Epidemiology of Bovine Mastitis in Selected Major Milk Shade Districts of Arsi Zone, Ethiopia

The data for the study on the epidemiology of bovine mastitis were collected from November 2015 to February 2016 to investigate the prevalence of mastitis in administrative towns of three selected districts of Arsi zone, Oromia Regional State, namely Iteya, Sagure, and Asella, Ethiopia; and to analyze the association between the risk factors and mastitis occurrence. Sample cows were selected and examined for clinical mastitis and at the same time, milk samples were evaluated both clinically and using the California Mastitis Test (CMT). Some parameters of the cows which have biological relation with this disease together with other potential risk factors were collected along with primary data collection. The analysis of the primary data showed that overall mastitis prevalence in the locations covered by the study was 28.6% of which 83.3% was subclinical, while 16.7% were clinical mastitis cases. Analysis related to the investigation of the existence of an association between risk factors and mastitis revealed that there was a significant association between the occurrence of mastitis and two of the risk factors evaluated, breed and age of the cow. The other risk factors investigated (i.e. parity, location, and lactation stage had no significant association with mastitis occurrence). Since mastitis is a complex disease that results from the interaction of different factors, it may be difficult to limit it to only some given factors. Environment and management factors and their combination may also have an additional impact on the epidemiology of the disease. A prevalence with the indicated magnitude together with the added effect of a lower level of awareness of the society about milk-associated zoonoses implies that this disease causes significant public health problems. In addition, mastitis may cause silent economic loss through unnoticed production loss along with reduced milk yield from subclinical and clinical (e.g. blind teats) cases. Due to a lack of awareness about mastitis and/or negligence, farmers in all corners of the country are suffering from economic and public health problems imposed by mastitis. This lack of awareness about mastitis-associated problems and/or negligence could be a possible reason for the use of raw milk which is a common scenario particularly in pastoral areas and the selling of adulterated milk which is becoming increasingly practiced in Ethiopia which is also true in these specific localities.


Background
The dairy industry is facing a great set back due to the high prevalence and incidence of mastitis in dairy cows. In addition to causing huge economic losses, this disease also possesses the risk for the transmission of zoonotic diseases, such as tuberculosis, brucellosis, leptospirosis, and streptococcal sore throat. Mastitis is becoming a major health and production constraint in dairy farming. Dairy animals are seen as frequently affected by this disease (Rahmeto et al., 2016).
Up to now, several pieces of research have been conducted but the problem persists (Rahmeto et al., 2016). This may be because of an absence of intensive research that deeply investigates the epidemiology of the disease which can aid in planning to develop tools for effective prevention and/or control measures to be taken and overcome the demand-supply imbalance which becomes a reason for adulteration and deterioration of quality of milk.

Problem Statement
The frequent occurrence of cases of mastitis, reduction in milk yield that does not support the consumer's demand, milk quality problem, and fear of zoonoses (diseases shared by animals and people)

Objective General Objective
❖ To investigate the prevalence of mastitis across the study sites and to identify if there is an association of the disease with risk factors.

Specific Objectives
❖ To screen cows with subclinical mastitis and identify cows with clinical mastitis among study animals ❖ To investigate the prevalence of mastitis across the study sites ❖ To assess the association of mastitis with some of the host and environmental factors 2. METHODOLOGY

Study Area and Study Population
This observational study on the epidemiology of bovine mastitis was conducted on selected three districts of the Arsi zone which has relatively larger dairy herds, and cows have a higher milk yield compared to other Ethiopian, and that is relatively located close to the school complex, the former School of Agriculture, Adama Science and Technology University. In each of the districts, the study was limited only to their administrative towns which are again potential dairy sites within the districts.

Selection and sampling of animals
The study was designed so that all lactating cows in each of the districts at the time of the survey were included in the sampling process. Besides, dry cows were selected to widen the scope for the investigation of subclinical mastitis. Based on these a total of 126 local and crossbreed cows were sampled. The following parameters were recorded for each animal at the time of sampling; age, breed, parity, stage of lactation, and milk yield.

Data collection method
Data about mastitis among the sampled dairy cows were gathered by collecting a 000-500,000, 1 = 400,000-1,500,000, 2 = 800,000-5,000,000, 3 = >5,000,000) (Miller and Kearns, 1967). The present study was hoping to search for pathogenic agents as the cause of mastitis which are the dominant cause or sometimes maybe the only cause of the disease.

Data Analysis
The data were recorded in the Microsoft Excel spreadsheet and coded before statistical analysis. Descriptive statistics and logistic regression analysis were computed. In the logistic regression analysis, the outcome variable was the presence of mastitis, while the predictors were the different host and environmental risk factors. The association between the dependent variable, cow mastitis status (0 = negative and 1 = positive), and categorical

RESULTS AND DISCUSSION
A survey on the epidemiology of bovine mastitis to investigate the magnitude of mastitis among different risk factors and its association with the factors showed the under mentioned results. The study also showed that mastitis was a significant problem for dairy cows in the study area.

Prevalence of Mastitis
Prevalence of mastitis was computed at different levels such as overall prevalence for the total sample from the three study locations. The prevalence for each location, and for the tested risk factors, and also the prevalence of clinical and subclinical mastitis are described in Table 1. The overall prevalence of mastitis was calculated as the proportion of mastitispositive cows (clinical and subclinical) against the total number of cows investigated. A cow was denoted as positive for mastitis if at least a single teat (quarter) with clinical mastitis or CMT positive result was detected.

Association of Mastitis with Risk Factors
In the logistic regression analyses conducted, statistical significance was set at P < 0.05.
Several cow-level and environmental factors like the breed of the cow, age of the cow, parity number of the cow, location (study site), and lactation stage of the cow were considered in the logistic regression analysis for the presence of their association with mastitis. Among those factors, the breed was found to be significantly (P < 0.05) associated with the presence of mastitis while age had a significant statistical association with mastitis just at 0.05 indicating a slight association. On the other hand, parity, location, and lactation stage did not have a significant effect (p > 0.05) on the occurrence of mastitis.
Excluding the non-significant factors and rerunning the logistic regression only for the significant risk factors, breed and age, yield the following out put ( Table 2). There is also a significant effect of the age of cows alone on the incidence of mastitis.

Table 2: Multivariable Regression Analysis for the Potential Risk Factors of Mastitis
Because the p-value for age is 0.05365 which is approximately equal to 0.05 level of significance.
Ln(odd of mastitis) = -2.1069 + 0.6081*Age For every one-year increase in age of the cow, the odds of incidence of mastitis increases by 83.7 units. Because exp(0.6081) =1.836938 (Appendices). This implies that the age of the cow has a significant effect on the incidence of mastitis, which means that when the age o f the cow increases by one year, the odds of incidence of mastitis increases by 84 units. T hat means there is a positive relationship between the age of cow and odds of incidence of mastitis (also seen from the logistic regression model, positive coefficient of age). The od ds of getting mastitis increase by 84% as the age of cow increase by one unit (year).
There were different researches conducted so far in Ethiopia on mastitis. The research output of some of these researches was consistent with the present study while others were not. Beyene et al., 2017 have shown that at the analysis of chi-square and univariable logistic regression, four factors namely breed, lactation status, age and number of parity were statistically significant while at multivariable logistic regression breed, age and lactation status showed to be the main risk factors that contribute for occurrence of mastitis. Rahmeto et.al., 2016 have noted that in the multivariable logistic regression model, breed, parity, and stage of lactation have a significant effect on mastitis occurrence. The occurrence of bovine mastitis and lactation stage was significantly (p<0.05) associated.
That is, higher infection in cows in early lactation stage followed by late and medium lactation stages. Mastitis prevalence was found to be higher in early lactation and lower in mid-lactation stages. The early lactation stage infection might be due to the carryover of infection from the dry period. Rahmeto et al., 2016 also showed that animals over 8 years old were more frequently affected with mastitis and those younger than 5 years were rarely affected. Mastitis is more common and has a serious impact on older lactating animals than in first lactation heifers Also the highest prevalence of mastitis was observed in animals with parity of more than 6, followed by 3-6 and 1-2 parity.
Mulugeta and Wassie, 2013 also found that all the potential risk factors considered for a univariate logistic regression, i.e. age, parity, breed, and lactation stage were found significant (P<0.05). However, age became insignificant when tested with multivariate logistic regression. The prevalence of mastitis varied significantly (p<0.05) among breeds.
It was also appreciated that the lactation stage and parity significantly (p<0.05) influenced the occurrence of mastitis.

CONCLUSIONS AND RECOMMENDATIONS
The frequent occurrence of cases of mastitis, reduction in milk yield that does not support the consumer's demand, milk quality problem, and fear of zoonoses are some of the problems that call attention for the research. The source of mastitis infection may be regarded as contagious or environmental. Contagious pathogens spread during milking.
Environmental agents are opportunistic invaders from the cow's environment.
Prevalence and distribution of mastitis might be due to differences in the breeds of cattle

Conflict of Interests
The author declares that there is no conflict of interest regarding the publication of this paper.

Acknowledgments
I would like to thank all those individuals and an organization who have got significant contributions to this particular work either directly or indirectly. Particularly, my gratitude goes to the former Adama Science and Technology University School of Agriculture for the financial support for the project.