K.P. Waller
Department of Obstetrics and Gynaecology, Faculty of Veterinary Medicine, Swedish University of Agricultural Sciences, Box 7039, SE-750 07 Uppsala, Sweden
Mastitis is a multifactorial and costly problem worldwide and can occur in all milk-producing ruminants. It affects not only animal welfare, but will also result in decreased milk production and deterioration of the milk quality. A healthy mammary gland is a result of a complex balance between the cow, the micro-organisms and the environment. Decreased disease resistance in the cow, increased virulence of the micro-organisms or increased burden from the environment will result in mastitis. Mastitis can be difficult to treat successfully. Therefore, it is important to identify risk factors in order to be able to prevent the disease from occurring. It is important to make the farmer aware of the problem, and to make him/her understand the importance of good management and environment. In many countries some kind of control plan is recommended. The yak is an important source of milk for millions of people in Asia even though the production per cow is low. So far, there are few reports on mastitis in yak. Whether this is due to a high resistance against infections in the mammary gland, or to a lack of investigation into the problem, is not clear. Crossbreeding of yak with high-producing breeds will increase the milk production, and the risk for mastitis. Therefore, it is important to increase the knowledge about mastitis in yak and the risk factors involved in yak milk production. In this paper, the etiology of mastitis is discussed, together with predisposing factors such as housing, feeding, genetics and milking, as well as ways to control the disease.
Keywords: Control, etiology, mastitis, predisposing factors, ruminants, yak
Mastitis is a multifactorial and costly problem, which occur in all milk-producing ruminants. It affects animal welfare and results in decreased milk production and deterioration of the milk quality. Mastitis is a common problem in most countries with dairy production. In Sweden 18% of the dairy cows are treated by veterinarians each year for clinical mastitis. However, a large number of mastitis cases are subclinical and can only be detected by high milk somatic cell counts (SCC). The yearly incidence of all cases of mastitis in Sweden (both subclinical and clinical) is estimated to be around 64% (Svensk Mjölk 1999). The major costs involved are decreased milk production and increased culling of animals. Current programmes for control of bovine mastitis have improved the situation substantially, but the incidence of clinical and subclinical mastitis is still high.
Mastitis can be difficult to treat successfully. Therefore, it is important to understand the underlying mechanisms and to identify risk factors in order to prevent the disease. It is also important to make the farmers aware of the problem and the importance of good management.
The yak is an important source of milk and other products for millions of people in Asia even though the milk production per cow is low. However, crossbreeding with cattle breeds with a higher production has become more common (Joshi et al. 1994; Rongchang et al. 1994). As a result the milk production increases, and thereby also the risk for mastitis. So far, there are few reports on mastitis in yak and yak hybrids. Whether this is due to a high resistance against infections in the mammary gland, or to a lack of investigations, is not clear. With increasing production and the introduction of milking machines, it is important to increase the knowledge about mastitis and the risk factors involved in yak milk production.
Mastitis means inflammation of the mammary gland and is an important defence reaction of the body to a local damage. Pre-inflammatory damage to the udder is most often due to some kind of trauma, bacterial toxins, or a combination of trauma and bacterial infections. The risk for such damages is influenced by predisposing factors in the cow, environment and/or management.
Most cases of mastitis are associated with bacterial udder infections. The most common mastitis pathogens are different staphylococci (Staphylococcus aureus and coagulase negative staphylococci) and streptococci (Streptococcus agalactiae, Streptococcus dysgalactiae and Streptococcus uberis). Other important bacteria are coliforms like Escherichia coli and Klebsiella spp. The route of infection is almost always via the teat canal.
A healthy mammary gland is a result of a complex balance between the cow, the micro-organisms and the environment. Decreased disease resistance in the cow, increased virulence of the micro-organisms or increased burden from the environment will result in a disturbance of the balance, which can result in mastitis. The balance is affected by many factors, which predispose for damage and udder infections by reducing the immune defence of the cow or increase the risk for infections in different ways.
Factors predisposing for udder infections and mastitis can be genetic or environmental. Examples of such factors in the cow are breed, age, milk production, teat and udder shape, temperament and genetic potential for milk production. Other cow factors, like viral infections, ectoparasites, claw health, lactation stage and immune status can also be of importance. Likewise, there are a number of factors in the environment and management that can predispose for disease. Such examples can be found in the climate, housing, feeding, and in milking routines and equipment. Some of these factors will be highlighted below.
The immunological status of the animal will affect the risk for disease. During certain stressful periods, e.g. around calving, the immune functions of the dairy animal are suppressed (Sordillo et al. 1997; Mallard et al. 1998). This period, i.e. the peri-partum and early lactation period, is associated with a high susceptibility to udder infections and mastitis (Sordillo et al. 1997). High blood levels of glucocorticoids, as well as of other hormones, are present around parturition, and the risk for metabolic stress is high. Examples of stress factors during this period are parturition, onset of lactation, and changes in feeding and management regimes. Other diseases, like different viral infections, can also cause immune suppression, which increases the risk for other health problems like mastitis (Niskanen et al. 1995). Local stress due to injuries to, or various skin diseases of, the teats and udder is also an important risk factor for udder infections.
Research on genetic resistance against udder infections and mastitis is important. However, heritability of clinical mastitis has been estimated to be low (Emanuelson et al. 1988; de Haas 1998). The SCC, as an indirect measurement of mastitis, has a higher heritability, and might be a better selective tool than clinical mastitis (Emanuelson et al. 1988; de Haas 1998). A combination of these parameters is recommended for selection towards reduced susceptibility to mastitis. Philipsson et al. (1995) reported a linear relationship between sires' breeding values for clinical mastitis and for SCC.
High milk production and various udder traits, such as udder suspensory ligament, fore udder attachment and udder depth, are associated with increased incidence of mastitis (Uribe et al. 1995; Alexandersson 1998; Emanuelson et al. 1988). A similar relationship was observed between mastitis traits and milk protein production (de Haas 1998).
In the Nordic countries, udder traits, cell counts and clinical mastitis are included in the breeding values of bulls in order to improve disease resistance. Other countries have recently started to include mastitis selection parameters in the breeding programs.
Most literature deals with heat stress and its detrimental effect on milk production. However, some studies also address the effects of heat on immune functions and udder health. Exposure of cattle to high temperature can increase the stress of the animal (Johnson and Vanjonack 1975). The total cell counts in both blood and milk were higher in heat stressed cows (Webster 1983; Elvinger et al. 1991). The results also indicated that heat stress depresses leukocyte responses in vitro (Elvinger et al. 1991), and migration of leukocytes into the mammary gland (Elvinger et al. 1992). However, Paape et al. (1972) did not observe any increase in the SCC due to thermal stress.
There are few studies on the effects of cold stress on mammary gland immunity, but it is well recognised that cold conditions can cause damages to the teat skin, especially when the teats are wet after milking and teat dipping. This will increase the risk for bacterial infections.
Different housing systems may be associated with a variety of risk factors. In free-stall systems the design and management of the cubicles is essential. If the cubicles are not comfortable the cows will not use them but lie down elsewhere. Likewise, special risk factors are present when using pasture, a straw yard or a sand yard, and if a tie-up system is used where the cows have limited ability to move. In all housing systems, high stocking density, dirty bedding or ground, and high humidity are important risk factors. Housing and poor hygiene are associated with a higher risk for both clinical (Barkema et al. 1999) and subclinical mastitis (Ekman 1998). The feeding environment is also important. All cows must be allowed equal access to both feed and water.
The attitudes and actions of the farmer are essential in order to keep disease to a minimum. The importance of management has been shown in several studies. For example, Ekman (1998) showed that low bulk milk SCC was associated with having clean cows that were well clipped with trimmed claws. Other significant factors were that the farmer was a patient, confident and sufficiently considerate person who also liked cows. The importance of an early detection of problems/diseases by the farmer/milker is obvious, and is the prerequisite for good health and welfare of the animals.
The milking procedure is one of the most important risk factors for both clinical mastitis and high SCC (Ekman 1998; Barkema et al. 1999). Malfunctioning machine milking can induce damage to the teat tissue increasing the risk for udder infections. Examples of risk factors are faulty vacuum level, pulsation rate and ratio, and liner design (IDF 1994). The milking routine is also very important. It is essential that the milking be performed in a hygienic way, in a calm environment and with the same routine each time. It is important to identify cows, which have mastitis. These cows should be milked last, or separately, to avoid spread of bacteria among animals.
Stress can lead to milk retention by inhibiting oxytocin-mediated reflexes (McCaughan and Malecki 1981; Johansson 2000). New environment, new milkers, painful milking technique and pain due to udder oedema are factors, which can be stressful for the animal. Inhibition of milk-let-down can cause subclinical mastitis to become clinical and will affect milk production.
Adequate nutrition is essential for overall health and the ability of the animal to resist diseases. The balance between protein and energy, supplementation of micronutrients, and feeding routines are factors influencing animal health by potentially increasing the risk for metabolic disturbances. Also, the hygienic quality of the ration is important.
At the onset of lactation dairy cows are metabolically stressed due to negative energy balance leading to the mobilisation of considerable amounts of tissue reserves. This could also be associated with the increased incidence of infectious diseases during this time. Increased levels of ketone bodies can have a negative effect on immune functions and influence the severity of mastitis (Klucinski et al. 1988; Kremer et al. 1993; Hoeben et al. 1997).
A balanced supply of micronutrients (vitamins, minerals and trace elements) is essential during periods of immune suppression. Deficiencies in selenium (Se), vitamin E, vitamin A, copper (Cu) and zinc (Zn) have been associated with a negative influence on the immune response in association with mastitis (Reddy and Frey 1990; Harmon and Torre 1994; Smith et al. 1997). If the contents of such minerals as Se, Cu and Zn are low in soils and pastures, supplementation may be necessary. Likewise, dietary supplementation of vitamin E and A may be necessary as the concentration in fodder decreases with length of storage.
Mastitis can be rather difficult to treat efficiently. Antibiotics are used in various ways, but with limited success, for example due to the development of resistance against antibiotics. Therefore, it is important to find means to prevent the disease as well as have a plan for handling cases of mastitis when they occur. In many countries some kind of mastitis control programme has been adopted. To make these programmes work it is essential that the farmer/milker understands the association between management and mastitis. It is also important to identify the problems on each herd and adjust the control programme for its specific needs.
A typical control programme will include a number of recommendations for good management in order to prevent the disease. In such a plan the following are important:
Most often mastitis is not a problem only in one cow of a herd. Commonly a herd problem, like high SCC, can be observed. In order to find the best control plan for each herd, it is necessary to do a thorough herd investigation to identify its specific problems. To make a correct diagnosis of the herd the management has to be carefully studied and problem cows must be identified. It is of great benefit to do bacteriological investigations of milk samples to further evaluate the problem. Depending on the bacteriological findings and the results from the herd investigation the udder health problems can often be identified as either due to cow specific or environmental bacteria. As these bacteria have a different pathogenesis the recommendations for herd control are also somewhat different.
Udder health problems caused by cow-specific bacteria, like Staphylococcus aureus, Streptococcus dysgalactiae and Steptococcus agalactiae, are often associated with problems at milking. These bacteria are transmitted between cows and udder quarters at milking, either by the milker or the milking machine. In contrast, environmental pathogens, like E. coli, Klebsiella spp and Streptococcus uberis, often infect the udder between milkings. Such udder health problems are often associated with bad hygiene.
The yak is very important for the local population, especially in remote and cold high-altitude areas. This is due to its many functions, production of milk, meat, hair/wool, leather and manure for heating, as well as transportation of both people and goods (Rongchang et al. 1994; Sasaki 1994).
Yak milk is consumed fresh or preserved in different ways, like cheese. In many areas, the milk is consumed locally, but in other areas yak milk products are an important source of income, and is, for example, sold to tourists. The milk production per yak cow is low and varies depending on feed, environmental conditions and season of calving. A typical lactation period would last for 100–150 days and produce 500–800 kg per lactation (Sasaki 1994; Long et al. 1999). However, the calf suckles most of the milk, so only about 1 kg/day is available for human consumption (Rongchang et al. 1994; Long et al. 1999). The yak is either milked once or twice per day. Milking twice a day increases both the daily production and the length of the lactation period but may not be recommended when the feed resources are poor (Long et al. 1999). The rear quarters have a higher production than the fore quarters (Xin and Luojun 1994). Milk let-down may not be elicited unless preceded by suckling and the calf is at foot (Xin and Luojun 1994). However, in the dzo (Yellow cattle × yak) cows, majority of cows could be hand milked without the calf or in the presence of the calf (Xin and Luojun 1994).
Milk production depends highly on feeding. Traditionally, yak depend on grazing and supplemental feeding is not given. During certain periods, underfeeding is common and this will influence milk production, reproductive performance and disease resistance. Long et al. (1999) showed that calving rates and live weight losses were influenced by whether grazing was done with or without supplements. Most yak only produce a calf every second year, but under good forage conditions they can produce a calf every year.
Crossbreeding of yak with different breeds of cattle has become popular in certain areas to increase milk production and to produce an animal that is more adapted to the intermediate altitude zone. However, the hybrids cannot stand harsh environments as well as the purebred yak. The male hybrid is sterile but the female is fertile and can be crossed back to cattle or yak bulls (Joshi et al. 1994; Rongchang et al. 1994). The crosses have a substantially higher daily milk production and their lactation period is longer. Moreover, they often produce a calf each year.
To my knowledge limited information is available about the incidence of mastitis in purebred and crossbred yak. According to Xin and Luojun (1994), streptococcal mastitis has been reported in yak in China. In one study, 4 out of 40 yak (10%) had subclinical mastitis, as measured by the SCC (Ma 1987). In another study, 224 yak in 4 counties in Hainai were investigated (Tang and Cai 1990). According to the SCC, 3.6% of the yak and 1.6% of the quarters had subclinical mastitis. The results differed somewhat between counties and the number of cases seemed to increase with lactation number. Bacteriological investigations were not performed in either study.
In order to avoid mastitis and its costs in milk producing animals it is important to identify risk factors that can negatively influence the defence mechanisms of the mammary gland. The importance of good management and adequate nutrition for the prevention of mastitis are some factors to consider. The possibilities for genetic selection of individuals with a well-developed resistance against udder infections and mastitis could also be evaluated.
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