrease viral exposure by isolating newly imported horses for two weeks. 

It has been recorded that an influenza vaccine from a recombinant canarypox virus protects against the EI. 

However, adult horses at risk should receive booster shots every six months, as the existing vaccinations' lifetime protection is limited.

•    Equine Herpesvirus (EHV) / Rhinopneumonitis

Equine herpesvirus-1 and equine herpesvirus-4 are common in the equine population worldwide and are major causes of respiratory infection. These viruses spread through indirect contact with infected miscarried fetuses, nasal secretions, placentas, or fluids from the placenta. 

Factually, the horse's susceptibility to infection depends on virus strain, the animal's immune system, and age. Usually, horses with immunological sensitization to the virus from a prior infection have a minor or undetectable illness. 


Commonly, a horse suffering from Rhinopneumonitis shows the following symptoms. 

•    Fever
•    Cough
•    Pharyngitis/throat inflammation 
•    Poor appetite
•    Nasal discharge
•    Enlarged lymph nodes.
•    Depression


Equine herpesvirus has no particular treatment. However, it is possible to reduce the likelihood of infection by providing the horse with extensive nursing care. In some situations, such as horses with fevers higher than 104°F, fever-reducing drugs can be a better option. 

Management practices are effective to adopt for the prevention of disorders associated with EHV-4 and EHV-1. Before introducing new horses to your farm, especially pregnant mares, quarantine them for three to four weeks.

•    Equine Infectious Anemia (EIA)

Equine infectious anemia, also known as "swamp fever," is a non-contagious infectious disease that is transmitted by biting insects in low-lying "swampy" environments. The viral strain's capability for replication or pathogenicity in the infected host is a major factor in its ability to cause illness.

Although cases of the disease have been reported worldwide, the numbers vary greatly between horizons. For example, there were significant epidemics in France, Japan, and America during the beginning of the 20th century.

However, in the southeast of the US, in northwest Alberta, and sometimes in other regions of North America, the illness is regarded as endemic.


•    Fever
•    Depression
•    Muscle weakness
•    Jaundice
•    Increased heart and respiration rates
•    Epistaxis
•    Hemorrhages on mucous membranes
•    Thrombocytopenia
•    Anemia


There is currently no authentic cure or treatment for EIA at all. Therefore, advocating for supportive care might not be of help in the case of equine infectious anemia. However, there are some measures to limit the severity and spread of the disease. 

•    Move the infected horse at least 200 yards away from other horses 
•    Contact your veterinarian as soon as possible
•    Limit exposure to biting insects
•    Equine Viral Arteritis (EVA)

Equine Viral Arteritis (EVA), caused by the equine arteritis virus, is a highly infectious disease that can result in miscarriage or even death in horses. Moreover, it can cause flu-like illnesses of variable severity. Horses all over the world are prone to viral infection. Majorly it is endemic in North American equines.

Depending on the virus strain, host genetic characteristics, and other exposure-related factors, EAV infection may manifest clinically or, more commonly, asymptomatically.


The symptoms of EVA may vary depending on the above-mentioned factors. However, the common symptoms of this viral disease in horses include

•    Fever
•    Limb edema and stiffness
•    Depression 
•    Appetite loss
•    Conjunctivitis and nasal lining irritation

Additionally, during these horse diseases, raised patches may form over the stallions' face, body, and scrotum. 


EVA infection does not have a particular treatment. Mostly, recovery is as quick as a "cold" and simply requires separation from the vulnerable horses. Moreover, when horses are clinically unwell, supportive care, including fluids, antibiotics, and painkillers, can help recover horses' health. Unfortunately, there is currently no cure for carrier stallions or pregnant mares.

•    West Nile Virus (WNF)

West Nile Fever is a newly discovered arthropod-borne infection that affects a wide range of animals, especially the equine family. In 2002, veterinarians in Canada identified the first complication of WNV in horses. Basically, the virus is transmitted through mosquito bites.

The number of WNV-infected horses is under 10%; however, the clinical illness has death rates close to 50%. Since reaching a peak of more than 15,000 cases in 2002, the number of West Nile viral diseases in horses has been falling in the US.


The WMF's symptoms and progression might resemble those caused by other viruses in horses. However, not all infected animals go on to acquire the disease, and some may die without exhibiting any symptoms of sickness.

The common symptoms of this viral infection in horses include 

•    Ataxia 
•    Inability to stand
•    Limb paralysis
•    Wobbly gait 
•    Muscle fasciculation
•    Teeth grinding
•    Lip droop/paralysis
•    Fever 


It is crucial to immediately speak with your veterinarian and administer supportive care because there is no particular antibiotic to combat the West Nile Virus.

Although corticosteroids and nonsteroidal anti-inflammatory medications are frequently recommended, symptoms don't always improve. Similarly, equine experts prescribe serum or plasma products containing antiviral antibodies; still, there’s no significant proof of recovery. 

The majority of horses that recover from WNF resume their regular activities within a year. However, in some cases, long-term impacts have been observed.

•    Equine Viral Encephalomyelitis

Equine viral encephalomyelitis, a contagious disease in equine that spreads through mosquitoes, is clinically characterized by paralysis and some symptoms of neural dysfunction. Climate and agricultural techniques that promote the proliferation of mosquitoes have a significant impact on the disease's spread.

This viral disease in horses includes different strains, such as Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE), and Epidemic Venezuelan equine encephalomyelitis (EVEE).

Some parts of North, Central, and South America are home to these strains. However, another notable factor is that the EEE has never broken out in the United Kingdom.


Clinical symptoms may vary depending on the specific type of virus strain. In some cases, the infected horse does not show any sign of the disease. However, the common symptoms at the onset of EVE are as follows.

•    Depression
•    Lack of appetite
•    Loss of weight and condition
•    Fever for an extended period 
•    Stiffness and weakness,

However, the late-stage symptoms of the infection are different, such as

•    Behavioral changes 
•    Blindness
•    Inability to move
•    Wandering
•    Seizures
•    Hyperexcitability


Equine viral encephalomyelitis, especially EEE, has no known treatment. Nevertheless, horses with clinical symptoms often get supportive treatment, such as intravenous (IV) fluids and anti-inflammatory drugs.

The EEE vaccination is a basic vaccine that the American Association of Equine Practitioners (AAEP) suggests. For optimum results, it’s necessary to maintain an annual vaccine schedule for the horses. When outbreaks take place, injecting boosters can be an effective form of prevention. 

To prevent your horse from getting EEE, you must follow appropriate equine vector management procedures in addition to immunization.
Furthermore, at night, when mosquitoes are most active, use insect repellents and confine horses indoors.

•    Equine Rotavirus Infection 

Rotavirus is one of the most frequent causes of diarrhea in foals younger than six months. It causes damage to the intestinal lining of the baby horse, which prevents food from proper digestion.

However, the severity of the infection depends on the foal's immunological condition and the viral dosage, along with some other factors. The most severe symptoms of the infection affect horses under two months.

Factually, rotavirus affects up to 50% of foals. Infection in foals occurs when they consume feces that contain the Rotavirus. Although adult horses are mostly asymptomatic, they can get and spread this viral disease.


The Rotavirus-infected foals show the following symptoms. 

•    Diarrhea
•    Lethargy
•    Reluctance veterinarians 
•    Distended abdomen
•    Dehydration


Equine rotavirus infection is curable with supportive measures, such as GI safeguards, intravenous (IV) fluids, and probiotics.

In pregnant mares, immunization can boost the foals' viral antibodies and prevent infection.

In most cases, it is wise to keep sick foals apart from normal ones, as the Rotavirus is highly infectious. Also, avoid spreading contaminated horse dung on pastures since the virus might linger in the environment and infect healthy horses.

•    Equine Morbillivirus Pneumonia

The Equine Morbillivirus Pneumonia (Hendra virus infection) has a Level 4 bio-safety designation, which means that there is a high possibility that it may infect humans in addition to horses. If it does so, there will be a life-threatening condition for those who contract it. The first onset of equine morbillivirus pneumonia was reported in Australia in 1994. 

Certain types of fruit bats carry the Hendra virus. Mostly, oral or nasal exposure results in infection. The disease begins as a feverish infection that develops into an acute respiratory illness or a serious neurological condition.

However, the Hendra virus doesn't seem to be particularly infectious. According to research from the field and lab, direct contact with secretions,  tissues, or urine infected by a virus causes this equine infection.


Usually, the symptoms of Equine Morbillivirus Pneumonia appear suddenly and demand prompt medical intervention. So, make an emergency appointment with your veterinarian when you observe any of these signs in your four-legged friend.

•    Sudden high fever
•    Abnormal muscle twitching
•    Labored breathing
•    Extreme discomfort
•    Sudden blindness
•    Rapid death
•    Bloody nasal discharge
•    Weakness
•    Loss of balance
•    Head tilting


Infection with the Hendra virus is not treatable with antivirals. Most horses that show clinical symptoms suddenly die from this infection. Therefore, treatment is primarily supportive for equines with modest clinical indications.

A vaccine that contains a noninfectious protein component of the Hendra virus was released in November 2012 and is only accessible via licensed veterinarians. Healthy horses can start receiving vaccinations at four months in two doses separated by 21 days, followed by boosters every 12 months.

Prevention of Equine Morbillivirus Pneumonia focuses on avoiding contact with fruit bats' bodily fluids. Burying infected samples deeply, isolating and limiting animal movement are the key methods of controlling Hendra viruses in horses.

•    Rabies in Horses 

The lyssavirus causes rabies and affects a horse's nervous system and salivary glands. A horse that has been exposed to the virus through a bite to the face may show symptoms earlier than a horse that has been exposed through a bite to its limbs. The reason is that the virus travels up the nerves near the exposure site and to the brain.

According to the Centers for Disease Control and Prevention(CDC), the number of rabies cases in horses reported each year in the US ranges from 42 to 82.

The research proclaimed that 20 percent of rabies cases in 2014 were found in Oklahoma, while forty-four percent were found in Texas. 


A range of symptoms can appear in horses with rabies. In some cases, the disease advances gradually, even if the early signs are not apparent. Your rabid horse may exhibit the following symptoms.

•    Depression
•    Aggression
•    Sudden death
•    Abrupt changes in behavior
•    Lack of interest in eating
•    Painful erection without sexual interest
•    Lack of muscle coordination
•    Paralysis
•    Itchiness 
•    Recurrent twitching
•    Trouble urinating 
•    Circling and head pressing
•    Muscle tremors 

Usually, a horse dies within five to seven days after exhibiting symptoms. 


Horses with clinical rabies (vaccinated or not) cannot be treated effectively. However, an immediate booster dose can prove effective for previously inoculated horses. 

To stop human exposure to the virus, horses with clinical rabies symptoms should be quarantined. In addition, only qualified personnel who have had the necessary rabies vaccinations should handle animals suspected of having the disease. Call your veterinarian immediately if you believe your horse may have rabies.

To Conclude

From the above discussion regarding horse diseases, especially the highlights on viruses in horses, we can say that prevention is better than cure. The reason is that most Viral Diseases in Horses have no specific treatment. 

Therefore, as a horse owner, you must take strict preventive measures against these deadly equine diseases. Moreover, contacting your veterinarian with immediate effect when you notice symptoms of an illness in your horse can prove helpful in saving your animal.

For such research-based articles on common diseases in horses and other animals, VetandTech is one of the best sources. We have created a separate section where you can find authentic material about diseases in pets and large animals


What are the common viruses in horses?

The most common horse viruses include equine influenza, equine herpesvirus, west Nile virus, rotavirus, equine morbillivirus, Hendra virus, and lyssavirus, causing rabies.

What are the strains of equine viral encephalomyelitis?

The three strains of equine viral encephalomyelitis include Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE), and Epidemic Venezuelan equine encephalomyelitis (EVEE).

What is the best practice to limit horse viral diseases?

Following strict preventive measures and contacting your vet in emergency cases are the most effective ways to limit viral diseases in horses.