A Study on the Prevalence of Subclinical Mastitis in Lactating Cows and Associated Risk Factors in Wolmara District, Oromia Regional State, Ethiopia

A Study on the Prevalence of Subclinical Mastitis in Lactating Cows and Associated Risk Factors in Wolmara District, A cross-sectional study was conducted to estimate the prevalence and associated risk factors of mastitis in dairy cows. The present study was carried out to investigate prevalence of subclinical mastitis (SCM) in milking cows and risk factors associated with sub-clinical mastitis. A total of 384 lactating cows were randomly selected to identify the SCM from 12 PAS of Wolmera Woreda and cow information was collected from the farmers’ thought interview. Milk samples were collected aseptically from California mastitis test (CMT) positive cows. Result showed that 172 lactating cows (44.8%) and 358(93.2%) quarters were positive to CMT. The prevalence of SCM was (p < 0.05) significantly high in European crossbreds (56.5%) compared to local breed (36.3%). The highest prevalence found in the age group (4-7) years and the highest prevalence in parity more than 4 year was 37.2%. The prevalence was significantly (p < 0.05) higher in cows with earlier lactation (41.3%). This study indicated a higher prevalence of mastitis linked with several risk factors. Thus, early diagnosis and regular screening of cows for subclinical mastitis together with proper therapeutic management of clinical cases are of paramount importance. Moreover, control and prevention strategies should be designed and implemented with great emphasis given to risk factors to reduce bovine mastitis and its impact on milk production and food security.


Introduction
In Ethiopia, livestock represents a major national resource and form an integral part of the agricultural production system. The country has the largest livestock population of any African country with estimated 43.1 million heads of cattle and cows representing the largest proportion of indigenous cattle of the country [1]. Milk produced from these animals provides an important dietary source for the majority of rural as well as a considerable number of the urban and peri-urban population. However, milk production often does not satisfy the country's requirements due to a multitude of factors, out of which disease of the mammary glands known as mastitis is among the various factors contributing to reduced milk production [2]. Mastitis is defined as an inflammation of the parenchyma of mammary gland, which can reduce milk yield and alter milk composition [3]. There are two main classes of mastitis.
The first is clinical mastitis, which manifests signs observed from the animal or the milk. The other is subclinical mastitis, which produces no visible signs from the udder except when using diagnostic tools.
Since the quality and quantity of the milk is influenced by mastitis, it is considered to be one of the most important causes of economic losses in the dairy industry worldwide. Mammary gland infections cost the US dairy industry approximately 2 billion dollars annually and have a similar impact in Europe [4].
Somatic cells are part of the natural defence mechanism and include lymphocytes, macrophages, polymorphonuclear cells and some epithelial cells [5]. Somatic cell count (SCC) can be measured quantitatively by California mastitis test (CMT). It is a simple, easy and low-cost screening test for subclinical mastitis at dairy farms.
Validity of CMT in diagnosis of infected quarters was established in various milking stages [6,7]. Mastitis is the most frequent diseases of dairy cow and reduces quality of milk. About 140 to 200 USD / Cow/Year ease next to reproductive disorders and one of the major causes for economy failure in Ethiopia [2]. In Ethiopia mastitis prevalence rate was 85.6% and 81.2% using CMT and somatic cell count (SCC), respectively [8]. An over all of 30.2 and 5.5% for subclinical and clinical mastitis respectively [9]. Most of the previous studies in Ethiopia were concentrated on the investigation of the prevalence and few risk factors for mastitis at cow level and no or little effort has been made to assess the prevalence, management and hygienic practices at herd/farm-level. A focused study on contagious mastitis with emphasis on subclinical type is lacking. Besides, teat morphology, which is inherited anatomical feature of the cow that may affect mastitis occurrence and could serve as a marker trait for selection to reduce mastitis in dairy cattle, is less investigated. It is therefore important to assess the effect of the variable on clinical and sub-clinical mastitis under Ethiopian condition and recommend preventive measures to reduce loss attributed to the disease. Moreover, given the huge economic relevance due to lack of clinical visibility and subsequent effects, investigation of sub-clinical mastitis at herd-level is of paramount importance for designing feasible prevention and control strategy [10,11].
Despite high cattle population and milk production potential of the study area, sub clinical mastitis is insufficiently investigated and information relating to its magnitude, distribution and risk factors is scant. Thus, the objectives of the study will be:

a.
To evaluate the prevalence of subclinical mastitis in apparently healthy dairy cows in Wolmera district.

b.
To evaluate associated risk factors affecting on subclinical mastitis

Study Area
The study was conducted in and around Holeta town located 45km away from Addis Ababa in the south west direction, 9° 3' N and 38° 30' E, at an altitude of 2,400m above sea level in central highlands. The area is characterized by mild subtropical weather with minimum and maximum temperature ranging from 2 to 9 °C and 20 to 27 °C, respectively. The area receives annual rainfall of 1060mm [9].

Study Population
A total of 384 dairy cows were examined in different kebeles

Sample Size and Sampling Method
Simple random sampling technique was followed to select the study animal, and the desired sample size had been calculated according to the formula given in [12]. Since there is no previous report on this issue 50% expected prevalence is used.
Accordingly, a total of 384 dairy cows were examined in different kebeles of Wolmera district.

Sample Collection and CMT Test
The udders and especially teats were cleaned and dried before

Physical Examination of Mastitis
Udder attachment, parity number, any physical abnormalities such as swelling of the udder, presence of lesions, anatomical malformations and tick infestation were recorded. The milk was examined for its colour, odour, consistency and other abnormalities.

Data Analysis
The

Different Risk Factors
According to different risk factors categories the highest prevalence was recorded in cross breed, young age group, fourth parity, and first lactation stage (Table 4). There was a statistically significant association (P<0.05) between the infection of sub clinical mastitis and breed, age group, parity, lactation stage, teat and barn hygiene, mammary gland cleaning and teat deep (Table 5).   [14], who reported that the overall mastitis prevalence in the farm was 66.6% in Assella Dairy Farm in Oromia Region, Ethiopia which could be due to lack of awareness, housing and milking practices they are accustomed in the area.
Subclinical mastitis is a complex disease and the prevalence could be affected due to variation in management system, age, milk yield, body condition, parity, environment and other conditions. Thus, these factors might have contributed to the observed differences in prevalence of subclinical mastitis.
The risk factor considered for this study was breed, age group, parity and lactation stage. In the present study from selected potential risk factors breed, age, stage of lactation, and parity had statistically significant effect (P < 0.05) on the occurrence of mastitis which agrees with the report by [15]. Similarly, the environment risk factors considered for this study were teat and barn hygiene, mammary gland cleaning and teat deep. The occurrence of mastitis based on these risk factors showed statistically significant effect with the occurrence of sub clinical mastitis. This was also reported by several investigators to have association with the occurrence of sub clinical mastitis mastitis [16][17][18][19]. The study showed that there were significant statistical associations ( <0.05) with the prevalence of mastitis and the parity number of animals; cows with many numbers of calves were with higher prevalence of mastitis and the risk of subclinical mastitis increases with increasing parity number which agrees with the finding of [19][20][21][22] who found that the risk of clinical and subclinical mastitis increase significantly with increasing parity number of the cow.
The higher prevalence in cows at three and above calved could be due to increase ease of penetration of the teat duct by pathogens and accumulated previous infection [23]. It is postulated that younger animal is less susceptible; through a more effective host defence mechanism. Older cows, especially after four calving are more prone to mastitis [24]. The present study indicated sub clinical mastitis is a prevalent disease in lactating cows in Wolmera district. Breed, age, parity, and management system are the main factors that were associated with mastitis. The subclinical cases are by far more frequent than the clinical ones. Unequivocally, silent loss of milk production due to subclinical mastitis along with blind teats could cause adverse effect on the dairy activity of the study area that guarantees serious attention in creating awareness and prevention strategy. Ethiopian dairy farmers are not well informed about the invisible loss from subclinical mastitis and thereby there is no practice of subclinical mastitis screening tests. In conclusion, in Ethiopia, the subclinical form of mastitis received little attention and efforts have been concentrated on the treatment of clinical mastitis, however high economic losses often come from subclinical mastitis [24,25].

Authors Contributions
BG: participated in the design of the study, conducted laboratory work, involved in data analysis and manuscript preparation. DA: participated in the design of the study, involved in data analysis and manuscript preparation. Both authors read and approved the final manuscript.