FACTORS CONTRIBUTING TO MASTITIS

FACTORS CONTRIBUTING TO MASTITIS

Mastitis is a difficult problem to comprehend because it is a disease caused by many factors. Microorganisms are responsible for the infection, but for them to enter the mammary glands and establish themselves to the point that they cause an infection, a multitude of factors may be involved (e.g. hygiene, housing, climate, milking machines, feed, genetics) acting simultaneously. It is even more difficult to generalize about the relative importance of each one, as certain factors affect certain microorganisms.
Klastrup and his coworkers²¹ estimate that 25% of the susceptibility to infection is attributable to environmental factors, 20% to genetic factors, and 50% to herd management.

ENVIRONMENTAL FACTORS

Climate
Climate may have a direct or indirect influence on the onset of mastitis. Older texts^7,43 insist a great deal on the fact that exposure to intense cold, draughts, excessive humidity or heat predisposes cattle to mastitis.
Just as they foster our colds, rapid changes in temperature can encourage mastitis. Research on how temperature influences the incidence of mastitis indicates that temperature extremes interact with other factors to cause mastitis but rarely will temperature alone cause the disease²¹. Temperature extremes may also affect somatic cell counts. Therefore the incidence of mastitis increases with extreme temperatures. In Florida, a higher rate of clinical mastitis was observed three years in seven during very hot periods²⁸.
A particular type of mastitis, often called summer mastitis, is caused by biting insects that contaminate the udder with the bacteria Corynebacterium pyogenes and other anaerobic bacteria. The frequency of this type of mastitis varies according to the regions, with humid valleys being most susceptible.
Climate may also have an indirect influence. For example, muddy conditions outdoors caused by abundant rainfall will increase the number of microorganisms and thus increase risks of infection.
Housing
Just keeping cows indoors increases the incidence of mastitis. When cows are inside, the risk of udder injury increases. There are also microorganisms indoors whose populations are less concentrated outdoors. In Australia, where cows go indoors only to be milked, mastitis caused by coliforms is rare.
Although the question is often debated, it would appear that mastitis is less common with loose housing systems than with tied housing systems. It might be thought that mastitis is more frequent with loose housing because the microbes are easily transmitted from one cow to another. However, cows are usually happier in loose housing. They are also less likely to injure themselves or come into contact with soiled bedding and so are less subject to mastitis. The social adjustment of cows within the herd is also clearer with loose housing. According to a Serbian study²⁷, there were 27% fewer cases of subclinical mastitis and 42% fewer cases of clinical mastitis in loose-housed herds than in herds kept tied.
In tied housing, there is greater likelihood of mastitis with some types of stalls. The longer and wider a stall, the more the cow can move, thereby reducing the number of injuries and the incidence of mastitis¹⁹. The greatest disadvantage is that the animal's vertical movements are restricted, particularly when getting up or lying down. Partitioning between the stalls reduces the incidence of mastitis arising from abrupt movements of neighbouring animals and tramped teat.
Quality of indoor air
In a barn, draughts, excessive humidity and frequent changes in temperature are factors that lead to increased incidence of mastitis. Indirect effects on an animal's immunity have not been seriously studied. However, effects on the concentration of pathogens in stables have been. For example, the bacteria Klebsiella pneumoniae causes more infection when relative humidity is low⁴⁸, while the number of infections caused by E. coli does not vary with humidity changes.
Bedding
Whether with loose or tied housing systems, bedding plays an important role in the incidence of mastitis. This is easy to understand when considering the mastitis-infected milk that ends upon the ground, the humidity that favours the development of microbes on bedding, and that cows often spend 14 hours out of 24 in contact with their bedding. In an experiment where cows were housed with and without bedding, the level of mastitis infections doubled where there was no bedding. Inadequate bedding in loose-housed herds, particularly large herds, may lead to serious situations of contagious mastitis.
Different materials used as bedding may affect the growth of different microorganisms. Straw is generally most recommended. There is less rapid development of pathogenic microorganisms with chopped straw and cedar sawdust than with newsprint⁴. Chopped straw, however, is more favourable to Klebsiella than sawdust¹⁶. Sawdust and shavings, particularly if heated, encourage the rapid development of coliforms in general and are often responsible for "epidemics" of coliform mastitis³⁶.
Stress
The more an animal is stressed in its environment, the less efficient its immune system is, and the less it can resist microbial infestations. Therefore, the more stress there is, the greater the chance of mastitis¹⁰. Giesecke¹⁰ has even demonstrated that stress affects the integrity of intramammary cells, which is yet another factor contributing to mastitis. The following are some sources of stress:

- Excessive density of animals. Proximity of cows encourages microbial exchanges and tense relations between animals;

- Irregular management, unpredictable behaviour on the part of the farmer;

- Noise;

- Stray voltage.

Genetic factors

Recently there has been a lot of research on how hereditary factors influence susceptibility to mastitis. The different dairy cattle breeds are not equally susceptible. High-producing cows are more likely to be affected. Selective breeding that focuses solely on milk production is undoubtedly an important factor in higher rates of mastitis. According to different sources, hereditary factors account for 12% to 20% of susceptibility to mastitis in a single breed.
Cows selected for several traits have a higher somatic cell count (better immune response), requiring almost two times less treatment, and their milk is thrown away half as often as the milk of cows selected for one trait, although the latters produce more milk⁴⁹.
Genetically, there is a correlation between the percentage of milk fat and the incidence of clinical mastitis. The more a line of cows produces fat milk, the more it will be susceptible to mastitis. It is therefore important to not select on this basis only.

NUTRITIONAL FACTORS

Despite several serious studies on the subject, the links between diet and mastitis still raise questions in scientific circles. There is agreement on two practices that increase the risks of mastitis - rapid changes in diet and excess or imbalance in the different components of rations.
Nitrogen and proteins
Excessive nitrogen or protein in feed is often mentioned as one of the factors causing mastitis. According to a Danish study²⁵, there is no definitive link between protein content in diet and the incidence of mastitis. However, there is more evidence regarding the harmful effect of nitrogen that is not in a protein form (e.g. urea and ammonia) on disease incidence.
Nonprotein nitrogen (NPN) is particularly hard on white blood cells, or leucocytes, which protect the udder. Abrupt changes to rations based on NPN-rich high moisture corn or alfalfa silage should be avoided. Increases, however modest, of ammonia in the blood have repercussions on metabolism. If such rations are used, sufficient fibre should be included to feed microorganisms in the rumen that will convert the nonprotein nitrogen into bacterial protein.
According to an experiment realized in Germany⁸, there is a significant relationship between the level of urea in the blood and bacterial colonization in the udder. In another experiment, the addition of urea to rations increased susceptibility to infection and increased the number of infections by more than 16%⁴⁵. The effect on the immune system is particularly evident when the urea is given in large quantities (over 18Og/day more than nitrogen requirements)².
Concentrates and energy
It is recommended that reduced quantities of concentrates be given to a cow with mastitis. It appears this is also true for preventing mastitis, according to a German study²² conducted on 1038 first lactation cows and 572 cows of successive lactations. When the cow rations contained 25% concentrates rather than 40%, the incidence of mastitis was 7% compared to 36% for first lactation cows and 19% in comparison to 37% for other cows.
The same study also compared different energy levels in rations. A high energy content in rations increased the incidence of mastitis in first lactation cows whereas it had the opposite effect on the other cows.
Calcium-phosphorus ratio
An inadequate calcium to phosphorus ratio in rations results in problems with milk fever at calving⁴⁰. In large herds, up to 50% of animals lacking calcium in their rations will develop coliform mastitis a few hours after calving. This hypocalcemia generally results from an inadequate calcium to phosphorous ratio in rations during the dry period. See page XX for more information on rations to give during the drying off period.
Silage and hay
Poor quality silage has a very negative effect on the immune system. The overheated proteins and sugars may kill the white blood cells protecting the udder. Cows fed with hay and grain have greater resistance in every way to several pathogens than cows fed with silage³⁸. In some cases, Pseudomonas and Proteus are the only microorganisms that survive the high temperatures produced in silage. Although rare, silage contaminated this way may then be a source of mastitis caused by these organisms. Mouldy hay and mycotoxins also harm white blood cells and therefore weaken the immune system.
Alfalfa and other legumes
Legumes, particularly alfalfa, contain estrogenic substances whose concentration varies depending on plant maturity. Turning legumes into silage does not reduce their estrogenic properties. Through a physiological mechanism, that is still not well understood, these external estrogenic substances (that is, they are not produced by the cow herself) tend to foster mastitis. Several studies indicate that alfalfa added to the rations of cows with chronic mastitis exacerbates the infection. What is important is not to feed silage with a high legume content to heifers. This estrogenic intake encourages premature development of the udder and increases the incidence of environmental mastitis²¹.
Selenium and vitamin E
In the last ten years, several researchers have looked into the use of supplements and the role of selenium and vitamin E in the prevention and treatment of mastitis. Maintaining an adequate level of selenium in the cow helps prevent mastitis, reduce the severity of infection and shortens its duration. Selenium serves to reinforce the immune system response by increasing the release of leucocytes and increasing the efficiency of phagocytes⁹. Selenium and vitamin E work together in the cow. Thus, a vitamin E supplement alone of 1000 lU/day reduces the somatic cell count but not the incidence of mastitis³.
With both selenium and vitamin E supplements, infections may be reduced by approximately 40% at calving, by 60% for the entire duration of the infection and by 30% for clinical mastitis. The role of selenium is considered to be most significant in the case of subclinical mastitis³⁰.
Selenium supplements may play a particularly significant role in cases of mastitis caused by E. coli. For example, cows that are given a selenium supplement of 0.35 mg/kg dry matter are better able to resist mastitis caused by E. coli²⁴. The duration of this type of mastitis is even shorter when cows receive 2 mg of selenium per day per kilo of ration⁹.
Recommended blood levels are 0.2-1.0 g/ml for selenium and more than 4 g/ml for vitamin E. Rations should provide 3 mg of selenium per day in the case of dry cows and 6 mg per day for producing cows. Rations should provide 1000 IU of vitamin E per day for both categories of cow⁴⁴. Supplementation with vitamin E has a greater effect on dry cows than on lactating cows, where a good part of vitamin E supplements is eliminated in the milk.
Important: It is not useful and even harmful to give large doses of selenium only (that is, without vitamin E), because the effect can be toxic. A selenium dose of 16 mg/day results in higher levels of mastitis unless vitamin E supplements are administered at the same time⁵³.
Silica
Finnish researchers³⁴ noted that the level of silica in mastitis-infected milk was only 0.39 mg/litre whereas it was 0.81 mg/litre in normal milk. Also, the level of silica in the blood serum of cows infected with mastitis is 1.02 mg/litre rather than 1.63 mg/litre for uninfected cows. Silica, whose role is similar to selenium, has a marked effect on the formation of free radicals, lipid peroxidation and macrophage activity. The silica content in rations may be increased by giving high silica-content feed like cereal straws.
Other nutritional factors
Vitamin A deficient rations reduce immunity. An Italian researcher experimented with vitamin A and beta-carotene supplements to control mastitis¹¹.
According to Katholm¹⁸, iron also plays an important role in the prevention of mastitis. It is linked to the protein lactoferrin.