No abstract

Compared with clinical mastitis, the subclinical form of mastitis (SCM) is more common and thought to cause more economic losses to the dairy industry. The current study aimed to investigate the prevalence, risk factors of SCM, and effects on reproduction of dairy cows in major milk-producing areas of Sri Lanka. A total of 1,357 cows of selected farms in 3 regions were examined in the study. California Mastitis Test was conducted for individual cows, and a score of 2 or more for any quarter without any clinical symptoms and abnormalities in milk was considered as positive for SCM. Samples from infected animals were collected and subjected to bacteriological analysis. A pretested questionnaire was used to collect data on individual cows and herds. Risk factors associated with SCM were analyzed using binary logistic regression in generalized linear mixed models. The effect of SCM on calving to conception interval and days from calving to artificial insemination were analyzed by survival analysis using Cox's proportional hazards regression and Kaplan-Meier survival function estimates, respectively. A Poisson regression model was run to determine the effect of SCM on number of artificial inseminations per conception. The prevalence of SCM was 57.5, 11.8, and 45.5% in the regions A, B, and C, respectively. The most common pathogen was Staphylococcus aureus, with 87.1, 56.5, and 92.3% in the regions A, B, and C, respectively. Logistic regression analysis showed that parity, farming system, milking area, region, and herd significantly affect the prevalence of SCM. Subclinical mastitis during the pre-breeding period was associated with 14% increase in the chance of having a greater number of artificial inseminations per conception. Likewise, median days from calving to artificial insemination was longer in cows with SCM compared with normal cows (79 and 64 d, respectively). Therefore, SCM affected the inseminated proportion of studied cows. However, SCM was not associated with the calving to conception interval. The results revealed that the cow factors and milk hygiene play a significant role in the prevalence of SCM.

Mastitis is the economically most important disease in lactating cows and the prevalence under any management condition is considerably high. It causes economic losses due to reduction of both quantity and quality of milk. The groups of microorganisms causing mastitis are categorized as bacteria, fungi, mycoplasma and nocardia. Among the several cow side tests to trace intra-mammary infections (IMI) at early stage, i.e.sub-clinical mastitis (SCM), California Mastitis Test (CMT) is commonly used in which somatic cell count (SCC) is indirectly taken into account. The SCC of milk is an indicator of mammary infections because SCC positively correlates with the severity of infection. The SCC of >200,000 cells/ml is considered to be an indication of IMI. However, SCC in the milk can also vary with some other factors such as breed, age of the cow, stage of lactation, body condition score, etc. A few studies have shown that high SCC in milk affect the composition, organoleptic properties and keeping quality of raw milk and heat treated milk, yoghurts and cheese. One could argue that low SCC milk (sub-clinical mastitis) will not have a significant effect on product quality. But it should be emphasized that the natural infection occurs with various types of microorganisms that can precipitate product defects despite the low SCC. Also, attention must be paid to the bulk tank somatic cell count (BTSCC) rather than individual animal SCC. The quality of raw milk collected from different parts of the country is reported to be low with high bacteria counts mainly due to unhygienic milking and field practices. Milk quality directly influences the income of the small scale milk producers which inturn affects the sustainable dairy production. In Sri Lanka the majority of dairy farmers are small scale producers and they practice minimum milk hygiene practices compared to medium and large scale producers. Therefore, it is essential to make them aware of hygienic milking practices and implement milk quality based payments (MQBP) with added premium and penalties for the existing milk price, with the objective of encouraging clean milk production.

The assay procedure for N-acetyl-β-D-glucosaminidase (NAGase) was considered to be the most reliable, simple, and rapid enzymatic method for estimating the severity of mammary tissue damage. This study was carried out to assess the relationship of NAGase enzyme activity, California mastitis test score (CMTS) and somatic cell counts (SCC) in milk and the influence of breed, parity and age of the cow on NAGase activity. The results showed that prevalence of sub-clinical mastitis (SCM) was 74.5% (41/55) in the population of cows tested. The isolated pathogens were Staphylococcus spp. (34%, 14/41), Escherichia coli (34%, 14/41) and Streptococcus spp. (32%, 13/41). The SCC showed a significant (p<0.05) positive correlation with the activity of NAGase (R2= 0.796) and CMTS (R2= 0.709). A significant correlation was observed between the NAGase activity and SCC in SCM positive milk, where the correlation was stronger (R2= 0.748) in the former than in the latter. The parity (R2= 0.843) and age (R2= 0.758) of the cow also showed a significant (p<0.05) positive correlation with enzyme activity. The mean SCC and enzyme activity of different crossbreds of the study population did not show a significant correlation, but mean values of SCC and enzyme activity were higher when an animal was phenotypically more related to European breeds (779-1848 x103 SCC/ml;0.64-0.89 µmole/min/ml) than tropical breeds (427-534x103 SCC/ml; 0.26-0.43 µmole/min/ml). It could be concluded that, the prevalence of SCM in crossbred cows in NorthWestern Province of Sri Lanka is considerably high. The determination of NAGase enzyme activity in milk could be used as a method for early detection of SCM more accurately than CMTS and SCC.