Abstract
Aim/background
Mastitis may be caused by microbial, thermal or physical causes and is one of the most negatively impacting problems in dairy industry worldwide. Streptococcus agalactiae (S. agalactiae) has been associated with both clinical and subclinical mastitis, with the potential to cause a drop in milk yield and quality from an infected cow, also with suspected zoonotic potential. The bacteria have been reported to spread easily from infected to susceptible cows’ udders during milking. In Nigeria, mastitis prevalence has been reported to range from 8.10 to 85.33% in the Northwestern zone of the country. However, it is not clear what proportion of this prevalence is associated to S. agalactiae as there are few publications in this area. The pathogen has been reported as a major pathogen of mastitis and the second most predominately isolated Streptococcus species (next to Streptococcus uberis) from mastitis cases in Nigeria. This paucity of information on the prevalence and risk factors of S. agalactiae associated with mastitis may affect the achievements of the National Control Strategies on bovine mastitis. This work aimed to understand the status of subclinical mastitis in Zaria and the proportion of subclinical mastitis associated with S. agalactiae and to identify some possible risk factors associated with the disease in lactating cows in the study area.
Methods
A cross-sectional study, involving 400 lactating cows from Fulani herds, Institutional, and Commercial farms, was carried out to estimate the status of subclinical mastitis and the proportion of S. agalactiae- associated subclinical mastitis in Zaria and its neighborhood. California mastitis test was used to screen the milk samples and S. agalactiae was isolated using plate culture on blood agar media. The phenotypic characteristics of the pathogen was appreciated using microscopy. Lactose, maltose, mannitol and sorbitol (Oxoid, Basingstoke, UK) were used to test for the ability of S. agalactiae to ferment sugars in Sugar fermentation test. Lancefield Sero-grouping of the isolates was performed using latex agglutination test in broth method described by the Manufacturer (Oxoid, Basingstoke, UK) for grouping of the Streptococcus into groups A, B, C, D, F and G.
Results
Prevalence of 29.8% for subclinical mastitis and 7.5% for S. agalactiae associated with subclinical mastitis in Zaria and its neighborhood were obtained. Risk factors such as herd size, breed, age, parity, previous history of mastitis and management type were studied, however, only parity and previous history of mastitis had significant association with S. agalactiae infection in this study. Primiparity had protective effect on infection with S. agalactiae {OR: 0.1, CI (0.0-0.5)}. This means primiparous cows are less likely to be infected with S. agalactiae than multiparous. Also, cows with previous history of mastitis were nine times more likely to come down with mastitis than those without history of previous mastitis (OR 9.3, CI (3.8-22.5).
Conclusion
This study revealed the persisting challenge of S. agalactiae-associated subclinical mastitis in Zaria and its Neighbourhood, which necessitates the development of a better management plan, especially for dairy farms with large herd size and also for those with multiparous cows

Key words: bovine, subclinical mastitis, risk-factors, Streptococcus agalactiae.