Effect of dry cow therapy on AMR
Antimicrobial residues in milk have been discussed as a possible selector for Enterobacteriaceae that produce extended-spectrum β-lactamases (ESBL) in dairy herds. Such residues are found in waste milk after antibiotic treatment of mastitis, but antibiotic dry cow therapy (DCT) might also lead to antibiotic residues in colostrum and in milk during early lactation. While it is known that feeding waste milk selects ESBL bacteria in calves, there was, until recently, little known about the effect of DCT via colostrum, which is supposed to contain much lower antibiotic concentrations than waste milk. In this observational prospective case study on two farms undertaken by Tetens et al (2019) (Journal of Dairy Science doi:10.3168/jds.2019-16659) the authors hypothesized that blanket dry cow treatment with β-lactams would have more selective, increasing, effects on ESBL concentrations than selective, individually chosen, antibiotic DCT. Thus, they compared concentrations of ESBL-producing Enterobacteriaceae in faeces of calves (n = 50) at two dairy farms with different management of antibiotic dry cow therapy.
Considerably higher concentrations of ESBL-producing Escherichia coli were observed in blanket antibiotic dry cow therapy on day 3 of the calf 's life (7.6 vs. 5.3 log cfu/g of calf faeces). Both farms used narrow-spectrum penicillin combined with aminoglycosides for drying off, and the majority of ESBL isolates (93%) were co-resistant to aminoglycosides. No waste milk was fed to calves and no calf was treated with β-lactam antibiotics or aminoglycosides during the first 3 days of life, thus differences were considered to be associated with different frequency of antibiotic dry cow therapy on farms (19 of 25 mother cows on farm A, 9 of 25 on farm B). Even though the presumable selection effect of antibiotics used in DCT decreased within the next 3 weeks, the authors concluded that this result emphasises the need for the reduction and prudent use of antibiotic DCT.
Mycoplasma bovis in colostrum
Staying with colostrum, as well as its incredible benefits to calf health, we all know there are also disease risks. In herds with Mycoplasma bovis infection, colostrum is often considered infectious. However, in contrast to milk, the presence of M. bovis in colostrum has not previously been evidenced. In a survey by Gille et al (2020) (Veterinary Researchdoi.org/10.1186/s13567-020-00778-w) the presence of M. bovis DNA was determined with real-time PCR in 368 colostrum samples from 17 herds recently infected with M. bovis. Only 1.9% of the samples tested positive, with 13 herds having no positive samples and an over-all within-herd prevalence of 3.2% (SD: 4.9%; range: 0–30.0%). These results show that in infected herds M. bovis DNA can be retrieved in colostrum. However, to what extend colostrum is infectious remains to be determined.
Drinking water and the calf microbiome
In the UK access to water from birth is considered a welfare requirement for calves, however this is not the case in all countries and even here very young calves can sometime be found without water. Wickramasinghe et al (2020) (Journal of Dairy Sciencedoi.org/10.3168/jds.2019-17258) have previously demonstrated that dairy calves having access to drinking water from birth (W0) achieved greater body-weight, fibre digestibility, and feed efficiency than those that first received drinking water at 17 days of age (W17). Since gut microbiota composition could be linked to growth and development of animals, the objective of this study was to examine the effect of offering drinking water to newborn calves on composition of bacteria in the gut using a faecal microbiota analysis. Fresh faeces were collected directly from the rectum of calves in W0 (n=14) and W17 (n=15) at 2, 6, and 10 weeks of age. All calves were fed pasteurised waste milk, weaned at 7 wk of age, and offered tap water according to the treatment. The DNA was sequenced using 16S rRNA gene-amplicon sequencing. The sequences were clustered into operational taxonomic units (OTU) with a 99% similarity threshold. Treatment effects on α-diversity indices and relative abundance of the 10 most abundant genera were analysed. Statistical significance of treatment effects on the 50 most abundant OTU was determined. At 2 wk of age, W0 had a greater number of observed OTU (5908 vs. 4698) and species richness than W17. The number of OTU and richness indices increased from wk 2 to 6, but the increment of W17 was greater than that of W0. The Firmicutes to Bacteroidetes ratios were also similar at every time point but decreased markedly when calves were weaned. The relative abundance of genera Faecalibacterium and Bacteroides was greater in W0 than W17 at 2 wk of age. The genus Faecalibacterium continued to be more abundant in W0 than W17 at 6 wk of age but had similar abundance 3 wk after weaning (10 wk of age). The abundance of Faecalibacterium at wk 6 was positively correlated with apparent total-tract digestibility of acid detergent fibre at 10 wk of age. Calves receiving water since birth had greater abundance of OTU related to Faecalibacterium prausnitzii, and Bifidobacterium breve at 6 wk of age. These species are known to improve growth in preweaned calves. The abundance of none of the genera and OTU was different between W0 and W17 at 10 wk of age. Overall, beginning to offer drinking water at birth has the potential to modulate gut microbiota composition and thereby positively affect performance of young dairy heifer calves ≤10 wk of age.