Exploring Dynamics of Infection with Mycoplasma ovipneumoniae in Domestic Sheep: Potential Risk Factors and Management

Klara McKay

Mycoplasma ovipneumoniae is a respiratory bacterium that affects members of the subfamily Caprinae including domestic sheep (Ovis aries), domestic goats (Capra hircus), and bighorn sheep (Ovis canadensis). M. ovipneumoniae is commonly found in the upper respiratory system of domestic sheep and goats, often without showing clinical signs of infection. In wild sheep, spillover of M. ovipneumoniae from domestic sheep or goats can result in an outbreak of fatal bronchopneumonia, leading to all-age die-offs and years of poor lamb recruitment. Preventing spillover events is key to protecting bighorn sheep health, and this requires an astute understanding of the dynamics of M. ovipneumoniae in domestic sheep. Understanding what risk factors might influence the probability of detecting M. ovipneumoniae in domestic sheep can inform the development of effective management strategies to mitigate transmission and infection.For my first chapter, I investigated whether certain environmental factors and life history traits could predispose domestic sheep to be more vulnerable to contracting M. ovipneumoniae. M. ovipneumoniae is known to adversely affect domestic sheep lambs but less is known about variation in detection of the bacteria throughout the adult lifespan. I hypothesized that age, reproductive effort, travel history, and seasonality might each play a role in the probability of detecting M. ovipneumoniae. This was a longitudinal study consisting of the serial collection of nasal swab samples from 40 Suffolk sheep and 55 Polypay sheep over the course of about two and a half years at the University of Idaho Sheep Center (UISC) in Moscow, Idaho. M. ovipneumoniae was detected from nasal swabs via real-time polymerase chain reaction (PCR) conducted by the Washington Animal Disease Diagnostic Laboratory (WADDL). I used ordinal regression to model potential risk factors that may influence M. ovipneumoniae PCR detection. I found that travel history and age were important risk factors that could be predictors of M. ovipneumoniae detection in Suffolk breed domestic sheep, and that reproductive stage (i.e., maintenance, gestation, or lactation) was an important predictor in Polypay domestic sheep. In the Suffolk sheep, M. ovipneumoniae was detected more in animals that had recently traveled longer distances. Further, a U-shaped relationship was observed with age where younger and older ewes had a higher probability of detection in the Suffolk cohort who were ages 2-12 years old at the beginning of the study. In the Polypay sheep, I observed a lower probability of detecting M. ovipneumoniae in pregnant and lactating ewes than non-reproducing ewes, and this relationship was not influenced by seasonality or litter size. Effective treatment and vaccine options are currently not available for M. ovipneumoniae in domestic or bighorn sheep. Test-and-remove is a management strategy that has proven successful in recent years as a means to reduce and even eliminate M. ovipneumoniae in bighorn sheep herds by removing chronic carriers who are responsible for the continued circulation of M. ovipneumoniae in a herd. I suspected that, in a similar way, a few domestic sheep that are persistently infected (PI) were responsible for maintaining M. ovipneumoniae circulating in a domestic sheep flock. I hypothesized that when not persistently infected (NPI) sheep are isolated from PI sheep, the probability of detecting M. ovipneumoniae would be lower in the NPI group over time. The second chapter of this thesis comprises an experiment that tests this hypothesis. Forty Suffolk sheep from the UISC in Moscow, Idaho were sampled up to four times within a year and half. Ewes that had three consecutive detections or indeterminate M. ovipneumoniae results detected via PCR from serially collected nasal swabs were considered PI ewes and ewes that did not fit that criterion were considered NPI ewes. The NPI ewes were isolated at a separate facility on the University of Idaho campus for one year away from contact with any other sheep to ensure they would not encounter any infected sheep including the PI group. Every 1-3 months during the year of isolation, samples were taken from the NPI group, the PI group, and thirty randomly selected sheep from the UISC as a measure of M. ovipneumoniae circulating in the general population (GP). I used ordinal regression to assess the relationship between the PCR detection of M. ovipneumoniae and whether sheep were experimentally isolated as well as other covariates of interest. I found that isolation was the best predictor of M. ovipneumoniae detection in the NPI group, with a lower probability of M. ovipneumoniae detection post-isolation than pre-isolation. However, when I compared prevalence across sampling occasions between the NPI and GP groups, I found no difference between the groups despite isolation of the NPI ewes. A key issue that might explain this discrepancy is the apparent sensitivity of the PCR: when I compared the original PCR results to PCR results that were rerun using an updated PCR protocol, I found that some ewes that were included in the NPI group at the beginning of isolation would have met the criterion to be excluded from isolation as a PI ewe based on these new PCR results. These “sneaky carriers” might have weakened the effect of isolation, so, although I observed a shift towards lower detection in the NPI group post-isolation, it was not significantly different from the baseline prevalence observed at the UISC. This study provides an important starting point for exploring further the role of the presence of PI domestic sheep in maintaining M. ovipneumoniae in flocks and how strategic management could reduce M. ovipneumoniae in domestic sheep operations. Taken together, my thesis shows that key biological and environmental factors can impact the detection of M. ovipneumoniae in domestic sheep and creative management solutions show promise for mitigating infection, preventing spillover, and managing disease.

Exploring Dynamics of Infection with Mycoplasma ovipneumoniae in Domestic Sheep: Potential Risk Factors and Management

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