Antimicrobial resistance
Although 90% of BRD relapses are reported to receive retreatment with a different class of antimicrobial in the USA, studies examining the impact of antimicrobial selection (i.e. bactericidal or bacteriostatic) on retreatment outcomes and the emergence of AMR are deficient in the literature. A survey was conducted by Coetzee et al (2019) (PLOS ONE doi.org/10.1371/journal.pone.0219104) to determine the association between antimicrobial class selection for treatment and retreatment of BRD relapses on antimicrobial susceptibility of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni. Pathogens were isolated from samples submitted to the Iowa State University Veterinary Diagnostic Laboratory from January 2013 to December 2015. A total of 781 isolates with corresponding animal case histories, including treatment protocols, were included in the analysis. Original susceptibility testing of these isolates for ceftiofur, danofloxacin, enrofloxacin, florfenicol, oxytetracycline, spectinomycin, tilmicosin, and tulathromycin was performed using Clinical and Laboratory Standards Institute guidelines. Data were analysed using a Bayesian approach to evaluate whether retreatment with antimicrobials of different mechanistic classes (bactericidal or bacteriostatic) increased the probability of resistant BRD pathogen isolation in calves. The posterior distribution calculated suggests that an increased number of treatments is associated with a greater probability of isolates resistant to at least one antimicrobial. Furthermore, the frequency of resistant BRD bacterial isolates was greater with retreatment using antimicrobials of different mechanistic classes than retreatment with the same class. Specifically, treatment protocols using a bacteriostatic medication first followed by retreatment with a bactericidal medication were associated with a higher frequency of resistant BRD pathogen isolation. In particular, first treatment with tulathromycin (bacteriostatic) followed by ceftiofur (bactericidal) was associated with the highest probability of resistant M. haemolytica among all antimicrobial combinations. While one would hope ceftiofur was not now being used in this way in the UK, these observations suggest that consideration should be given to antimicrobial pharmacodynamics when selecting drugs for retreatment of BRD.
Surveillance studies are needed in the UK to determine the clinical relevance to antimicrobial stewardship programmes if the effects of prescribing decisions on AMR are to be fully understood.
Bovine TB transmission
Disease-causing microbes that infect more than one type of animal can be difficult to control. This is especially true when they infect wildlife. For example, Mycobacterium bovis causes bTB in tens of thousands of cattle in Britain every year and also infects badgers and other wildlife. Controlling the infections in cattle is essential, as it helps prevent the bacteria from infecting humans, improves cattle welfare and reduces the substantial costs to the livestock industry. M. bovis persists in cattle populations, often where potential wildlife reservoirs exist. However, the relative contribution of different host species to bTB persistence is generally unknown. In Britain, the role of badgers in infection persistence in cattle remains contentious, despite decades of research and control efforts.
A study by Crispell et al (2019) (eLife doi. org/10.7554/eLife.45833) applied Bayesian phylogenetic and machine learning approaches to bacterial genome data to quantify the roles of badgers and cattle in M. bovis infection dynamics in the presence of data biases. They showed that complex patterns of contact between cattle and badgers likely drive the persistence of bTB. In three separate analyses, Crispell et al compared the genomes of M. bovis found in cattle and badgers, the animals’ locations, when they were infected, and whether they could have been in contact. The analyses found that M. bovis was likely to have been transmitted more frequently from badgers to cattle (10.4x in the most likely model) than from cattle to badgers. They also showed that transmission within each species happened more often than transmission between species. If representative, these results support the case for control operations targeting both cattle and badgers.
Digital dermatitis in grazed cattle
Bovine digital dermatitis (DD) is an important infectious cause of cattle lameness worldwide that has become increasingly prevalent in New Zealand pastoral dairy herds. In a study by Yang et al (2020) (Veterinary Research doi.org/10.1186/s13567-020-00750-8), a simplified DD scoring system after considering both M and Iowa DD scoring systems was applied to explore the transmission dynamics of DD in a typical spring calving pastoral New Zealand dairy herd. The modified model only included three categories: normal skin, early stage lesions and advanced lesions. Lesions regressing after treatment were excluded as DD lesions are rarely treated in New Zealand. Furthermore, sub-classes within each lesion class were not defined due to the lack of variability in DD lesion presentations in New Zealand. The model was validated based on longitudinal field data from three dairy herds in the Waikato region during one lactation season (2017–18). The model suggested that in infected dairy herds, although DD prevalence will tend to increase year-on-year it is likely to remain relatively low (<18%) even after 10 years of within-herd transmission. It is likely that the low transmission rate during the late lactation results in more cases resolving than developing during this period and therefore results in the low prevalence of infectious cattle at the start of each subsequent lactation. Cattle with advanced lesions had a stronger influence on the establishment and maintenance of DD than cattle with early stage lesions highlighting the importance of targeting these animals for intervention. In New Zealand at least, on-going monitoring of DD is recommended to assess the long-term progression of the disease in grazing herds.