Transmission dynamics of 2 pathogens (Mannheimia haemolytica and Mycoplasma bovis) in Young Cattle at the beginning of the fattening period.

National and/or international context/issues/issues

More than 50% of European and North American beef comes from male cattle raised from weaning in feedlots (USDA, 2011, FranceAgriMer, 2010). Infectious bronchopneumonia (IPB) is the main disease encountered in these workshops (Assié et al., 2009, Cusack et al., 2003, Smith, 2009). These are infectious diseases caused by viruses and bacteria. Among bacteria, Mannheimia haemolytica (M. haemolytica) and Mycoplasma bovis (M. bovis) are considered important agents. In addition to these infectious agents, various factors related to animals, husbandry conditions and the environment favour the development of clinical signs.

Measures to prevent the development of IPBs are based on the control of risk factors and targeted vaccination against some of the infectious agents. These measures reduce the incidence of cases but do not completely prevent the occurrence of IPB (Taylor et al., 2010). IPBs are difficult to detect early and completely (Timsit et al., 2011), resulting in growth retardation and mortalities that can have a significant impact on the economic performance of feedlots (Griffin, 1997, Smith, 1998).

The treatment of IPBs requires the use of antibiotics (Radostits et al., 2007). To circumvent the difficulty of detecting young cattle that could benefit from treatment, an alternative strategy to curative treatment of only cattle detected by visual inspection is the use of metaphylaxis, i.e. treatment of all animals in a batch after detection of the first cases that may require treatment (Schwarz et al., 2001). As with any non-targeted antibiotic treatment, metaphylaxis results in the treatment of healthy animals that may not require treatment and therefore potential overuse of antibiotics. Indeed, one justification for the use of metaphylaxis is the protection it could confer by preventing the occurrence of new patients secondary to the transmission of pathogenic bacteria within batches of animals. With the overall objective of reducing antibiotic consumption, the use of metaphylaxis in the treatment of IPB must be very cautious. At present, however, there is insufficient knowledge about the horizontal transmission of bacteria involved in IPBs, particularly during or just after batching, to assess the real danger posed by these secondary cases.   

The objective of our work was to determine whether one or more clones of M. haemolytica and M. bovis are present in the lungs of the various cattle in a batch during IPB episodes. Isolates of M. haemolytica and M. bovis were characterized by pulsed field electrophoresis (PFGE). 

Results

Sixteen episodes of IPB occurred naturally in 12 batches of 8 to 12 young cattle (n=112) newly arrived in 3 fattening plants. M. haemolytica was isolated during 14 of these episodes. 175 isolates of M. haemolytica were collected. Two to 3 clones of M. haemolytica were found in 10 episodes while only one clone was found in 4 episodes. Isolates of M. bovis were found in 8 of these 16 episodes. The intra-lot prevalence of M. bovis positive cattle was 8-100%. The PFGE revealed that, although the cattle came from different breeder farms of origin, only one M. bovis clone was present in a batch during IPB episodes with a high prevalence of M. bovis infection.

Perspectives/term impact

The significant diversity of M. haemolytica strains during IPB episodes indicates that IPB in which M. haemolytica is involved is not primarily due to the spread of a single virulent clone among cattle and highlights the importance of factors that predispose to the occurrence of cases by helping the resident flora in the nasopharynx of cattle to overwhelm their immune system.

Although M. bovis can occur in carrier animals after a stressful event such as transport or allotment, the increased prevalence of M. bovis lung infection observed in IPB episodes occurring early in the feeding period appears to be primarily due to horizontal transmission of a clone among young cattle.

Valorisation

Timsit E., Christensen H., Bareille N., Seegers H., Bisgaard M., Assié S. 2012. Transmission dynamics of Mannheimia haemolytica in newly-received beef bulls at fattening operations. Veterinary Microbiology.http://dx.doi.org/10.1016/j.vetmic.2012.07.044

Timsit E., Arcangioli M.A., Bareille N., Seegers H., Assié S. 2012. Molecular epidemiology of Mycoplasma bovis during bovine respiratory disease outbreaks in newly-received beef bulls at fattening operations. Journal of Veterinary Diagnostic Investigation. In press.

Bibliography

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National and/or international context/issues/issues