Page 55 - July 2017

Pigeon Fever research
Spier has been treating horses with this disease and performing research for more than 30 years to find out more about it and how it
is transmitted. Most recently, she participated in a study to try to determine how prevalent this disease might be across the U.S. “Several researchers from University of California Davis, Texas A&M, and the University of Kentucky collaborated on a study that was published in the Journal of the American Veterinary Medical Association. This paper documented increasing incidence of pigeon fever in the U.S. over a 10-year period (January 2003 through December 2012),” says Spier.
UC Davis School of Veterinary Medicine Department of Medicine and Epidemiology Professor
Sharon Spier, DVM, PhD., Diplomate ACVIM
To get an idea of frequency and prevalence, the researchers invited 44 state veterinary diagnostic laboratories throughout the U.S.
to participate in the study to provide data on the number of positive cultures identified each year, the geographic locations from which the positive samples were submitted, the year of submission, the breed and age of horses, and whether the clinical manifestation was inter- nal infection, external abscess, or ulcerative lymphangitis.
Of the 44 laboratories invited to participate, 15 state veterinary diagnostic labs provided 2,237 positive samples from 23 different states. The labs that participated were in California, Colorado, Iowa, Kansas, Kentucky, Montana, Nebraska, Oregon, Pennsylvania, South Dakota, Texas, Wisconsin, and Wyoming.
The only labs that did not have any cases
to report were both in Pennsylvania. Four states had more than 100 cases. Texas had 1,559 cases, Colorado had 211, Oregon had 104, and California had 103. Louisiana, New Mexico, and Montana had prevalence similar to California, considering the number of cases and total number of horses in those states.
“It was interesting to find that pigeon fever was occurring in several states that were
previously reported as being free of this disease, such as Louisiana, Michigan, North and
South Carolina, South Dakota, Vermont, and Wisconsin,” says Spier.
Occurrence of infection varied greatly among years and seasons, with a seeming association between number of cases and climatic condi- tions favorable for the breeding, hatching and survival of the various insect vectors suspected
of mechanically transmitting these bacteria. As stated by Spier, historically the highest number of cases seen in California occur during dry months (late summer and fall).
Numbers of cases fluctuated over the 10 years of this study, but were significantly increasing, with the greatest increase during the final two years of the study in 2011 and 2012. No significant association between the clinical form of the disease and age or breed of horse was observed.
“The data in this study were only from horses where a bacterial culture was submit- ted to a lab by a veterinarian, so our numbers dramatically underestimate the actual number of cases,” says Spier. “California numbers were very low in this report, most likely because California veterinarians typically don’t culture external abscesses; they just treat them, know- ing the symptoms. If the horse has a pectoral or midline abscess with non-odorous pus, most veterinarians familiar with the disease just assume it is pigeon fever, especially since the cost for a culture is more expensive now than in the past.”
Spier published an earlier paper describing this disease, titled “Corynebacterium pseudotu- berculosis Infection in Horses” which appeared in the January/February, 2007 issue of the California Veterinary Medical Association Journal. In that paper, she described the three forms of this disease. The most common form is external abscesses in the pectoral region (breast muscles) or ventral abdomen along the midline of the belly. Much less common is involvement of internal organs, with abscesses appearing in the liver, kidneys or spleen. Even less common in North America is infection of the limbs, known as ulcerative lymphangitis, with multiple draining lesions.
In an early study of infections in California, Spier found that external abscesses made up about 90% of cases, internal abscesses about 8%, and ulcerative lymphangitis (where the abscesses rupture along chains of lymphatics) about 1%.
“In our recent study, most of the culture- positive horses for internal infections were from California (and not very many cultures from external abscesses). The lower numbers from external abscesses suggests that California vet- erinarians are diagnosing external infections on their own, without submitting pus for culture.
Most of the cultures from internal abscesses of the liver, kidneys, lungs and other internal organs, came from California veterinarians or from the veterinary school at Texas A&M where ultrasound-guided aspirations were performed. I am sure that our paper underes- timates the total number. Internal infections are difficult to diagnose; it’s technically more complex to aspirate pus from a liver or kidney than from an external abscess,” says Spier.
Insects transmit the disease, but the reservoir for the bacteria is soil where it can survive more than 8 months and up to 2 months in hay or bedding.
An interesting finding was that during 2011 and 2012 there was an increase in number of ulcerative lymphangitis cases, and all but one of these were in Texas. This might indicate
a different vector population, or perhaps a reporting bias; veterinarians might be more likely to take bacterial samples/cultures for the horses with these less common clinical forms of the disease than from a horse with a typical pectoral abscess.
“The reservoir for the bacteria is soil. The organism has been shown to survive for up to
2 months in hay or bedding (straw or shavings) and more than 8 months in soil samples. Insect vectors can transmit the disease, but the insects themselves are not the reservoir. They only assist in spreading bacteria from horse to horse. A pre- vious study showed that the equine biovar is able to survive in a variety of soil types under a wide range of environmental conditions,” says Spier.
“The addition of feces to soil in horse pad- docks and pastures enhances the multiplication and survival of these bacteria. They can persist for months and survive in different soil types and moisture content, and this may be a factor accounting for why the disease is now being found in regions where it was not previously recognized—after introduction of these bacte- ria to the environment,” Spier says.
Incidence of this disease fluctuates from year to year. This may be due in part to dif- ferences in herd immunity and environmental factors, such as temperature and rainfall. Horses kept outdoors or with access to an outdoor paddock seem to be at higher risk than stabled horses. Foals less than 6 months old
SPEEDHORSE, July 2017 53
equine health