TRENDS IN UDDER HEALTH AND EMERGING MASTITIS-CAUSING PATHOGENS IN SOUTH AFRICAN DAIRY HERDS.

Discipline: mastitis; 

Mastitis remains a major cause of economic loss in South African dairy herds, even though technological and management advances have been made over the past decade. Therefore, knowledge of mastitis-causing pathogens is important. By categorizing them their basic epidemiology can be studied and understood, which could contribute to proactive management in dairy herds. Current information about pathogens is also needed due to changing trends over time. The prevalence of mastitis and its pathogens has been investigated in South Africa since the late 1970s. However, no official national survey has as yet been done to determine trends in prevalence of mastitis, occurrence of udder pathogens and drug-resistant bacteria. During this period the number of dairy herds in South Africa decreased, while herd size increased. Consequently, the risks for the spread of udder pathogens increased due to amalgamation of herds and movement of cows. The aim of this retrospective study was therefore to investigate trends in the bacteria isolated from milk samples obtained in the National Milk Recording Scheme as well as the prevalence of udder health from a national mastitis diagnostic service carried out over the period 1996 to 2007 in South Africa. 

Milk samples were obtained from 7 of the 9 provinces. Although there are limitations to a country wide survey, such as variation in herd size, management skills, parity, milk yield, milking frequency and other parameters, the size of the database helped to get a fair indication of general udder health in South Africa. Cytology and routine bacteriology were performed on 379 000 milk samples of lactating cows and bacteriology on 11 946 samples from non-lactating cows. The results showed that mastitis have not decreased over the test period, although the prevalence and teat canal infection was lowest in 2002. Both mastitis and teat canal infection increased from 2002 to 2006, from 8.1% and 24.1% to 15.4 and 30.0% respectively. The percentage of mastitis-causing pathogens isolated from cows over these years also varied. Previously unknown or almost eradicated pathogens such as α,β haemolytic Staphylococcus aureus, which is thought to be of human origin, Streptococcus agalactiae and Enterococcus canis were responsible for numerous mastitis outbreaks. Coagulase-negative staphylococci were the most frequently isolated bacteria in milk samples from both lactating and dry cows, followed by Staphylococcus aureus and Streptococcus agalactiae. Although Staph. aureus remains the principal mastitus-causing pathogen in South Africa, owing to its chronic nature and resultant economic losses, most cases of mastitis were caused by coagulase-negative staphylococci. This finding increases the importance of coagulase-negative staphylococci (formerly described as a minor pathogen) significantly. Isolations of Streptococcus agalactiae peaked between 2000 and 2005 and decreased again by 2007. Coagulase-negative staphylococcal isolates increased from 2002 and were still on the increase in 2007. Streptococcus agalactiae, Streptococcus uberis and Enterococcus canis were isolated more frequently from milk samples of lactating cows compared with dry cows, while Enterococcus faecalis was isolated more frequently from dry cow samples. 

In conclusion: Despite the limitations of descriptive studies, they have important practical applications for the evaluation of the efficacy of udder health management programmes on dairy farms. Changes in udder pathogens that occurred over the 11-year period (1996 to 2007) were pronounced. Of particular importance was that the number and type of microorganisms isolated from milk samples and information on the pathogenic-status of the isolates could be determined. It also became evident that contagious mastitis was still responsible for about 50% of mastitis cases. Therefore the dairy industry must be alert to the increasing risk of zoonosis, which makes it crucial that such a study should be repeated soon.

Reference: 

Petzer I-M, Karzis J, Watermeyer J C, van der Schans T J, van Reenen R, 2009. Present trends in udder health and emerging mastitogenic pathogens in South African dairy herds. Journal of the South African Veterinary Association 80: 17–22.