Bibliographic Synthesis on the Influence of Cow Mastitis on Fresh Milk Quality

: Mastitis in lactating cows lead to a decrease in the overall cow production; make milk unfit for human and calf consumption; deteriorate milk quality. However, data are not updated on the influence of mastitis on the physicochemical and microbiological quality of milk. Therefore, the present review was made to highlight the current state of knowledge on prior research on the impact of mastitis on milk quality, and to identify the shortcomings of these studies. Two themes were addressed to achieve this. We started by reviewing the prevalence, aetiology and factors associated with the occurrence of mastitis in the African continent. Then, we compared the variation of chemical components of mastitis milk in several studies. Our analysis shows that subclinical mastitis is predominant in Africa, with most prevalence higher than 50%. The majority of authors (83.33%) claimed that Staphylococci are the main cause of mastitis. The breed (83.33%) and lactation stage (66.66%) were the two most cited factors contributing to the occurrence of mastitis. Data collected on the chemical components of mastitis milk came mostly from studies prior to the year 2000. Several contrary opinions (increase/decrease/no variation) were stated by authors on the variation of chemical components of mastitic milk, specifically on fat, lactose and crude protein contents. Overall, it is challenging, if not impossible, to draw a definitive conclusion about the influence of mastitis on the chemical composition of milk for most chemical parameters, because the data examined in relation to the chemical components of milk with mastitis seem rather inconsistent.


I.Introduction
Bovine mastitis is most often due to an udder infection, characterized by a decrease in milk yield and quality, as well as a possible deterioration of the animal's general health (Benhamed, Moulay, Aggad, Henni, & Kihal, 2011). Two types of mastitis are frequently encountered in dairy production, including subclinical and clinical mastitis (Wallace, 2007). According to Rakotozandrindrainy and Foucras (2007), Staphylococcus aureus, Escherichia coli, Streptococcus agalactiae, and Klebsiella are the primary microorganisms that cause mastitis in dairy farms. Indeed, negative effects of mastitis in cows are numerous, including a decrease in the overall production level of the animals; makes the milk unfit for human consumption and that of the calf; deteriorates the quality of milk both nutritionally, microbiologically and physicochemically (Millogo,  Hence the interest of this review which aims to bring out the state of the art of previous studies on the influence of mastitis on the quality of raw milk. Specifically, the review will focus on the prevalence and factors associated with mastitis in Africa; the main aetiological agents involved in mastitis; variation in the chemical components of mastitis milk.

II.1. The Research Strategy
In this analysis, various study types (including longitudinal studies and cross-sectional surveys) were taken into consideration. Results were gotten from investigations carried on mastitis screening and on the analysis of the microbiological and physicochemical quality of milk from local and improved breeds of well-defined milking cows' populations. Articles included in this review were searched using PubMed and Google Scholar databases published up to June 2021. The search was conducted using the following search terms: -Bovine mastitis, prevalence, aetiology, associated factors, cow milk quality‖.

II.2. Selection criterion
The following papers were included in this review: -Studies with a clear description of the various mastitis screening methods used; -Articles with a clear description of the laboratory procedures used for microbiological and chemical sample analysis; -Articles published in either English or French; -Research conducted across Africa in general and in Cameroon in particular.

II.3. Data extraction
To find potential studies based on titles and abstracts, data extraction was done using the inclusion criteria. A standardised Excel spreadsheet was created by extracting pertinent data from the chosen publications. Data were extracted using a predefined form, which included the following information: Publication data (journal, authors, study period, year of publication); country of investigation in Africa; Mastitis type and its prevalence; Major aetiological agents; associated factors, Mastitis' impact on the chemical components of milk; references.

II.4. Critical Assessment
The critical appraisal was based on a weighted tool developed by Folegatti et al. 2017; tool based on the modified checklists proposed by Downs and Black and the NOS (Table 1).
Articles were evaluated against a score-based system that combined elements of both scales, Articles with a score > 60% were included in this review.   (Bradley, 2002). However, they are often quickly eliminated from the udder after repeated milking. This is the case for Streptococcus agalactiae and Streptococcus uberis (both pathogens with an environmental and mammary reservoir) (Oviedo et al., 2007). It is therefore necessary firstly to emphasise various hygiene measures in the farm and secondly to insist on udder hygiene before, during and after milking.

III.1.3. Factors associated with mastitis in dairy farms
The authors most commonly listed breed, lactation stage and age of animals as the main factors related with the www.cmhrj.com development of mastitis in livestock, accounting for 83.33%, 66.67%, and 50% of cases, respectively (Table 2). Improved breeds with high dairy performance are more prone to mastitis than local cows (Shyaka, Kadja, Kane, Kaboret, & Bada Alambedji, 2010). As for age and lactation stage, according to a study by Poutrel, the frequency of udder infections and clinical mastitis increases with age and, more specifically, with the number of lactations (Poutrel, 1983).

III.2. Impact of mastitis on the chemical quality of raw cow's milk
According to several authors, a decrease in milk fat concentration during mastitis is unavoidable (Table 3), due to a reduction in the synthesis and secretion capacity of the mammary gland (Rowland et al., 1959;Seelemann, 1963). However, results from other authors are contradictory, since some authors found an increase in fat concentration in clinical mastitis, explained by the presence of fat in the milk, resulting from lysis of the lipoprotein membranes of bacterial cells (Andreatta et al., 2007;Bansal et al., 2005). In addition, an increase in volatile fatty acids (Auldist et al., 1996;Lee, Yu, Back, & Yoon, 1991) and certain proteins (albumin, immunoglobilin G and lactoferrin) was reported in the milk of mastitis cows (Anderson & Andrews, 1977; Baranova & Belov, 1993;Hagiwara et al., 2003). This could be explained by alteration of the milk fat globule membrane by leukocyte lipases or by hydrolysis of lipoprotein membranes, two phenomena that can promote lipolysis. As for the influence of mastitis on lactose concentration, some authors found that lactose levels in milk tend to decrease during mastitis (Auldist et al., 1996;Klei et al., 1998), while other authors found no significant variation (Teute, 1961). Overall, the results discussed seem somewhat contradictory and it is difficult, if not impossible, to draw a clear conclusion on the impact of mastitis on the chemical composition of milk for most parameters. This could be partly explained by the sampling methods used to take different milk samples. In fact, some studies are carried out on mixed milk, while others are carried out on milk taken from one or more udder quarters. As a result, the analyses carried out on these different milk samples could lead to variable results. What's more, these divergent results can also be explained by the different pathogens responsible for mastitis. The symptoms of mastitis and their impact on milk yield and composition can differ considerably depending on the bacterial species involved in the mastitis outbreak (Marechal et al., 2011). Among the pathogenic bacteria involved in mastitis, some cause changes in milk composition, while others have little or no effect on milk composition. For example, C. bovis does not alter the composition of milk, whereas the changes in milk are more marked in the case of E. coli mastitis (Coulon et al., 2002).

Table 3 : Effect of mastitis on the chemical components of milk
Arrows indicate an increase (↑) or decrease (↓); a question mark (?) indicates that the relationship is suspected but not clearly demonstrated; a dash (-) indicates that no variation was observed .

IV. Conclusion
The aim of this review was to bring out the state of the art of previous studies on the influence of mastitis on the quality of raw milk in several African countries. The prevalence rate noted by several authors is a warning sign indicating how insufficient the current anti-mastitis initiatives are across the African continent. In addition, given the primary aetiological agents responsible for mastitis, it is essential to underline various farm-wide cleanliness practises and to insist on udder hygiene before, during, and after milking. Indeed, this disease is the greatest world health and economic concern in dairy farms. Therefore, an accurate knowledge of the epidemiological data on cattle mastitis and its effect on milk quality would help to create a better mastitis control program, hence increasing milk quality and quantity throughout the African continent in general and throughout Cameroon in particular.
In perspective to this review, the lack of contemporary information on the effects of mastitis on the nutritional content of raw milk justifies the importance of further in-depth investigation on the chemical characteristics of raw cow's mastitis milk.

V. Conflicts of Interest
The study was conducted without any conflict of interest. VI.