Bovine mastitis in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science at Massey University, Palmerston North, New Zealand

Abstract

This thesis represents an aggregation of knowledge on bovine mastitis in the New Zealand dairy industry. Firstly, the thesis reviews the factors influencing the economic impact of bovine mastitis. Secondly, it provides information on the incidence of clinical and subclinical mastitis, as a prerequisite for estimating these costs. Thirdly, it investigates the effects of experimentally-induced Streptococcus uberis mastitis early in the dry period on milk production in the subsequent lactation. In the review of factors influencing the cost of bovine mastitis, it was clear that neither farmers nor farm advisors have a good understanding of its full economic impact. In order to better understand these costs, it is necessary to have a clear idea of the incidence and consequences of clinical and subclinical mastitis: areas of knowledge which were identified as being deficient. Hence, two studies were conducted to investigate these areas. In the first study, the incidence of clinical mastitis in Northland, New Zealand, was estimated. Furthermore, the aetiological agents causing mastitis were elicited and their chronological distributions in lactation were described. The average incidence of clinical mastitis was 0.19 cases per 305 cow-days at-risk, which is higher than previously reported in New Zealand. There were approximately equal numbers of isolations of Staphylococcus aureus (23.7%), and Strep, uberis (23.3%) from clinical cases: a pattern that is remarkably different to elsewhere in the country. Clinical mastitis due to S. aureus or Strep, uberis differed between age groups, with the highest incidence of S. aureus isolations from older cows (0.043 cases per 305 cow-days-at-risk) and lowest from 2-year old cows (0.014). The incidence of Strep. uberis was similar in first calving (0.034 cases per 305 cow-days-at-risk) and older cows (5 year-old: 0.039 cases, 6 year-old: 0.030 cases). Overall, 12% of cows were temporarily removed from supply and 1% were culled for mastitis. The differences between the study in Northland and these reported elsewhere from NZ highlight the need for a national survey on the aetiology and epidemiology of bovine mastitis. A second study evaluated the effects of Strep. uberis clinical mastitis in the early dry period on milk production in the subsequent lactation. In a previous study, Strep. uberis mastitis was experimentally induced and then promptly treated. This experiment provided a data set from which the impact of Strep. uberis clinical mastitis early in the dry period on milk production in the subsequent lactation could be estimated. Results of this study indicated that an early dry period clinical mastitis due to Strep. uberis, when promptly treated, did not affect production in the subsequent lactation. For cows that suffered mastitis episode during early dry period compared to those that did not, there was no difference in milk yield ( 5126 vs. 5010 litres), fat yield (267 vs. 264 kg), and protein yield (182 vs. 179 kg), respectively. It was considered that the short duration of intramammary infection did not cause permanent damage to the mammary secretory tissue. It was concluded that the current estimates of the economics of mastitis in New Zealand are probably under-estimating the real cost of mastitis to its dairy industry. This was based on the higher incidence of clinical mastitis in Northland than elsewhere in the country and a failure of previous studies to take into consideration the costs associated with animals that were temporarily removed from supply (i.e. rather than culled). Additionally, as the highest frequency of new intramammary infections occurs in the first week or two after drying off, it may prove beneficial for farmers to pay more attention to checking for clinical mastitis during the early dry period.