Researchers at the Kansas State Veterinary Diagnostic Laboratory and the Center of Excellence for Vector-borne Diseases, both in the university's College of Veterinary Medicine, have established a set of models to evaluate the space-time pattern and environmental drivers of a devastating cattle disease, bovine anaplasmosis, in Kansas.
The results appear in PLOS ONE, "Bayesian Space-Time Patterns and Climatic Determinants of Bovine anaplasmosis." Data used in the study were from diagnostic samples submitted to the diagnostic laboratory between the years 2005-2013. The study results indicate the number of bovine anaplasmosis positive submissions in Kansas have steadily increased and have originated from newer geographic areas during the same period.
Bovine anaplasmosis affects beef and dairy production in almost all the U.S. states, causing significant economic losses to producers. The control of this disease currently relies primarily on infection-avoidance, although an experimental vaccine is used in many areas of the U.S. The causative bacterium Anaplasma marginale lives in red blood cells and causes a hemolytic disease in cattle, which manifests as anemia, abortion, icterus and lethargy. It can cause death, primarily in older animals. Cattle that survive infection are persistent carriers of the bacteria and are a source of infection for other cattle through inadvertent mechanical transmission via blood-contaminated multiuse needles and surgical equipment, as well as via tick and fly transmission.
"The number of positive anaplasmosis samples submitted to the Kansas State Veterinary Diagnostic Laboratory has increased over the years, and the geographical area from where these samples originated has expanded," said Gregg Hanzlicek, director of production animal field disease investigations for the diagnostic laboratory. "Over time, these changes may have occurred because veterinarians have become more aware of the disease, but this study suggests environmental conditions and management practices may have also played a role."
Ram Raghavan, a spatial epidemiologist at the diagnostic laboratory, worked closely with Hanzlicek in evaluating the space-time patterns of this disease.
"Some of the increase in the expansion of tick-borne diseases in the Midwestern region may be attributed to geographic expansion of tick populations over time," Raghavan said. "Other evidence suggests a growing potential threat for bovine anaplasmosis in newer areas, but a quantitative evaluation of whether or not bovine anaplasmosis has spread to previously unreported areas over time is not readily available. Likewise, information on any potential environmental and climatological drivers behind the space-time expansion of bovine anaplasmosis cannot be easily found, which has disease management implications."
The article noted that Bayesian space-time disease mapping models are particularly appropriate tools to describe disease patterns and to identify unusual clusters of incidence in space and time-trends or both.
"Such models help detect any localized clusters that may be linked in time, for instance due to a set of favorable environmental drivers or cattle movement," Hanzlicek said. "Another way to strengthen inference from Bayesian space-time models is by including relevant ecological factors that often explain additional variability in disease prevalence. This is particularly relevant in the case of tick-borne diseases since the spatial distribution of ticks is largely determined by physical environmental and climatological conditions."
Hanzlicek said that the presence of higher space-time interaction for bovine anaplasmosis infection within a cluster of central and south central counties in Kansas indicates the possible influence of similar risk factors, and are potential areas for targeting prevention/management efforts.
The study also has produced some new information about the effects of climate change on vector-borne diseases, which is a research focus of Raghavan.
"Three climate change indices — minimum land surface temperature, diurnal temperature range and relative humidity — are drivers of the space-time pattern for bovine anaplasmosis," Raghavan said. "This finding is significant in the context of climate change implications on infectious diseases and adds to the mounting evidence of climate change linkages to animal health."
The researchers noted two immediate questions they were led to ask based on this finding: How might the associations of these climate factors be further quantified under field and laboratory conditions for the tick host and the pathogen? And, how might these factors influence further geographic expansion of this disease under different climate-change scenarios?