New PhD-graduate Talita Resende described in this open-access publication from the Journal of Veterinary Diagnostic Investigation conjunctivitis cases in 3 wean-to-finish barns. After extensive investigation, Mycoplasma hyorhinis was identified as a causative agent.
Three conjunctivitis outbreaks were identified in 3 unrelated wean-to-finish barns in the Midwest. Clinical signs were described as redness of the conjunctiva, swelling of the eyelid and watering eyes in pigs between 8 to 10 weeks of age. In the later stages of the disease, clear tears changed into a mucopurulent consistency. Prevalence was high with between 30% and 60% of the pigs were affected. These signs affected the general health of the pigs which refused to eat and failed to gain weight.
Samples and testing
Thirty-six eye swabs were collected from both diseased and healthy pigs. Twelve tissue samples -eyes and conjunctiva – were collected after necropsy. All samples were then submitted to the University of Minnesota Veterinary Diagnostic Laboratory. Tissues were examined by histopathology and by immunohistochemistry and/or PCR for several agents such as Classical Swine Fever, African Swine Fever,Pseudorabies, Chlamydia spp., Mycoplasma hyopneumoniae, Mycoplasma hyorhinis, Influenza A, Porcine Reproductive and Respiratory Syndrome, and Porcine Cytomegalovirus.
Pigs tested negative for Classical Swine Fever, African Swine Fever, Pseudorabies, Chlamydia spp., Mycoplasma hyopneumoniae, , Influenza A, Porcine Reproductive and Respiratory Syndrome.
Porcine Cytomegalovirus was inconsistently detected by PCR in affected and healthy pigs but no lesion was seen by histopathology, ruling it out as a causative agent.
Mycoplasma hyorhinis were detected by PCR in affected animals as well as unaffected pen-mates. Histologic lesions in the conjunctival tissue were positive for M. hyorhinis by in-situ hybridization (ISH), placing the pathogen within the lesion. Bacterial isolation of M. hyorhinis was attempted but did not succeed due to an overgrowth of Staphylococcus aureus.
Although Koch’s postulate was not fulfilled in this study, PCR-positive results with lower Ct-values in affected pigs as well as ISH-positive samples in affected tissue are strong indicator that Mycoplasma hyorhinis was the primary agent in this conjunctivitis case.
Conjunctivitis is an uncommon finding in commercial swine herds, and the etiology of the disease is rarely studied. We investigated cases of conjunctivitis in 3 wean-to-finish swine farms. Eye swabs and tissues were obtained from clinically affected pigs (8–22wk of age), from unaffected pigs in contact with affected pen-mates, and from age-matched pigs from an unaffected herd. Real-time PCR (rtPCR) testing for Mycoplasma hyorhinis demonstrated consistent detection and high bacterial load in samples from affected herds (clinically affected animals and non-clinical pen-mates). Ct values in affected pigs were 18.9–25.3; values were 36.4–38.6 in unaffected pigs from unaffected herds. Additionally, M. hyorhinis was identified within inflamed palpebral conjunctivae by in situ hybridization. The association of rtPCR and in situ detection of M. hyorhinis, along with the lack of detection of other potential pathogens and noninfectious causes, suggests the involvement of M. hyorhinis in the etiology and pathogenesis of the reported swine conjunctivitis.