Climatic and environmental factors appear to be key drivers in assisting a soil-dwelling pathogen to cause pigeon fever in horses, American researchers report.
Pigeon fever, also known as pigeon breast or dryland distemper, is a distressing condition in horses caused by the bacterium Corynebacterium pseudotuberculosis.
Researchers from Kansas State University set about trying to identify potential environmental and climatic risk factors for the disease, following what they described as an unprecedented outbreak in the state during the fall of 2012.
Their analysis showed that environmental factors (low levels of soil moisture and landscape fragmentation) and hot and dry weather (in particular, land surface temperatures of 35 degrees Celsius or above) were associated with the disease.
Mixed forests, grassland/herbaceous cover and habitat fragmentation significantly increased the odds of infection in horses.
They suggested that infected horses exposed to these highly fragmented habitats may have been infected through biting insects. It may even involve a transmission cycle that included wildlife reservoirs for C. pseudotuberculosis, they said.
The best-performing model used by researchers Courtney Boysen, Elizabeth Davis, Laurie Beard, Brian Lubbers and Ram Raghavan indicated a protective effect for higher soil moisture content.
Their modeling, which used medical records and publicly available environmental and climate data collected between 2005 and 2013 in Kansas and neighboring states, indicated that age, gender and breed had no influence on the likelihood of C. pseudotuberculosis infection in horses.
C. pseudotuberculosis is a soil-borne bacterium with worldwide distribution which can cause disease in cattle, sheep and goats, as well as horses.
Horses are believed to acquire it from inhaling infected soil or through contamination of damaged skin, and also through biting flies.
The disease has three form in horses, the most common involving the formation of external abscesses that contain a lot of pus. They are typically found in the pectoral region, ventral abdomen, sheath, or mammary gland.
The second form, which occurs in fewer than 10 percent of cases, involves an internal infection, with the development of either single or multiple abscesses in the abdominal or thoracic cavity. A reported mortality risk of 40% is seen, even with aggressive antimicrobial treatment. Without treatment, the mortality rate increases to 100%.
The third and rarest form involves ulcerative inflammation of the walls of the lymphatic vessels.
The study team said pigeon fever was traditionally considered to be an arid region disease, perhaps due to its first identification among horses in drier areas of California. Outbreaks outside this region have been recorded periodically.
“Large outbreaks in the Midwestern US, such as the one Kansas witnessed in 2012, are so far rare.
“However, the number of cases seen within this region, particularly in Texas, has been increasing over the years, indicating a plausible eastward expansion of this disease’s enzootic range.”
Between 2005 and 2011, the number of confirmed cases tested at the Texas Veterinary Medical Diagnostic Laboratory increased by an annual average of 177%. Most of these cases clustered around the panhandle and Central Texas regions, which experience semi-arid to subtropical climate, unlike the arid west.
The 2012 outbreak in Kansas also occurred in areas that received relatively higher amounts of rainfall, but it followed an exceptionally warm winter and hot and dry summer seasons that year.
The researchers said that understanding environmental and weather-related influences may well prove useful in terms of preventing infection and managing horses.
The researchers, who reported their findings in the open-access peer-reviewed journal, PLOS ONE, said the physical environment and climatic factors had clearly influenced C. pseudotuberculosis infections among horses in the study region.
“It is likely that horses of all ages are equally susceptible to C. pseudotuberculosis under outbreak conditions in naïve populations.
“In the present study, areas with soil that can hold higher moisture levels had a protective effect from C. pseudotuberculosis infection for horses, and conceivably this effect is similar for horses in other regions as well.”
They found that horses diagnosed with pigeon fever in the study had experienced weekly average land surface temperatures greater than 35°C for at least a seven-day period or more, eight to twelve weeks prior to diagnosis.
The seasonal influence in cases was noted in the study, with most occurring during late summer and fall, most likely due to infections acquired in spring and early summer.
“The Great Plains region in general, and Kansas in particular, experienced prolonged drought conditions during the spring and summer months in 2012, and recorded historically high temperatures combined with low rainfall,” they noted. “Such circumstances may have favored an outbreak.”
Soil is highly prone to wind erosion under drought conditions, potentially favoring inhalation of C. pseudotuberculosis in airborne dust.
Weakened immune systems among drought-stressed horses and any soil processes that boost C. pseudotuberculosis infectivity could also be factors, they added.
The study was funded by the Kansas State Veterinary Diagnostic Laboratory and Veterinary Health Center at Kansas State University.
Boysen C, Davis EG, Beard LA, Lubbers BV, Raghavan RK (2015) Bayesian Geostatistical Analysis and Ecoclimatic Determinants of Corynebacterium pseudotuberculosis Infection among Horses. PLoS ONE 10(10): e0140666. doi:10.1371/journal.pone.0140666
The full study can be read here.
The study was published under a Creative Commons License.