Chlamydiae in cattle

Chlamydiae are minute gram-negative bacteria with an obligate intercellular developmental cycle, in which they compete with their host for intracellular nutrient pools. In cattle, Chlamydia (C.) pecorum is the dominant species. Additionally, C. abortus, C. psittaci, C. suis, as well as chlamydia-like organisms (Parachlamydia acantamoebae, Waddlia chondrophila) were found. Mixed infections involving two or more chlamydial species occur frequently at herd level and in individual animals. Detection of Chlamydia species is not part of routine veterinary bacteriological diagnosis, and species-specific serological tests for chlamydiae are not commercially available. Intermittent shedding and the capability of subverting the host's immunological response make detection even more difficult. Typically, chlamydial agents are more readily detectable in young animals, while antichlamydial antibodies are diagnosed in older animals. Investigations over the last decade using highly specific nucleic acid-based detection assays have identified ubiquitous low-level chlamydial infections in bovine herds that remain without obvious clinical signs in most cases. Nevertheless, these asymptomatic infections correlate highly significantly with reduced growth rates in calves, and with reduced milk yield, milk quality, and fertility rates as well as reduced musculosceletal and hoof health in dairy cows. Underlying these economically significant losses is a systemic inflammatory status that is mediated by circulating inflammatory mediators released from the sites of mainly asymptomatic chlamydial infection. Additionally, local inflammatory insults may further reduce performance of affected organs (intestinal and genital tract, lung, joints). Management factors (e.g., herd hygiene and density, nutrition, herd microclimate, mixed infections) modulate the extent of performance reduction by, and clinical presentation of bovine chlamydial infections. Currently, optimal herd management is the preferred strategy to minimize economic losses due to asymptomatic chlamydial infections. Antibiotic therapy cannot eliminate chlamydial infections. In the future, recurring herd vaccinations with an effective vaccine may become an effective control strategy.