Mycoplasma ovipneumoniae is a respiratory bacterium associated with economically impactful pneumonia in domestic sheep and goats since 1972, when it was first described.
Although M. ovipneumoniae was previously reported to affect species of Caprinae (sheep, goats, and muskoxen), recent studies have identified the bacterium in animals outside Caprinae, highlighting the knowledge gaps in the basic epidemiology of this bacterium. To date, M. ovipneumoniae has been reported in Beira antelope, bighorn sheep, caribou, Dall sheep, moose, mountain goats, mule deer, caribou, muskoxen, white-tailed deer, and domestic cattle, sheep, and goats via a variety of diagnostic methods. The prevalence and epidemiology of M. ovipneumoniae is unknown for most of these species. In addition, it is unknown which species are susceptible to disease, which carry M. ovipneumoniae as a commensal organism, and what role susceptible species play in transmission. M. ovipneumoniae has been associated with polymicrobial pneumonia in both domestic sheep and bighorn sheep in the western United States. Although epizootics and high death losses of bighorn sheep associated with M. ovipneumoniae was first described in 2008, the epidemiology of this bacterium in bighorn sheep remains unclear. Other pathogens commonly detected in bighorn sheep pneumonia include lungworms (Protostrongylus species), Fusobacterium necophorum, Trueperella pyogenes, and members of the Pasteurellaceae bacterial family. M. ovipneumoniae was recently detected for the first time in Dall sheep, moose, and caribou in Alaska; however, the epidemiology of the bacterium in these species is not known.
In 2011, a USDA National Animal Health Monitoring System (NAHMS) study detected M. ovipneumoniae by PCR test in one or more domestic sheep on 88.5% (401/453) of sheep premises and 29.4% (1,199/4,073 ) of individual sheep tested. An unpublished Alaskan study detected M. ovipneumoniae in 57% (4/7) of Alaskan domestic sheep premises and 7.6% (13/171) of domestic sheep, which suggests that the prevalence in Alaska may be lower than in the conterminous 48 states.
Prevalence in domestic goats is less understood and studies differ. An unpublished Alaskan study detected M. ovipneumoniae in 4/32 domestic goat premises (12%), and in 12/485 domestic goats (2.5%), while a Washington study sampled 84 goats from 16 premises and detected M. ovipneumoniae in 7/16 (44%) premises (individual animal results not reported). An unpublished study, sampling goats on pack goat premises in 12 states, found M. ovipneumoniae in 14/83 premises (17%) and 46/571 of all goats sampled (8%), with significantly higher prevalence in those less than 1 year of age. An ongoing USDA NAHMS national study will report M. ovipneumoniae prevalence in U.S. domestic goats.
Infection with M. ovipneumoniae in domestic sheep may be subclinical, and generally causes low mortality with variable morbidity. Clinical signs vary from mild respiratory disease to severe pneumonia and sudden death; this variation is believed to be related to differences in strain virulence, host immune response, and secondary pathogens. Clinical signs include lethargy, fever, decreased appetite, coughing, nasal discharge, and decreased milk production in ewes. A coughing syndrome with rectal prolapse and decreased rate of gain occurs in domestic lambs infected with M. ovipneumoniae. M. ovipneumoniae can be pathogenic but also predisposes animals to other respiratory infections, resulting in polymicrobial pneumonia. Outbreaks of severe M. ovipneumoniae-related pneumonia have been reported when domestic sheep were moved into higher density housing conditions or commingled.
Reports of M. ovipneumoniae infections in bighorn sheep describe a range of clinical signs from coughing, nasal discharge, ear paresis and headshaking, to fever, lethargy, and sudden death.
M. ovipneumoniae has been found in respiratory tracts of healthy animals and in animals with respiratory disease. Transmission occurs via respiratory droplets or secretions when animals are in close contact. Pathogen, environment, and host factors contribute to the manifestation of respiratory disease in domestic and wild ruminants. These factors include exposure to multiple viruses and bacteria (including M. ovipneumoniae and others), harsh climate and weather, nutritional status and deficiencies, genetics, immune system status, concurrent disease conditions, forage quality, population/stocking density, population distribution, handling/shipping stress, habitat fragmentation and destruction, and predation. Relative contributions of factors may shift temporally and seasonally and vary between geographic populations of a species, resulting in different disease outcomes between different animal populations. Recent detections of M. ovipneumoniae in wildlife species highlight knowledge gaps in the epidemiology of M. ovipneumoniae, such as the number of species that are susceptible or commensal carriers, M. ovipneumoniae prevalence among different species, and what role individual animals, including non-diseased carriers, may play in transmission.
Factors that influence M. ovipneumoniae virulence and pathogenicity in susceptible species are unknown. Studies indicate that strains of M. ovipneumoniae differ genetically and in protein expression. More than one M. ovipneumoniae strain type has been identified from single domestic sheep farms, bighorn sheep herds, and even in individual domestic sheep. Causes for this variability are unknown but may be related to strain virulence, host immune responses, and other factors. Knowledge gaps remain regarding the relationship of different strain types to the development of disease in different hosts.
Impacts of M. ovipneumoniae on bighorn sheep populations differ among studies. Studies in Hells Canyon (Washington, Idaho, and Oregon) describe population-limiting pneumonia epizootics, where M. ovipneumoniae is reported to be an important factor. Other states where M. ovipneumoniae-associated bighorn sheep pneumonia epizootics occur include California, Nevada, Nebraska, Arizona, Utah, Colorado, Montana, North and South Dakota, and Wyoming, though some herds with M. ovipneumoniae in Montana and Wyoming also have stable to increasing populations. This contrast suggests that other factors, aside from M. ovipneumoniae presence, also influence the onset of pneumonia epizootics in bighorn sheep. These factors are not fully understood and may be related to strain virulence, host, environment, and other unidentified risk factors.
Impacts of M. ovipneumoniae on other U.S. wildlife populations are unclear. Evidence suggests M. ovipneumoniae has been present in Dall sheep in Alaska since at least 2004, and it has been detected in caribou and moose populations, but reports of pneumonia deaths in Alaskan wildlife are sporadic. It is unknown if the M. ovipneumoniae strain(s) detected in Alaskan wildlife are part of the normal respiratory commensal flora, if the animals have adapted to the strain(s), and/or if the strain(s) are virulent.
Various diagnostic tests for M. ovipneumoniae identification have been developed, including bacterial culture, antibody-based tests including indirect hemagglutination (IH) and competitive enzyme-linked immunosorbent assay (cELISA), and DNA based tests like polymerase chain reaction (PCR). Sensitivity and specificity vary with test type and protocol. Protocols between laboratories may be different and change over time, which can result in variability in test results between laboratories.
An official antemortem diagnostic test for M. ovipneumoniae does not exist, and currently available tests have demonstrated some variability between laboratories. Regulatory tests must be standardized and validated and the definition of disease freedom must be clearly stated regarding test results and interpretation. The World Organization for Animal Health (OIE) criteria for assay development and validation include defining the assay purpose, assay optimization, assay standardization, repeatability, analytical sensitivity, analytical specificity, assay thresholds, diagnostic sensitivity, diagnostic specificity, reproducibility, and assay fitness for intended purpose. See The Manual for Diagnostic Tests and Vaccines for Terrestrial Animals for a detailed description
Recommended biosecurity measures to reduce likelihood of transmission of M. ovipneumoniae in domestic sheep and goat herds include avoiding overcrowding, improving ventilation in confinement, and using management strategies that reduce risk factors for respiratory tract disease.
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