- In the western United States, bighorn sheep (Ovis canadensis) have suffered major die-offs in every state since the mid-1800s, and disease from domestic sheep (Ovis aries) has been a primary factor in these events. Beginning in the early 1900s, poly-factorial, poly-microbial pneumonia was identified as a major disease affecting bighorn populations, and within the last 12 years Mycoplasma ovipneumoniae (M. ovi) has been identified as a primary causal agent associated with pneumonia outbreaks; other pneumonic pathogens that have been widely implicated include leukotoxigenic Mannheimia (formerly Pasteurella) haemolytica (M. haemolytica), Bibersteinia (formerly Pasteurella) trehalosi (B. trehalosi), and Pasteurella multocida. Pneumonia outbreaks typically follow from direct contact between domestic animals and wild sheep, but as yet, respiratory disease dynamics in bighorn populations are not completely understood. My research identifies factors that are correlated with variation in the expression and occurrence of the disease both spatially and temporally within bighorn populations of California.
In Chapter 1, I investigated post-outbreak survival of adult female bighorn across 9 populations and evaluated the relationship between M. ovi infection and survival, while testing effects of range factors that could potentially drive differences in adult female survival (i.e., forage quality, winter precipitation, and population abundance). I used survival data from radio-collars on 115 females that were captured and screened for M. ovi infection between 2013 and 2015, and I evaluated potential effects on survival from November 2013 to March 2017 using the known-fate model in Program MARK. Annual survival was negatively correlated with positive infection status at capture but varied across populations with respect to differences in range conditions. Summer and autumn forage quality, as measured using the normalized difference vegetation index (NDVI), was positively correlated with overwinter survival, while winter precipitation (a proxy for winter severity) was negatively correlated with overwinter survival. I also found that population abundance was negatively correlated with annual survival, suggesting a potential density-dependent effect. My findings suggest that summer and autumn forage quality, indexed by NDVI, may partially offset the negative effect associated with M. ovi infection on host survival. Additionally, the negative effect associated with population abundance suggests that density-dependence further added to effects associated with infection.
Reduced neonate survival has been identified as a major factor limiting population recovery for bighorn sheep following pneumonia epizootics. In Chapter 2, I investigated lamb survival in 7 populations of desert bighorn (O. c. nelsoni) exposed to M. ovi. I applied beta regression to late-season (15 August to 30 September) lamb-ewe ratios from 2014-2016 to model lamb survival and evaluate effects of precipitation and population abundance, in order to better understand neonate survival trends in these populations. I detected a quadratic effect of precipitation in October-April, coincident with the period of mid-gestation to lambing for parturient females, whereby precipitation up to an optimum level was positively associated with lamb survival. In contrast, precipitation in May-August, coincident with the post-lambing period, was associated with a negative effect on lamb survival, but this effect was not robust to additional testing with yearly effects and may have been spurious. My findings suggest that higher precipitation in October-April likely increases forage availability and thereby nutrition for parturient females, although excessive precipitation might have a deleterious effect in terms of neonate survival.
In Chapter 3, I used GPS-collar data from 135 radio-collared bighorn (27 males and 108 females) from 14 populations to evaluate movement trends. Animals were captured in November 2013-2015 and March 2017 and location data were collected from November 2013 to December 2018. My objective was to model intermountain movement as a binary response and evaluate effects of sex, age, and M. ovi infection status at capture (determined from PCR testing of nasal swabs collected from animals at capture), in order to assess the potential transmission risk individuals posed between populations by inferring potential for contact from movement behavior. I also examined seasonal trends by calculating proportions of intermountain movements by season for males and females. In a secondary analysis, I tested effects on mean seasonal daily movement rates using linear regression. I found that males were more likely to undertake intermountain movements than females, older animals (> 5 years old) were more likely than younger animals (< 5 years old), and individuals that were PCR-positive for M. ovi at capture were less likely than those that were PCR-negative. Additionally, intermountain movements were more frequent during the breeding season (July-November) for males and during the period of mid-gestation to lambing (October-April) for females. The breeding season was also associated with higher mean daily movement rates for males, while effects of age and PCR status at capture were not supported. For females, positive PCR status and the period of mid-gestation to lambing were both negatively associated with mean daily movement rates and the effect of age was not supported. My findings suggest that the potential transmission risk posed within and across populations by an individual varies depending on time of year and sex in this system, while M. ovi infection may also inhibit animal movement and consequently potential transmission risk.
Results from this study corroborate findings from other studies regarding factors influencing survival and movement of bighorn sheep. We found evidence that M. ovi infection may have lingering effects on survival and movement as well, which has clear implications for disease transmission. Additional investigation may be needed to gain a better understanding of how the pathogen specifically interacts with host and environment to prolong its existence within and across populations. Findings from my research can help direct future studies and management efforts aimed at mitigating effects of respiratory disease in bighorn populations.