A recent study in the Veterinary Record (Pusterla et al 2011) described a surveillance study conducted across the US, focusing on equine herpesvirus types type 1 and 4, equine influenza and Streptococcus equi (strangles).
The study 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 collected from 761 horses, mules and donkeys from 95 veterinary clinics in 23 states between March 2008 and February 2010. Samples were tested using PCR, a molecular diagnostic test to detect bacterial or viral DNA/RNA.
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 2 different pathogens while one had 3 (I'd like to know 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 olds.
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 less than 70% of cases were diagnosed. Whether that indicates weaknesses in current tests (e.g. intermittent shedding of the pathogens by sick horses or tests that don't detect all horses that are shedding), or poor sample collection or the presence of other important but unknown/un-investigated causes of disease is unclear.