Effect of abiotic and biotic factors on subclinical mastitis occurrence in low-input dairy sheep production systems

Abstract

This study aimed to explore the effect of abiotic [microclimate, management (MS) and milking system (MLS)] and biotic risk factors (microorganism group, age class of ewes, lactation month and ewe genotype) on the occurrence of subclinical mastitis in low-input dairy sheep farms. Samplings were carried out in 10 extensively and 10 semi-intensively managed low-input dairy sheep farms, with three milking systems: milking parlour machine (MPM), portable milking machine (PM) and hand milking (HM). Over two consecutive lactations, milk samples (n = 9624) were collected monthly in each farm, from 20 ewes (10 primi- and 10 multi-parous ewes). The ovine major histocompatibility complex (MHC) DQA2 gene was genotyped for all sampled ewes (n = 800). Ambient air temperature and relative humidity were recorded hourly at each farm, in order to calculate the temperature-humidity index (THI) affecting each sampled sheep. Daily milk yield (DMY) per ewe, pH, total bacterial count (TBC) and somatic cell count (SCC) of milk samples were measured. The fixed effects of microclimate, MS, MLS, bacterial pathogen (microorganism group), age class of ewes, lactation month, involved in milk SCC and subclinical mastitis occurrence, were explored with linear mixed-effects (LME) models. Milk samples with SCC 400 × 103 (cells/mL) and ≥250/mL Colony Forming Units of one or two udder pathogens (in the microbiological examination) were defined as subclinical mastitis positive. Subclinical mastitis was found in 53 % (n = 1336) of microbiologically examined milk samples, with Staphylococcus spp. being the most prevalent amongst isolated pathogens. A positive correlation was found between SCC and TBC values (r = 0.687, p < 0.001, df = 1598). LME models revealed that subclinical mastitis prevalence was related to the age class of ewes (p < 0.05) and lactation month (p < 0.001), but not to MS or MLS (p> 0.05). MLS (p < 0.01), age class of animals (p < 0.05) and lactation month (p < 0.05) affected SCC of milk samples. The LME model, with SCC milk values ≥400 × 103 as dependent variables, revealed a significant effect of microclimate (β = 0.014, SE = 0.004, p < 0.001) and microorganism group [F(1,162) = 4.29, p < 0.001] on SCC. The MHC DQA2 allele B1 ("0602") was found at a lower frequency in ewes with subclinical mastitis (p < 0.05). In conclusion, microclimate and MLS (from the abiotic factors) and microorganism group, age class of ewes and lactation month (from the biotic factors) had a significant effect on milk SCC, while age class of ewes and lactation month (from the biotic factors) had a significant effect on subclinical mastitis occurrence. To our knowledge, this is the first report exploring the effect of microclimate (THI), PM milking and DQA2 gene on subclinical mastitis of sheep reared under low-input management systems.