Epidemiology of <italic>Mycoplasma hyorhinis</italic> in U.S swine production systems

Abstract

<italic>Mycoplasma hyorhinis</italic> has recently emerged as one of the main concerns of swine veterinarians dealing with post-weaning morbidity and mortality. Since the introduction of PCR in 2009, <italic>M. hyorhinis</italic> is detected in about 50% of polyserositis and 12% of arthritis cases submitted to the Minnesota Veterinary Diagnostic Laboratory (MVDL) every year. In the field, swine practitioners are applying different antibiotic and autogenous vaccine protocols with varying degrees of success. Part of the minimal success is due to the lack of essential epidemiological information, which is scarce and obsolete. The main goal of this dissertation was to advance the knowledge on the infection dynamics and the epidemiology of <italic>M. hyorhinis</italic> in swine populations in the US. Such information will contribute to the better implementation of control and prevention strategies for this pathogen. The lack of accurate and thoroughly evaluated diagnostic tools represents a major limitation to study the dynamics of <italic>M. hyorhinis</italic> infection in swine populations. A high-throughput, sensitive and specific quantitative PCR was developed for the detection of <italic>M. hyorhinis</italic> DNA in diverse clinical samples. The qPCR showed suitable precision within and between runs. The qPCR will be a valuable tool to characterize infection dynamics and to quantify bacterial load in order to evaluate selected intcccerventions targeting <italic>M. hyorhinis</italic>. Essential information, such as the prevalence of pigs in commercial populations is absent. The prevalence of <italic>M. hyorhinis</italic> infection was estimated in different age groups across three commercial swine populations by means of a cross-sectional study. The results consistently showed an extremely low prevalence in the sows and suckling piglets. The prevalence of nasal infection after weaning increased in two distinct patterns. In two herds a high proportion of PCR-positive pigs were detected shortly after weaning while in the third herd the prevalence of nasal infection remained extremely low until the last week in the nursery when all pigs tested PCR-positive. In addition, this study provided the opportunity to confirm the role of <italic>M. hyorhinis</italic> as a cause of systemic disease. While <italic>M. hyorhinis</italic> was detected in the upper respiratory tract of both healthy and diseased pigs, it was only detected in systemic sites of diseased pigs and frequently associated with the typical lesions of fibrinous serositis. Information related to <italic>M. hyorhinis</italic> transmission and spread within infected populations is largely unknown. The prevalence and incidence of <italic>M. hyorhinis</italic> in naturally infected pigs and the relationship between sow infection and serological status with piglet infection was evaluated through a longitudinal study. While the prevalence of <italic>M. hyorhinis</italic> infection observed in suckling piglets and sows was low (<5%), over 90% of the pigs became colonized during the nursery phase. Moreover, the incidence rate estimations demonstrated a relatively high number of pigs acquiring the microorganism within a short period of time, indicating that a rapid transmission occurred after placement in the nursery.The development of an MLST-s typing scheme for epidemiological and genetic characterization of <italic>M. hyorhinis</italic> field isolates was described. This study demonstrated genetic variation within <italic>M. hyorhinis</italic> strains circulating in US swine production systems. While no clear relationships were observed between ST and state of origin, sample type and pig age, the majority of isolates from each system clustered together. The application of the MLST-s protocol described will allow studying <italic>M. hyorhinis</italic> epidemiology and dynamics of infection in U.S swine herds. Molecular typing will also aid in better understanding transmission routes, in assessing sources of infection and in evaluating interventions such as vaccination and use of antibiotics.The information generated in this thesis provided essential <italic>M. hyorhinis</italic> epidemiological information applicable to modern US swine production systems. Forthcoming <italic>M. hyorhinis</italic> research could focus on identifying risk factors for pathogen transmission and disease manifestation. The tools developed, evaluated and presented here can aid in such studies. Moreover, new information on the prevalence and incidence of infection could be used to better target control measures such as antibiotic and vaccine administration.