Milk L-lactate concentration is increased during mastitis Stephen R Davis 1 * †, Vicki C Farr 1 , Colin G Prosser 1 , Gina D Nicholas 1 , Sally-Anne Turner 1 , Julian Lee 2 and Alan L Hart 2 1 AgResearch Ltd, Ruakura Research Centre, P.B. 3123, Hamilton, New Zealand 2 AgResearch Ltd, Grasslands Research Centre, Palmerston North, New Zealand Received 31 March 2003 and accepted for publication 2 June 2003 A study was undertaken in cattle to evaluate changes in milk L-lactate in relation to mastitis. A healthy, rear quarter of the udder of each of ten cows in mid-lactation was infused with 1000 colony-forming units (cfu) of Streptococcus uberis following an afternoon milking. Foremilk samples were taken at each milking from control and treated quarters and antibiotic treatment was applied following the onset of clinical mastitis or after 72 h. One cow did not become infected. Six quarters showed clinical symptoms of mastitis within 24–40 h and this was associ- ated with a more than 30-fold increase in milk L-lactate (to 3 . 3mM) and an increase in somatic cell count (SCC) from 4 . 5r10 3 to 1r10 7 cells/ml. Three cows were subclinical, with cell counts ranging from 1 . 5r10 6 to 1r10 7 cells/ml. In these animals, milk lactate ranged from 0 . 7 to 1 . 5mM in the infected quarters up to 40 h post-infection, compared with less than 0 . 1mM in control quarters. Milk was examined from 137 cows in mid-lactation which were known to have mastitis. Foremilk samples were taken aseptically from control and infected quarters of cows on commercial farms. Mean milk L-lactate concentrations and SCC were 0 . 14±0 . 02 mM and 1 . 85± 0 . 3r10 5 cells/ml, respectively, in control (bacteriologically negative) samples. However, L-lactate concentrations exceeded 2 . 5mM in the presence of some types of infection, the level of the lactate response being closely related to the impact of the infection on SCC. L-Lactate con- centrations were relatively elevated in milk samples taken post partum, declining from 0 . 8 to 0 . 14 mM over the first few days of lactation. In conclusion, milk L-lactate has potential as an indicator of clinical and subclinical mastitis in dairy cows. Keywords : Mastitis, milk lactic acid, lactate, somatic cell count, serum amyloid. There are only two reports on the measurement of lactate in mammary secretions. Morr et al. (1957) determined the concentration of lactate to be around 0 . 1mM in raw milk. Mackie et al. (1977) found a relatively slow but substantial increase in lactate in milk during mammary involution with concentrations reaching approximately 5 mM, 5 d after the last milking. The aim of the present study was to evalu- ate firstly, whether changes in milk L-lactate (hereafter re- ferred to as lactate) concentration occurred during mastitis, and secondly, whether milk lactate measurement had any potential as a diagnostic of mastitis. Mastitis is usually detected from clinical signs such as milk clots in foremilk, udder inflammation and from changes in somatic cell counts (SCC). Difficulty in detection of mastitis has led to the development of several manual, automated or semi-automated methods, alone or in combination, to detect infections (Douglas et al. 1997). Such methods can be as simple as clot detection on fil- ters or can involve relatively complex systems measuring changes in electrical conductivity in milk (see for example, Hillerton & Walton, 1991; Milner et al. 1996; Woolford et al. 1998). In addition, there are a wide range of bio- chemical indicators of infection that have been evaluated, at least in research environments (Kitchen, 1981 ; Hold- away et al. 1996). Recent examples of these include nitric oxide (Bouchard et al. 1999) and the acute phase proteins, notably serum amyloid A (Hirvonen et al. 1999). In the context of milk lactate being a potential diag- nostic measurement for mastitis, Mayer et al. 1988 reported that milk oxygen concentrations were reduced during mastitis suggesting that, given a favourable supply of sub- strates, the metabolism of somatic cells in milk and/or the mammary epithelium may become anaerobic leading to release of lactate into milk. Others have reported that *Present address : ViaLactia Biosciences (NZ) Ltd, P.O. Box 109-85, Newmarket, Auckland, NZ †For correspondence ; e-mail : steve.davis@vialactia.com Journal of Dairy Research (2004) 71 175–181. f Proprietors of Journal of Dairy Research 2004 175 DOI: 10.1017/S002202990400007X Printed in the United Kingdom