Transmission of Mycoplasma bovis into suckler cow farms after introduction of breeding bulls

Technical Summary: Transmission of Mycoplasma bovis into Suckler Cow Farms via Breeding Bulls

Problem Statement
Mycoplasma bovis (recently reclassified as Mycoplasmopsis bovis) is a significant pathogen in cattle, causing respiratory disease, mastitis, arthritis, and otitis media. While transmission via the purchase of infected animals is a known risk, particularly in dairy herds, there is limited data regarding the occurrence and clinical impact of M. bovis in suckler cow herds following the introduction of breeding bulls. Asymptomatic carriers and intermittent shedding complicate detection, and the specific dynamics of transmission from breeding stations to suckler farms remain poorly understood.

Methodology
This study investigated the transmission of M. bovis from a purebred breeding bull station in Finland to 35 suckler cow farms after the introduction of breeding bulls in June 2023. The breeding station had experienced an acute M. bovis outbreak in the spring of 2023, confirmed by serology and bacterial culture.

• Study Population: 35 farms purchased bulls from the station. After exclusions (previous positive history, immediate removal of bulls, or lack of participation in testing), 27 farms were included in the final analysis.
• Data Collection: A comprehensive questionnaire (46 questions) was administered to farm owners regarding management practices, bull handling, and clinical signs.
• Diagnostic Approach:
  • Serology: Milk and blood samples were collected from cows (primarily primiparous) to detect M. bovis antibodies using an indirect ELISA (ID Screen®). Farms were considered positive if antibodies were detected in any sample.
  • PCR/Culture: Nasal swabs (NS) and deep nasopharyngeal swabs (NP) were collected from calves to detect the presence of the bacteria.
  • Follow-up: Two positive farms (Farm 22 and Farm 26) underwent extensive longitudinal monitoring, including repeated sampling of isolated groups and the main herd over 18 months.

Key Results

• Infection Rate: Following the introduction of bulls from the infected station, M. bovis antibodies were detected in 8 out of 27 farms (30%).
• Clinical Presentation: Clinical signs were generally mild or absent.
  • Respiratory signs in calves were the primary clinical observation in positive herds, though similar signs were also reported in negative herds, making clinical diagnosis unreliable.
  • One positive farm reported otitis and respiratory signs in calves after the grazing season ended and animals were commingled indoors.
  • No reduced fertility was reported. Mastitis was observed in individual animals on one positive farm.
  • Increased calf mortality was reported in two positive and one negative farm.
• Detection Challenges:
  • Nasal swabs from calves in the 8 positive herds were consistently negative for M. bovis via PCR.
  • The infection was detected primarily through serology (antibody testing).
  • In Farm 22, infection was initially confined to a specific contact group (bull, dams, and bull calves) and did not spread to the rest of the herd despite later commingling.
  • In Farm 26, a single primiparous cow tested antibody-positive; M. bovis was subsequently isolated from her tonsils post-slaughter, while the rest of the herd remained negative.
• Bull Status: All 12 tested bulls from the breeding station were antibody-positive. Of the 6 bulls tested upon arrival at the 5 participating farms, 5 were positive.

Significance and Conclusions
The study concludes that the spread of M. bovis in suckler cow herds via breeding bulls is difficult to predict and does not necessarily result in widespread infection or clinical disease, even when the source animals are infected.

• Low Transmission Efficiency: Only one-third of the exposed farms became seropositive. The authors suggest this may be due to limited or intermittent bacterial shedding by the bulls, or the ability of some animals to clear the infection rapidly.
• Role of Environment: The introduction occurred during the grazing season, where lower stress levels, fresh air, and reduced co-infection pressure may have limited the progression from colonization to clinical disease.
• Diagnostic Implications: Relying solely on clinical signs or nasal swabs from calves is insufficient for detecting M. bovis in suckler herds. Serological testing (ELISA) proved to be a more sensitive method for identifying infected herds in this context, particularly given the subclinical nature of the infection in adult cows.
• Management Factors: The study highlights that while animal contact is necessary for transmission, management factors (housing, stress, regrouping) play a critical role in whether an infection remains subclinical or progresses to clinical disease.

The paper emphasizes that while the risk of introduction exists, the outcome is variable, and effective monitoring in suckler systems requires serological surveillance rather than reliance on clinical observation or calf swabbing alone.