Hendra Vaccine Allergy in Horses

Hendra Vaccine Allergy in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

Most common symptoms

Anemia / Diarrhea / Difficulty Walking / Lethargy / Out of Breath / Poor Appetite


Rated as moderate conditon

1 Veterinary Answers

Most common symptoms

Anemia / Diarrhea / Difficulty Walking / Lethargy / Out of Breath / Poor Appetite

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Hendra Vaccine Allergy in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

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What is Hendra Vaccine Allergy?

The Hendra vaccine is given to horses to prevent a deadly disease called the Hendra virus that has killed close to 100 horses since it was recognized in 1994. This condition starts out as a respiratory virus and rapidly spreads to the central nervous system, eventually causing death. Not only is it deadly to horses, it can also be spread to humans, and has caused the death of four people out of the seven who were confirmed to have the disease. That is a 58% death rate in humans and approximately 75% death rate in horses. The Hendra vaccine allergy may be acute (starts right after first injection) or chronic (starts after booster and continues to get worse).

The Hendra virus disease is a serious and deadly illness that has killed many horses in Australia. A vaccination to help prevent this condition was created in 2012. However, it seems that this vaccine is being thought to cause some serious (and lethal) side effects as well. Many horse owners have claimed that their horses have had reactions such as severe neck pain, golf ball sized lumps, high body temperature, and inability to move the neck. If your horse has been vaccinated for the Hendra vaccine and is having any of the symptoms below, you should call a veterinary professional immediately.

Symptoms of Hendra Vaccine Allergy in Horses

There is a long list of symptoms that have been reported in horses after receiving the Hendra vaccination. Some of the most common are:

  • Swelling and pain at injection site
  • Oedema
  • Lethargy
  • Loss of appetite leading to weight loss
  • Abdominal pain
  • Stiffness of muscles
  • Lack of energy
  • Welts, rash, or lesions at or near injection site
  • Lameness
  • Changes in behavior
  • Inability to get up (recumbency)
  • Disorientation
  • Difficulty walking
  • Panting
  • Rapid heart rate
  • Tremor
  • Breathing trouble
  • Pale gums
  • Inability to move
  • Constipation
  • Diarrhea
  • Incoordination
  • Weakness
  • Death


  • Acute Hendra vaccine allergy is a severe reaction that occurs shortly after the first dose of the Hendra virus vaccine
  • Chronic Hendra vaccine allergy is a serious condition caused by a booster of the Hendra virus vaccine and continues to get worse over time

Causes of Hendra Vaccine Allergy in Horses

The cause is believed to be an allergic reaction to the Hendra vaccine, but this is still uncertain. Reactions may occur such as severe neck pain, panting, high body temperature, ataxia, and inability to move the neck, among many others. The exact cause of the adverse or allergic reaction has not been determined.

Diagnosis of Hendra Vaccine Allergy in Horses

Diagnosing a Hendra vaccine allergy is very difficult and your veterinarian will most likely rule out other disorders until the only cause left is the Hendra virus vaccine. Because the symptoms are so varied and the condition is so newly recognized and uncertain as to the reason for the reaction, the veterinarian will need to do a great deal of tests to narrow down what the cause may be. There are many conditions that share the most common symptoms of Hendra vaccine allergy, which include fever, neck pain, lump in the neck, appetite loss, confusion, and lack of energy. A complete and thorough physical assessment will be conducted which will likely include an overall body condition score, vital signs, and behavior. The veterinarian will need to know your horse’s complete medical history and immunization records. Please be sure to mention any medications your horse has been given. A lameness examination is needed as well, which means you will need to walk and trot your horse while the veterinarian observes your horse’s movements and attitude.

In addition, a host of blood tests will be run, which will probably include blood cultures, a biochemistry profile, complete blood count, blood urea nitrogen (BUN), and whatever else your veterinarian believes may help reaffirm the diagnosis. Also, radiographs (x-rays) are helpful to check for abnormalities of the liver, heart, stomach, and kidneys. Additionally, your veterinarian may do other specialized tests such as an endoscopy, colonoscopy, body scan, ultrasound, MRI, and CT scans.


Treatment of Hendra Vaccine Allergy in Horses

Treatments for a Hendra vaccine allergy are supportive in nature as there is no cure for this condition. Medications such as antihistamines, NSAIDS, and steroids are usually given to help with inflammation and fever. Also, fluid and oxygen therapy may be used if needed.


The medications that have been used by veterinarians to help treat horses suffering from a Hendra vaccine allergy vary since this condition is still relatively new and unconfirmed in many cases. However, some of the common drugs include NSAIDS or corticosteroids for inflammation, fever, and pain, antihistamines to counteract the allergy symptoms, and stomach protectants for colic.

Oxygen and Fluid Therapy

If your horse is having difficulty breathing, an oxygen cannula or mask will be used to help with breathing. Also, intravenous (IV) fluids will be provided to improve circulation and prevent dehydration.

Recovery of Hendra Vaccine Allergy in Horses

Because of the lack of information, recovery is guarded, but the mortality rate is less than 0.8% so chances are good that your horse will recover. However, some horses are reported to be unable to perform after this condition and some have been unable to continue with a normal lifestyle and were euthanized. Be sure to talk to your veterinarian about the side effects of the Hendra vaccine and know the pros and cons presented for each individual equine before vaccinating your horse.

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Hendra Vaccine Allergy Questions and Advice from Veterinary Professionals




Australian Stock


12 Years


Moderate condition


-1 found helpful


Moderate condition

Has Symptoms

Lethargic, Stiff, Fever,
Lethargic, Stiff, Fever

I've had my 12 year old gelding for approximately 4 months. He had been vaccinated for hendra before but 3 years had passed. 4 weeks ago he had his vaccination and he had no obvious side effects. 3 days ago he had his second shot and it's a whole different story. He can barely move his head due to the soreness in his neck, he is lethargic and has just about lost his appetite. More worrying is that tonight I noticed he is quite warm and sweaty in his hind quarters, along his back and under his tail.

Sept. 1, 2018

Buck's Owner