Corynebacterium pseudotuberculosis is a pleomorphic, gram-positive, facultative intracellular bacterium with a worldwide distribution that causes disease in cattle, sheep, goats and horses [1] [2]. The bacterium is soil-borne, and horses are believed to acquire it from inhaling infected soil or through contamination of abraded skin, and also through biting flies [1]. The bacterium causes three clinical syndromes in horses, the most prevalent of which is the formation of external abscesses that contain abundant purulent material and are characteristically located in the pectoral region, ventral abdomen, sheath, or mammary gland, which is commonly referred to as pigeon fever, pigeon breast or dryland distemper. The second clinical syndrome occurs less frequently (<10% of cases), as an internal infection with development of either single or multiple abscesses in the abdominal or thoracic cavity. Although internal infections are rare, a reported mortality risk of 40% is seen even with appropriate and aggressive antimicrobial treatment, and in the absence of treatment mortality rate increases to 100% [2]. The third and least common manifestation of disease associated with C. pseudotuberculosis is ulcerative lymphangitis.

Infection with C. pseudotuberculosis is traditionally considered to be an arid region disease perhaps due to its first identification among horses in drier areas of California, where the disease is now enzootic. However, outbreaks of this disease outside this region have been recorded periodically [2], [3]. Potential environmental or latent climatological influences associated with these outbreaks are seldom reported. 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 the years 2005–2011, the number of confirmed cases tested at Texas Veterinary Medical Diagnostic Laboratory (TVMDL) increased an annual average of 177% [4], [5]. Most of these cases clustered around the panhandle and Central Texas regions, which experience semi-arid to subtropical climate, unlike the epizootic arid west. The 2012 outbreak in Kansas also occurred in areas that received relatively higher amounts of precipitation but followed an exceptionally warm winter and hot and dry summer seasons that year [6], [7].

While there is a chance for new infections to occur outside the epizootic areas through contact with horses that were infected elsewhere, it is more or at least equally likely for the infections to originate locally from C. pseudotuberculosis that may be present in soils and/or other hosts. This is often suspected to be the source of infection for horses that have had no apparent contact with travelling or infected horses. Information on the ecology and spatial distribution of the pathogen is however lacking. Environmental and climatological influences such as a prolonged drought could trigger infections in new areas [5], [8]. Understanding such influences may have important preventative and management implications, and as well as help recognize the eco-climatological influences in the ecology and evolution of this disease. Doherr et al. [9] reported that young adult horses and those that had contact with other horses on summer pastures were at significant risk for pigeon fever in California. Besides this study, evaluations of environmental or climatological risk factors for C. pseudotuberculosis infection, which could vary over geographic regions cannot be found. Soil types and conditions such as moisture levels, and p^H could affect the survival and C. pseudotuberculosis infectivity, while a horse’s exposure to extreme climatic events and different ground cover or landscape patterns that support reservoir hosts could increase risk of infection.

In this study, we evaluated associations of such potential risk factors for C. pseudotuberculosis infection among horses that were seen between 2005–2013 period in Kansas and neighboring states. For this, we used medical records of horses that had a positive or negative diagnosis for C. pseudotuberculosis infection that were received at the Kansas State University’s Veterinary Health Center (VHC) and Kansas State Veterinary Diagnostic Laboratory (KSVDL), and publicly available environmental and climatological covariate data. Following univariate screening of candidate variables, models were constructed in a Bayesian hierarchical framework, which has advantages over traditional methods when there is a potential for spatial influence on disease data [10], [11].

The observed spatial distribution of C. pseudotuberculosis infection in Kansas based on VHC and KSVDL medical records is present in Fig 1. Most cases recorded during the study period were reported during the fall of 2012 (Sep–Nov) (Fig 2A and 2B), indicating that the infection had occurred during the preceding summer months or late spring in the region. From the univariate, deterministic, non-spatial logistic models, five variables retained their statistical significance and others were discarded. Higher soil moisture levels had a protective effect (OR = 1.22, 95% CI =1.08, 1.30) on C. pseudotuberculosis infection status in horses; and, mixed forests (OR = 1.18, 95% CI = 1.04, 1.12), grassland/herbaceous cover (OR = 1.22, 95% CI = 1.08, 1.30), total edge contrast index (OR = 1.33, 95% CI = 1.17, 2.21), and day time land surface temperature (≥35°C) (OR = 2.28 95% CI = 2.14, 3.31) had significantly increased the odds of infection status in horses. Of these variables, mixed forest and grassland/herbaceous cover did not retain significance in the Bayesian geostatistical model with covariates, and, individual host factor covariates (age, sex and breed) were not significantly associated with case status when they were inserted one at a time. Box plots showing differences in statistical distribution of significant univariate variables and those retained in the Bayesian geostatistical model surrounding case and control locations are present in Figs 3 and 4, respectively.

The Bayesian geostatistical model without any covariates had a DIC value of 740 and the value of the decay parameter for spatial correlation (ϕ) was 0.3, indicating that a high degree of spatial correlation of C. pseudotuberculosis infection prevalence was evident between locations with relatively large distances separating them. From the Bayesian geostatistical model which included all covariates, the removal of mixed forest and grassland/herbaceous cover one at a time did not significantly affect the model performance, but removing the remaining three covariates one at a time resulted in ≥10% increase to the DIC value and were therefore retained (Table 3). The final covariate model had a lower DIC value (681) relative to the model without any covariates, indicating that the presence of covariates accounted for additional spatial autocorrelation in the infection status among horses. The decay parameter for spatial correlation (ϕ), was still 0.3. Inclusion of host factors (age, gender and breed) did not change the posterior parameter estimates by ≥10%. The posterior distributions for all parameters followed a normal distribution pattern, and (MCESD) values for individual covariates revealed no convergence issues within the burn-in period and no mixing issues were noted.

Physical environment and climatological factors influenced C. pseudotuberculosis infections among horses in the study region; and, based on the spatial correlation estimate, it appears highly likely that pigeon fever infections in the study region were influenced by common factors. As in previous studies [1], [8], seasonality in case arrivals was noted in this study, most occurring during late summer and fall months and are likely due to infections acquired in the spring and early summer. Infection with C. pseudotuberculosis has been noted to be common among young adult horses [9] but like in the present study gender and breed were not significantly associated with case status in prior studies. The lack of significant pigeon fever association with a horse’s age in this study is similar to Hall et al., [25] in their epidemiological investigation of an outbreak in 1997 in Western Colorado. It is likely that horses of all ages are equally susceptible to C. pseudotuberculosis under outbreak conditions in naïve populations.

Non-spatial models used for analyzing data with inherent spatial structure (as in this study) can lead to biased regression parameters, underestimated standard errors, narrow confidence intervals and overestimation of parameter coefficients [26], [27]. We therefore used a generalized linear geostatistical model in a Bayesian framework to account for spatial autocorrelation and to incorporate uncertainties in the input data and model parameters. This approach likely provided more accurate parameter and significance estimates and increased predictive accuracy [10], [28], [29]. Cases of pigeon fever recorded over several years (2005–2013) were modeled in this study; however, most of the cases occurred during the 2012–2013 period spread over approximately four month period. Despite this, temporal or spatio-temporal interaction effects was not noted in the disease data during the initial screening of variables. This could be because of single mode in the outbreak data and the relatively wide-spread occurrence over a four month period.

Soil is a reservoir for C. pseudotuberculosis and at least one of its biovars, equi has recently been shown to survive in different soil types under wide ranging environmental (humidity, ambient temperature) conditions [30]. 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. Soil types that retain higher levels of moisture typically have higher clay content, and are perhaps covered by vegetation and permit less run-off [31]. Soil-borne bacterial ecology is highly regulated by soil physiochemical properties such as organic matter content, moisture and p^H levels, the significance for many of which are currently not known for C. pseudotuberculosis. Future evaluations of such effects are worth considering.

Horses that had positive diagnosis for C. pseudotuberculosis infection in the present study had experienced weekly average land surface temperatures (LST) greater than 35°C(95°F) for at least a seven day period or more, eight to twelve weeks prior to diagnosis. Higher LSTs for prolonged periods of time (variable on different ground cover types) is an indicator used by meteorologists for determining drought conditions, which has long been suspected to be associated with C. pseudotuberculosis infection. The Great Plains region in general and Kansas in particular experienced prolonged drought conditions during the spring and summer months in 2012 [32], and recorded historically high temperatures combined with low rainfall [7], [33], [34], circumstances that may have favored an outbreak. More proximal causes for drought related disease incidence may include multiple factors. Soil is highly prone to wind erosion under drought conditions [35], consequently favoring inhalation of C. pseudotuberculosis containing airborne dust. Weakened immune system in horses exposed to persistently high temperatures or drought stress and any soil biochemical processes that boost C. pseudotuberculosis infectivity could also be proximal factors. Soil warming and drought-like conditions have resulted in altered microbial community characteristics and soil enzymatic activity under field conditions [36–38], and improving our understanding on any such effects regarding C. pseudotuberculosis may help devising preventive methods for this disease.

Total Edge Contrast Index (TECI), a measure of landscape fragmentation [19] was significantly associated with C. pseudotuberculosis infection status among horses in the present study. Horses that were exposed to highly fragmented landscapes may have received infection through biting insects inhabiting such landscapes, potentially also involving a transmission cycle that could include wildlife reservoirs for C. pseudotuberculosis. Landscape fragmentation leads to more and smaller landscape patches, increased patch isolation, decreased complexity of patch shape, and higher proportions of edge habitats [39], conditions that lead to loss of biodiversity and “dilution effect” [40], that subsequently favor arthropod-borne disease transmission to wildlife and humans [41], [42]. Insects have been incriminated as mechanical vectors for C. pseudotuberculosis [9], [43], and increased activity of flying-insects was observed by horse owners weeks prior to their horses received infection [25]. It is possible for insects to act as vectors for this pathogen from infected to susceptible hosts, some of which could also include other ungulates that inhabit fragmented landscapes. Kelly et al., 2012 [44] reported the isolation of C. pseudotuberculosis from abscess from captive elks, and this bacterium has been suggested to infect multiple hosts [45], including deer that may inhabit fragmented habitats. Therefore, a possibility for a transmission cycle involving insects and wild ruminant hosts seems likely but this premise needs further evaluation.

It is important to discuss a limitation of this study. Our study population originated from medical records maintained at two facilities, Kansas State University’s VHC and KSVDL, and not private practices or other sources in the region where horses may have been treated for C. pseudotuberculosis infection. Despite these two veterinary medical facilities being the largest in the state with a broad clientele in the region, there does exists a statistical possibility for some of our estimates and risk factors to change if horses from a larger population were to be included; a problem typical of many retrospective case-control studies that can only be fully mitigated by conducting carefully designed prospective sampling. Future, more robust studies are therefore necessary to better understand pigeon fever outbreaks and ecoclimatic drivers of this disease.

Infection with C. pseudotuberculosis occurs in horses of all ages, gender and breed in the study region, and this disease is driven by environmental (soil moisture, landscape fragmentation) and hot and dry climatic conditions (LST ≥ 35°C). The identification of these risk factors for pigeon fever is important in the context of disease prevention. Also, it is noteworthy that soil moisture and land surface temperature are climate-change indexes that are projected to change under different IPCC (Intergovernmental Panel on Climate Change) scenarios as soon as next decade. Studies on the implication of such changes for the ecology and evolution C. pseudotuberculosis and similar soil-borne bacteria that have animal and public health significance are needed.