Page 56 - July 2017

Besides a form of pigeon fever that involves the internal organs, approximately 8% of the cases, the other two forms of the disease are . . .
horse manages to develop an immune response and kill the bacteria. Until then, the infection can course along through the lymphatic sys- tem. That’s how it goes deeper, into the lymph nodes,” she says.
External abscesses can be quite deep because bacteria inside the macrophages can travel to lymph nodes deep under the muscles in the pectoral (breast) region or forearm. Since the incubation period is variable and can be 3 to 4 weeks or longer, there may no longer be a break in the skin (such as the fly bite or ventral midline dermatitis) by the time the abscesses mature enough to be noticed.
Flies and the transport of horses around the country probably spread the disease. “Though pigeon fever is still thought to be a California disease by many horsemen, it is now present in more than 25 states in all regions of the U.S., including Hawaii. Based on what we’ve seen of epidemics in Texas, Colorado, and other states, it seems to increase in drought conditions.
The exact environmental conditions leading to epidemics are not completely understood. We know the bacteria survive very well when soils go from moist to dry conditions, which may allow the bacteria to spread more readily,” she says. Transportation of horses is also likely to be a way the disease is spread. An example of this is the outbreak of pigeon fever in Maui, Hawaii, last year.
“One theory is that, when weather is hot and dry, horses seek shade and pass manure where they are standing in the shade. These organisms thrive in soil mixed with manure. In dry conditions, the soil underfoot becomes
Since bacteria survive well and spread more readily when soils go from moist to dry, pigeon fever epidemics seem to increase in drought conditions when horses churn soil and manure into dust.
External abscesses in the breast muscles or midline of the belly make up about 90% of the cases.
are at least risk—which suggests that passive transfer of antibodies via the dam’s colostrum may provide protection to foals born in regions where the disease exists. Their dams may have been exposed to the pathogen and developed antibodies.
It takes longer for horses with internal abscesses to show signs of disease. Horses with internal abscesses are generally diagnosed 1 to 2 months after the peak number of cases with external abscesses, according to Spier.
Transmission
Spier says there is strong evidence to sug- gest that these bacteria can be transmitted via horse to horse contact, from infected horses
to susceptible horses via insects, or by contact with contaminated soil. Abrasions or breaks in the skin or mucous membranes are probably necessary for bacteria to enter the body. Insects are probably one of the most common ways the infection is transmitted, and a few years ago Spier was involved in studies at UC-Davis looking at this possibility.
“We went to a number of farms that were experiencing outbreaks, several different years, and revisited those same farms in years they were not experiencing any cases. We wanted to learn more about transmission and also how these bacteria persist in the environment. I worked with entomologists from UC-Davis
The rarest form of the disease (around 1% of the cases) is ulcerative lymphangitis, an infection of the limbs with multiple draining lesions.
and we trapped flies using drift traps that capture all the insects that fly through.
We also netted flies off the horses that had abscesses. When farms were having outbreaks with infected horses on the premises, we could easily find the bacteria in 3 different species of flies,” she says.
“The entomologists identified all the fly species for me and we tested them for the pres- ence of bacteria. We found Corynebacterium pseudotuberculosis in 3 very common spe- cies—housefly, stable fly and horn fly—and there could be some other vectors as well. The horn fly feeds on the ventral midline (under- side of the belly), which is often the area where abscesses start,” explains Spier.
On farms where horses had the disease, her team found that up to 20% of the houseflies were carrying the bacteria. This meant that
1 out of every 5 houseflies that lands on a horse could possibly transmit the disease. “We went back to those farms on years there were no cases, and found the fly populations were negative for the bacteria. This tells us that the reservoir is not the fly, but the soil. We took soil samples and found the bacteria can survive in many soil conditions,” she says.
After these bacteria get through the skin, they travel inside certain white blood cells— the immune cells known as macrophages. “They can survive inside those cells until the
54 SPEEDHORSE, July 2017
equine health
Sharon Spier
Sharon Spier