Mastitis is a multi-etiologic disease of the mammary gland characterized mainly by reduction in milk production and considered an economically important disease in the dairy subsector in developed and developing nations. The results from this study show a high prevalence of subclinical mastitis; a finding that was comparable to what was previously reported both in the country [33–35] and elsewhere [36–39]. The overall high prevalence could be attributed to the invisible and silent nature of subclinical mastitis which is usually given little attention by farms when it comes to treatment, and the lack of mastitis control program which has been proved to decrease the prevalence of mastitis in developed countries . However, the prevalence of subclinical mastitis reported in this study is lower than those reported in Uganda , Rwanda [31, 42, 43], Kenya , Ethiopia  and Vietnam . It has been pointed out by previous authors that subclinical mastitis is a complex disease and discrepancies in the reported incidences in different studies could be attributed to differences in animal breeds, management systems, parity, teat morphology, and milking hygiene [46, 47].
The bacterial pathogens isolated in the present study were dominated by Staphylococcus aureus (Table 2). The prevalence rate of S. aureus (66.15%) in the current study agrees with previous findings reported in Tanzania [34, 35, 48], Rwanda , Algeria , Italy  and Brazil . However, the prevalence of S. aureus in mastitic cows reported here is higher than 25.7% previously reported in Tanzania , 40% in Morocco , 24.1–48.75% in Ethiopia [39, 54] and 30.6% in Kenya . The high rate of isolation of S. aureus may be attributed to the fact that S. aureus easily localize the skin of the udder, and has the capacity to penetrate into the tissue, producing deep seated foci protected by a tissue barrier . This high frequency of staphylococcal mastitis is considered to be due to poor milking hygiene and lack of proper attention to the health of the mammary gland in general. Thus, hygiene at milking is of paramount importance in control of these infections because they are spread during the milking process causing intramammary infections.
The findings of the current study have shown a low prevalence (13.85%) of E. coli, the coliform environmental bacteria associated with wet and muddy conditions. These findings corroborate with those previously reported in Tanzania  and Rwanda . E. coli have been reported to be of more importance in clinical mastitis than subclinical mastitis, despite the environmental factors . This confirms that the prevalence of coliform bacteria in this study would be low, as few samples were of clinical mastitis. Although environmental Streptococcus agalactiae (6.5%) were ranked third in the current study, the microorganism have been reported as the most prominent bacteria in cow mastitis in some countries [26, 45]. The prevalence rate of S. agalactiae reported in the current study agrees with previous findings observed elsewhere [35, 59]. Bacillus spp., were present in 6.15% of cases in the current work. Bacillus spp. has also been identified as important pathogens in both CM and SCM in previous investigations [60, 61]. Nonetheless, these results contradict other studies showing that mastitis caused by Bacillus spp. is rare in dairy cows [36, 62]. The presence of Bacillus spp. could be explained by the poor hygienic conditions of milkers as the bacteria are widely distributed in dairy environment, including on teat skin, milkers’ skin and gloves, and farm floors, which represent reservoirs of bacteria associated with intramammary infections [61, 63].
The estimated financial milk losses per farm associated with the presence of subclinical mastitis in this study ranged from 1,326,000/ to 7,446,000/= Tanzanian shillings (577.00–3,239.00 US Dollar) while the average loss attributed to mastitis per quarter was estimated at 49,320/= Tshs (21.45 US Dollar). These estimated losses are in agreement with previous studies [20, 64, 65]. Subclinical mastitis is an invisible problem that can cause financial losses to producers through the reduction of milk production, lowered milk quality, discarded milk, veterinary fees, drug costs and extra labor to farmer [16, 17, 66, 67]. Estimation of the financial impact observed in this study could be used to advocate for the implementation of prevention methods that reduce the impact of subclinical mastitis in Tanzanian dairies.
The incidence of retained fetal membranes observed in the present study ranged from 4.0 to 16.4% (average of 10.35%). This finding is in agreement with other studies conducted in the country  and in other countries [2, 6, 8, 69]. Some other authors have however reported higher incidences than the observation made in the current study [10, 14, 70]). Nevertheless, significantly lower incidences of RFM have also been reported in Bangladesh crossbred and local cattle  and Ethiopian dairy cattle [72, 73]. The differences in the incidences of RFM reported by different authors could be attributable to various factors, such as age, breed, heredity, environment, hormonal status, nutrition, and immunity.
Higher incidences of retained fetal membranes cause considerable economic losses to the farm . Since the direct costs associated with RFM are difficult to derive because of a multitude of influences [75, economic losses are normally measured through establishing indirect costs to farmers resulting from periparturient disease, decreased milk production, extension of calving interval, reduced pregnancy rates, and increased risk of culling . In this study, the mean number of services per conception in normal control group was 1.9 ± 0.5, whereas the corresponding value for the retained fetal membrane group was 2.9 ± 1.0, which differed significantly (P < 0.05), and corroborated with findings reported elsewhere [8, 10, 11, 14, ]. Open days was longer (P < 0.05) in cows exhibiting retained placenta as compared to normal cows by about 507 days (Table 4). This is because more than 50% of animals that calved normally had days open within the three months post-partum, while more than 70% of cows with retained placenta conceived after 150 days. This finding concurs with the results reported by other researchers [8, 10]. Calving interval was significantly longer (P < 0.05) in cows exhibiting retained placenta as compared to normal cows by about 60 days. A similar trend has been reported by other authors [3, 14, 10]. Retention of placenta is associated with secondary uterine infections, which may be related to probability of subsequent conception . Cows with retained fetal membranes had longer intervals from calving to first service and to conception and required more services per conception and lower pregnancy rate and conception to first service.