Response to equine vaccination or a real disease? Can your veterinarian tell?

We've addressed how important it is to have your horse vaccinated, on several previous occasions. Particularly the importance of preventive care when dealing with newly emerging or mosquito borne disease. Available only through your veterinarian, vaccination is the only way to protect your horse against certain diseases. We understand that inoculation causes the horse's immune system to mount a response that, when tested, can look like natural infection. So, this presents a fascination question, 

When dealing with a dangerous mosquito-borne diseases, such as Eastern equine encephalomyelitis (EEE) or West Nile virus (WNV), how do you tell the difference between a horse that’s been vaccinated and one that is genuinely infected?

How Do Veterinarians Distinguish Between the Two?


Does a vaccine give a false positive for infection in horses?

Consider this example: a veterinarian vaccinates a horse. Shortly thereafter, the same horse comes down with the disease it has been vaccinated against or becomes ataxic  (un-coordinated.) The horse then tests positive for the disease. So, did the vaccine cause the positive titer (a way of expressing concentration levels) or is it genuinely an infection?


A study was carried out by a research team at Boehringer-Ingelheim Vetmedica (BI), to find out if there was a simple way to differentiate between the two. The team studied a killed-antigen vaccine (BI’s Vetera Gold,) designed to protect against EEE/WNV.  At present, both these viruses are endemic in the United States. They can result in a range of debilitating symptoms including ataxia and acute death, especially in the case of EEE.  First signs of infection can occur as little as only seven to 14 days or four to 10 days, respectively, after exposure to the virus.

The team specifically looked at two antibodies: immunoglobulin IgG and IgM, both produced by specialized white blood cells. 

So, Was it The Disease or The Vaccination? 

A positive diagnosis of WNV and EEE encephalitis is made based on both report of clinical signs and a serum capture IgM titer of >1:400. This means the concentration of specific antibodies found in a blood serum sample.

Vaccination against WNV and EEE may trigger an IgM antibody response that could falsely increase IgM in the blood. This would then be interpreted as a positive for disease on the capture ELISA (Enzyme-linked immunosorbent assay is a common laboratory technique which is used to measure the concentration of an analyte (usually antibodies or antigens) in solution,) diagnostic test, especially if horses and foals are tested after vaccination. 

The study included six adult horses (8-25 years old) and six 4-month-old pony foals who received a vaccine. The dams of the foals were not vaccinated during the trial but had been vaccinated annually and the pony foals had never been vaccinated. Blood samples were collected prior to vaccination, (Day 0) and on Days 7, 14, and 21 after the first vaccination. The foals were then vaccinated with a booster on Day 21. Blood samples were repeatedly drawn from both foals and dams per the parameters of the blood sampling regimen. The mares served as sentinels to help them detect any natural infection, since they were exposed to mosquitos that might transmit WNV and EEE.

Blood tests included a test used to diagnose active infection and a test to determine the protective antibody, (IgG,) response to the vaccine.

Testing results indicated the following: the IgM titers for WNV and EEE titers were <1:400 and IgG titers increased in all horses (remember a figure >1:400 indicates natural infection.) This proved that the IgM titers were not high enough to be confused with the diagnostic test for natural infection for either virus. Therefore, the horses were found to have experienced a healthy antibody response to the vaccine, which is likely protective.

All the pony foals developed IgM titers for WNV and EEE that were <1:400 and IgG titers, except for one foal, which did not develop measurable IgG titers after either vaccination. Thus, we can see that vaccination does not interfere with the diagnostic test for these viral infections. It seems most foals develop a good antibody response to vaccination but, in rare cases, some foals do not develop protective antibodies to these viruses and should be given a third booster.

As a control, the foals dams, who were not vaccinated during the trial, were tested. Their results did not show IgM titers >1:400, so was proven they were not become exposed to disease during the trial.


It can be concluded that vaccination against WNV and EEE in adult horses and pony foals does not result in a false positive diagnosis of acute disease. 

*Images courtesy of Dollar Photo Club