Septic arthritis in western Canadian feedlot cattle: a comparison of antimicrobial treatment options and a description of the bacterial communities present in infected joints

Septic arthritis (SA) presents health, welfare, and production concerns in western Canadian feedlots. Septic arthritis has the highest case fatality rate of lameness causes and little information exists on treatment options. Mycoplasmopsis bovis is often the primary etiological agent of SA in feedlot cattle and H. somni has also been implicated. Furthermore, animals with SA often have concurrent chronic pneumonia. As a result, SA treatment protocols commonly recommend parenteral administration of antibiotics that have label claims for bovine respiratory disease (BRD) but with little efficacy information available for treatment of SA. The objectives of this two-year investigation were to 1) compare the health outcomes of three commonly administered antibiotics for the treatment of SA and 2) determine the microbiota of synovial fluid collected from SA joints. The treatment study consisted of trained pen riders identifying SA cases from the fall-placed calf population over a two-year period at four commercial feedlots. Cases were randomly allocated to three antibiotic treatments: florfenicol (FFN), oxytetracycline (OXY), and tulathromycin (TUL). The health outcomes monitored to 90 d post allocation included SA relapse treatments (first and second), BRD treatment, infectious mortality (BRD, H. somni, SA), and SA only mortality. Additional outcomes were early shipment for salvage slaughter (railer) and the sum of infectious mortality events and railer events (total fall out). In order to describe the microbiota of synovial fluid from SA cases, synovial fluid samples (n = 137) were collected over the same two-year period in the fall placed calf population at 11 commercial feedlots. The samples were collected via arthrocentesis from joints with and without SA and from antemortem and postmortem specimens. After collection, the samples were frozen at -20°C and then -80°C until DNA extraction process was completed. Samples underwent 16S rRNA gene sequencing using an Illumina platform and analysed using DADA2, QIIME2R, and the R-package. Amplicon sequence variants (ASVs) were based on 99% similarity and taxonomy was assigned using the SILVA reference database. Samples with less than 20,000 reads were omitted from the final analysis. In total, 251 eligible cases were allocated to the treatment comparison and no differences (P > 0.05) were noted for the morbidity outcomes of first and second SA relapse and BRD. Likewise, no differences (P > 0.05) were noted in the mortality outcomes, railer events, or total fallout. Time to event analysis was conducted with no differences (P > 0.05) noted for any of the health outcomes. From the original 137 synovial fluid samples, 66 met technical criteria to be included in the final analysis. Mycoplasmatota accounted for 96.6% of the phyla abundance and Mycoplasmopsis accounted for 94.8% of the abundance at the genus level. Samples from SA cases were noted to have a higher abundance of Mycoplasmopsis (P=0.033) and a lower Shannon diversity index compared to controls (P=0.018). Postmortem samples had a higher abundance of Mycoplasmopsis (P = 0.015) and a lower Shannon diversity index than antemortem samples (P = 0.001). Only 8.5% of the ASVs assigned to Mycoplasmopsis were assigned to the species level with M. alkalescens and M. wenyonii present in 10 and 2 samples, respectively. Histophilus did not rank in the top 15 genera found and the Pasteurellaceae family (of which Histophilus belongs) only accounted for 1% of ASV assignments. The lack of differences in health outcomes by treatment indicate that there is no economic advantage to using a particular treatment; the most cost effective product would be the least expensive to purchase. A study limitation was the lack of production performance data, which may have revealed differences by treatment. The lack of difference observed in the three antimicrobial products also introduces a question of antimicrobial efficacy for SA, it is possible that the products were equally ineffective. Conclusions from the microbiota study of joints with SA in the western Canadian feedlots are that Mycoplasmopsis is the predominant genera. Also of interest was that Histophilus was not found in the top 15 genera from the samples in this study. The lack of Histophilus is surprising because it has been considered an etiological agent for feedlot SA cases. The presence of M. alkalescens and M. wenyonii in SA joints suggest further research is needed to understand the impact of Mycoplasmopsis species other than M. bovis in the role of SA.

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