The American Heritage Dictionary defines environment as the combination of external physical conditions that affect and influence the growth, development, and survival of an organism or group of organisms. Relative to bovine mastitis, the environment influences both of the principal groups of participants: bacteria and cows. Specifically, environmental conditions will affect the rate and magnitude of bacterial growth in the cow's surroundings. The other primary factor determining the incidence of environmental mastitis is the mammary gland host defenses. The same environmental factors that increase the growth of common mastitis pathogens often have a negative effect on mammary defenses.
The primary environmental mastitis pathogens include Escherichia coli, Klebsiella pneumonia, and Streptococcus uberis. These bacterial species are chemotropic organisms requiring organic material to utilize as food. Bedding materials commonly used for lactating and nonlactating cows provide and excellent environment for propagation of mastitis pathogens. Populations of the bacteria in bedding are related to the number of bacteria on teat ends and rates of clinical mastitis. Therefore, reducing the number of bacteria in bedding generally results in a decrease in environmental mastitis. Coliforms and streptococci can not live on teat skin for long periods of time. If these bacteria are present in large numbers on teat skin, it is the result of recent contamination from a source such as bedding. Therefore, the number of these bacteria on teat skin is a reflection of the cow's exposure to the contaminating environment. One of the environmental factors that has the greatest impact on bacteria in the cow's surrounding is the choice of bedding materials.
The bacterial view of life apparently is very simple: eat, drink, and reproduce. Unfortunately, many materials used to bed dairy cows allow for bacteria to accomplish these meager goals with astounding proficiency. Many organic materials provide adequate nutrition for both coliforms and environmental streptococci to reach populations in excess of 107 colony forming units per gram of bedding. Common organic bedding materials such as sawdust and straw usually contain very few mastitis pathogens before use as bedding. However, mastitis pathogens that contaminate the cow's environment establish residence in the bedding and often reach maximum populations within 24 hours after fresh bedding is added to stalls. The rapid increase in bacterial populations often preclude the soiled appearance of bedding. Therefore, the gross appearance of bedding has little correlation with bacterial load. Bacterial populations tend to remain in stationary growth phase for up to 7 to 10 days, then start to decline due to the exhaustion of nutrients in bedding. The common practice of adding fresh bedding to stalls or manure packs replenishes the essential nutrients and maintains bacterial populations.
Particle size of bedding influences bacterial populations. Finely chopped materials support greater bacterial numbers than the same bedding with larger particle sizes. Finely chopped organic material has greater surface area for attachment and colonization by bacteria. For example, chopped straw generally has higher counts than long straw, sawdust greater than shavings, and chopped newspaper greater than shredded paper. In addition, finely chopped materials adhere more readily to teat skin than larger materials, thus increasing exposure of the teat end to mastitis pathogens.
Growth rates of coliforms and environmental streptococci are greatest during warm, wet weather. The effects of season on bacterial populations in bedding are quite dramatic in regions that experience a wide variation of temperatures within a year. In general, the impact of bedding on exposure of cows in confinement housing decreases during cold weather and increases as temperatures and humidity increase. Previous trials have shown a strong correlation between bacterial counts in bedding and both ambient temperature and relative humidity. Therefore, proper ventilation of barns is essential to moderate the effects of heat and humidity in housing areas. Climatic factors affecting exposure in herds where cows are maintained on dry lots differ from those of traditional Midwestern and Eastern herds. Dry lots are used primarily in hot, arid areas where temperatures are seldom below freezing for an extended time. In these areas, the rainy seasons of late Fall through early Spring are when bacterial populations are greatest. Manure in dry lots during the Summer tends to be desiccated, thus limiting the moisture essential for bacterial growth.
The bedding material that we recommend most for controlling environmental mastitis is washed sand. Ideally, bedding should be inorganic materials that are low in moisture content and contain few nutrients for bacteria to utilize. Washed sand has little nutritive value to common mastitis pathogens, thus limiting their growth. Compared to organic materials such as sawdust, recycled manure, straw, and dirt, washed sand consistently contains fewer mastitis pathogens. Many free stall barns are forced to use organic bedding materials that are compatible with liquid manure handling systems. There appears to be little advantage in using one organic material over the use of another. For example, straw tends to have highest streptococcal counts, while sawdust and recycled manure have highest coliform counts in comparisons among these bedding materials.
Any material to be used as bedding should be stored in a dry area to prevent saturation by rain and ground moisture. Composting organic materials such as manure is an effective way to reduce bacterial counts before use as bedding. However, although many organic bedding materials have relatively few mastitis pathogens prior to use, the pathogen populations often increase 10,000-fold within a few hours when used as bedding. Fresh bedding tends to absorb moisture from the cows' environment for use by the great number of bacteria that are constantly present in manure and soiled bedding. Regardless of the bedding used, removing wet and soiled material from the back one-third of stalls will significantly reduce the bacterial counts. Stalls should be raked a minimum of twice daily when animals are moved to be milked. Spraying bedding with disinfectant and adding powdered lime to bedding has met with little practical success in reducing bacterial counts. These practices cause an initial decline in bacterial populations, but pathogen numbers quickly recover. Twice a day application of powdered lime may be necessary to sustain an advantage in lowering bacterial numbers. Avoid standing water and mud in free stalls, holding areas, and corrals.
Dirt and manure covered corrals are commonly used to house cows in semi-arid and arid areas. Exposure to pathogens generally is low during the dry seasons as moisture content of the dirt-manure mixture is low. However, as density of cows increase under shade structures and around feeding areas and water troughs, excess wet organic matter should be removed or spread out to be dried. Cow's access to dirt-manure lots should be limited during rainy seasons. Outbreaks of coliform mastitis are common during rainy seasons when cows are exposed to dirt-manure lots and alleys leading to the milking parlors.
Much like bacteria, the primary goals of a cow are to eat, drink, and the continued propagation of the species. The latter of these is an environmental factor that greatly affects the incidence of mastitis. Parturition, lactation, mammary involution, and lactogenesis (initiation of milk secretion) are each reproductive events that influence the susceptibility of the mammary gland to infection. Rates of new intramammary infections caused by environmental streptococci and coliforms are greater during the dry period than during lactation. During the dry period, susceptibility to intramammary infection is greatest the two weeks after drying off and the two weeks prior to calving. Many infections acquired during the dry period persist to lactation and become clinical cases. Research has shown that 65% of coliform clinical cases that occur in the first two months of lactation are from intramammary infections that originated during the dry period. Streptococcal infections during the dry period account for 56% of clinical cases during the first two months after calving. Rate of intramammary infections during lactation is highest at calving and decreases as days in milk advances. Therefore, the thrust of herd management strategies for controlling environmental mastitis should focus on reducing intramammary infections during the dry period and early lactation.
Housing and other environmental concerns for dry and maternity cows often are precluded by the comfort and housing needs of lactating cows. However, given the impact of intramammary infections acquired during the dry period on the subsequent lactation, providing cows with a clean and dry environment is not limited to during lactation. Dry cow and maternity facilities should be managed similar to lactating cow housing. Dry cow areas should be well drained and free of excess manure. Dirt covered areas can expose cows to pathogen levels comparable to those in free stalls. Box stalls and loose housing areas should be cleaned to the foundation base regularly. Manure packs are to be avoided because they generally contain extremely high counts of pathogens dangerous to both dam and calf.
The increased susceptibility of cows to environmental mastitis may relate to physiological events that render the cow immune suppressed. Many of these events arise from the dam supplying essential nutrients and protection against diseases to the calf. Management practices that allow for the maternal transfer of vital factors to the calf while not compromising the dam's well being have been developed. Two examples of these are vaccination and adequate supplementation of essential micronutrients.
Vaccination against coliform mastitis has become an accepted management tool to reduce the severity of clinical signs. Escherichia coli J5, and other rough mutants, are naturally occurring structurally modified strains that have unique antigenic properties that may cause enhanced immunity to not only themselves, but also other coliform strains. Use of E. coli J5 bacterin does not prevent intramammary infections. However, the use of E. coli J5 bacterin reduces the severity and duration of mastitis. Most immunization schemes include vaccination at drying off, mid-dry period, and calving to maximizes protection during the late dry period and the first month of lactation. Immunizing cows during lactation may have little value because the risk to intramammary infections decreases significantly as lactation progresses. Using coliform vaccines to treat clinical cases will not influence the coarse of the infection in most cases.
British researchers have reported the development of a Streptococcus uberis vaccine that may provide protection against some environmental streptococci. However, no vaccines effective against environmental streptococci are currently available.
Deficiencies in a number of essential micronutrients have been shown to cause cows to be more susceptible to disease. Among this list are vitamins A, D, and E, zinc, selenium and copper. One can easily rationalize that feeding diets deficient in essential micronutrients will eventually result in decreased resistance to mastitis. The two micronutrients that have been shown linked to bovine mastitis most often are vitamin E and selenium. Animals deficient in either or both of vitamin E or selenium have had higher rates of infections, more frequent cases of clinical mastitis, infections of longer duration, and more severe clinical signs that cows fed supplemented diets.
The recommended limit for selenium concentration in dairy cow rations is .3 ppm, corresponding to an approximate intake of 3 mg/day for dry and 6 mg/day for lactating Holsteins. Little data exist to suggest that dietary selenium greater than .3 ppm, in the absence of interfering elements such as sulfur and calcium, results in additional enhancement of host defenses against mastitis. Dry and lactating cows should consume 1000 IU/d of vitamin E. For cows fed stored forages, vitamin E may need to be supplemented at 1000 IU/day for dry cows and at 500 IU/day for lactating cows, dependent on forage quality and dry matter intake.
The recommended dietary and blood concentrations of vitamin E and selenium relate to maintenance of host defenses to protect against infections. Optimal blood concentrations may be greater during periods of stress. One such period of stress is calving. Plasma vitamin E concentrations in dairy cows are normally lowest when rates of mastitis are highest and when white blood cell functions are depressed around calving. Subcutaneous injections of vitamin E successfully elevates vitamin E concentrations in plasma and white blood cells during late gestation and early lactation periods. Cows injected subcutaneously with 3000 IU of vitamin E 10 and 5 days prior to anticipated calving have elevated plasma vitamin E concentrations and maintain intracellular kill of bacteria by white blood cells when dietary supplementation can not support these defenses.
An old, but popular, mastitis cliche is that environmental mastitis control is based on keeping cows clean, dry and comfortable. While this is true, the other half of the mastitis equation must also be accounted. Mastitis pathogens must be kept cold, thirsty and hungry. Engineering decisions and husbandry practices should consider the importance of maintaining cow comfort while simultaneously minimizing pathogen populations in the environment.