Equine Respiratory Surveillance Research Study

I found this study interesting, especially since BVEH helped collect some of the samples tested. Despite the common belief that older horse's do not get strangles, it was the most common pathogen in the 6-10 year old group. Only 26% of cases submitted identified a pathogen, but this number can be improved by handling of the sample and testing horses early in the course of disease.

Equine Respiratory Surveillance Research Study:

Upper respiratory tract infection is a common and important problem in horses. While typically relatively mild, it can result in lost training or competition days. Outbreaks can occur and cause widespread problems. Occasionally, serious disease or complications like secondary infections can develop. Equine herpesvirus (EHV) type 1 can also cause outbreaks of neurological disease or abortion, but is primarily transmitted via respiratory secretions and the respiratory tract. So, it's an area that clearly warrants attention.

A recent study in the Veterinary Record (Pusterla et al 2011) described a surveillance study conducted across the US, focusing on equine herpesvirus types 1 and 4, equine influenza and Streptococcus equi (strangles). They recruited veterinarians to submit blood and nasal swab samples from horses with clinical signs of upper respiratory tract infection or acute onset of neurological disease (EHV-1). Samples were tested using PCR, a molecular diagnostic test to detect bacterial or viral DNA/RNA.

Samples were collected from 761 horses, mules and donkeys from 95 veterinary clinics in 23 states between March 2008 and February 2010. One or more of the target pathogens was detected in 26% of animals.

• EHV-4 was most common, being identified in 10.8% of horses.


• Equine influenza virus was next at 7.9%.


• Streptococcus equi was detected in 6.4%.


• EHV-1 was found in 3.0%.


• 15 horses harboured two different pathogens while one had three different pathogens (I'd like to know about the infection control program of that farm...).


• Horses aged 1-5 years were most commonly diagnosed with the viral pathogens, while S. equi was most common in 6-10 year old horses.

Some other interesting findings:

• EHV-4 was most commonly found during fall and winter months. 32/52 horses whose vaccine status was known had been previously vaccinated against EHV-4 (specific timing of vaccination was not reported).


• Equine influenza was mainly found in winter and spring months. 9/19 affected horses whose vaccine status was known had been vaccinated against equine influenza.


• Streptococcus equi was most common in the winter and spring. 12/24 horses with known vaccine status had been previously vaccinated against strangles.


• EHV-1 was less common in the summer than other months. Most affected horses (10/12) whose vaccine status was known had been vaccinated.


• Standardbreds, mules and donkeys were over-represented in the EHV-1 infected group.


• Quarter Horses and American Paint Horses were overrepresented in the equine influenza group and, along with ponies, in the S. equi group.

This study provides some interesting information about causes of upper respiratory tract infections in horses, and risk factors for different pathogens. An important thing to consider, however, is that >70% of cases were not diagnosed. Whether that indicates weaknesses in current tests (e.g. intermittent shedding of the pathogens by sick horses, tests that don't detect all horses that are shedding), poor sample collection or preservation during transport, or the presence of other important but unknown/un-investigated causes of disease, is unclear.