Flu Vaccine Allergy in Horses

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

What is Flu Vaccine Allergy?

The equine influenza virus is one of the most common causes of respiratory disease in horses and can spread quite quickly from animal to animal. Although the vaccine is not required for all horses, it is recommended for any horse that has contact with other equines. Horses can develop allergies to vaccines after either just a few exposures to the medication or after many years of successful vaccinations and changing the manufacturer of the vaccine may change the outcome. Reactions to vaccine generally result in anxiety, rapid breathing, and fever as well as hives for horses that are allergic and on rare occasions, dangerous anaphylactic responses may be triggered which can be fatal without prompt medical treatment.

Vaccination against the equine influenza virus is recommended for most horses. In rare cases, equines can develop allergies to the compounds in the vaccination itself.

Symptoms of Flu Vaccine Allergy in Horses

Horses that experience an allergic reaction to vaccination may present initially with rapid breathing, anxiety, and sweating or a low-grade fever. Hives may also show up several hours after the vaccination, either at the site of injection or on the side of the neck, and then move across the throat and shoulders. Early welts, which may or may not itch, may start as small as half an inch in diameter, however, they frequently increase in size and often converge as the reaction progresses. 

  • Hives
  • Rapid breathing
  • Sweating
  • Low temperature elevation
  • Anxiety


There are three types of flu vaccines available to equines. These include:

  • Killed Virus Vaccine - In this kind of vaccine, the virus is killed chemically before injection. This version of the vaccine can utilize whole viruses or just the extracted DNA or RNA from the influenza virus
  • Live Modified Vaccine - The viruses used for the live vaccine have been modified to decrease the virulence using a process known as attenuation
  • Recombinant Vaccine - With this type of vaccine, specific genes from the influenza virus are inserted into another virus that does not cause diseases in horses, generally a canary pox virus in the case of vaccines against the equine influenza virus


Causes of Flu Vaccine Allergy in Horses

Mast cells are the specialized cells of the immune system that are tasked with the role of protecting the body from dangerous pathogens. When the horse’s immune system is stimulated by a specific protein, it may manufacture its own protein triggering mast cells to release histamine. Histamine is a naturally occurring compound that has an inflammatory effect on the tissues it comes into contact with. In rare cases, allergies that are related to vaccines may induce an anaphylactic reaction, in which the horse experiences respiratory distress due to swelling of the lung tissues and the blood pressure drops dramatically. This type of response occurs rapidly and is frequently fatal not treated quickly.



Diagnosis of Flu Vaccine Allergy in Horses

If your horse experiences an anaphylactic reaction shortly after exposure to the vaccine the timing will help your veterinarian make a preliminary assumption that the allergy is likely to be related to the vaccine. Signs of milder reactions may be delayed by several hours, making it more challenging to determine the origin of the symptoms. If standard blood tests are run, they may expose increased levels of eosinophils, a specialized type of white blood cell that may indicate an allergic reaction, although eosinophils may also be elevated in horses that are hosting parasites in their gastrointestinal system. Both intradermal testing and serum testing are available to help define an individual horse’s sensitivities. A sample of blood extracted from the animal is used to test against allergens in serum testing.

Intradermal testing is a more reliable but more invasive procedure that involves sedating the patient and shaving a patch on their necks, then introducing a minuscule amount of common or suspected allergens under their skin. These injections are placed in a grid pattern in order to quickly ascertain which of the specific allergens are triggers. The injections will then be evaluated at several intervals, starting at fifteen minutes from injection until twenty-four hours after being injected.



Treatment of Flu Vaccine Allergy in Horses

If the horse is showing signs of distress supportive treatments are likely to be started without delay. In some instances, equines experience an anaphylactic response to vaccines and breathing becomes impaired due to the swelling of the membranes in the lungs. When anaphylactic reactions do occur, the use of epinephrine injected directly into the animal’s jugular will be required to allow the horse to breathe. With any moderate to severe reaction, an IV line may be set up as well to provide the large volumes of water that are needed to adequately support the large animal’s circulatory system.

Cold water is frequently used as an effective method to reduce allergy-related fever and inflammation until your veterinarian has been able to examine the animal to ascertain an appropriate treatment plan. This treatment plan may include corticosteroids and antihistamines to help reduce the general symptoms of allergy, and anti-inflammatory medication will often be recommended to reduce fever, swelling, and pain.



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Recovery of Flu Vaccine Allergy in Horses

The equine influenza vaccine is considered a risks based vaccine, which means that it not a core required vaccine. That being said, influenza is one of the most common infectious respiratory diseases in horses and can be spread rather rapidly. The American Association of Equine Practitioners (AAEP) generally recommends this vaccine for all horses except those that are kept in closed herds with no contact with other horses. Animals with only occasional contact with others of their species may only require booster shots on an annual basis whereas horses with more contact should have their immunity reinstated every six months.



Flu Vaccine Allergy Questions and Advice from Veterinary Professionals






5 Years


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Has Symptoms

My horse got flu cabin afternoon come evening unwellit went on for 3wks not eating really unwell vet was called many times gave him booster blood tests went on for 3 wks till got vet horse was so week she came and that was it she put him to sleep she did say most likely flu jab killed him liver was damaged lungs were full of fluid he was only 5 fit and healthy

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