Mastitis in dairy cattle is very common during the dry period and early lactation. Intramammary infections (IMI) observed during the dry period can be due to a subclinical infection (presence of bacteria without signs of inflammation) carried into the dry period from the previous lactation (pre-existing infections), or those acquired at the period between drying off and calving (new infections).

Mastitis during the dry period leads to a decrease in milk production, poor milk composition and high susceptibility to clinical mastitis in the subsequent lactation, all of which dramatically impact dairy economics and animal health.

During the transition from a lactating to a completely non-lactating dry udder, the mammary gland undergoes a series of changes that influence the cow’s resistance to bacterial infection. With the cessation of milk removal, accumulation of milk into the udder will increase the pressure in the gland, causing the streak canal to widen and teat orifice to dilate, allowing bacteria into the gland. Once inside the gland, the milk acts as a rich environment for bacteria to multiply.

The mammary gland responds to an infection by increasing the influx of immune cells (somatic cells) that help to fight the infection. These cells migrate into the gland and release cytokines, which are proteins that initiate and regulate inflammation, immunity and play an important role in mammary gland defenses. The successful elimination of the infectious agent or establishment of an infection will depend on the degree of the inflammatory response, which is extremely variable among cows.

A common practice to control and prevent intramammary infections during the dry period is the infusion of antibiotics into the udder at drying off (dry cow therapy – DCT). The withdrawal time is different among the diverse DCT antibiotics, varying from hours to days after treatment.

Advertisement

However, there are increased concerns of public health and food safety about the use of antimicrobial drugs in food animals regarding antimicrobial drug residues in milk and dairy products, leading to antimicrobial resistance and selection of drug-resistant food-borne pathogens. A report from New Zealand shows that intramammary antibiotics made up more than 90 percent of all antimicrobials used in cattle in 2003 and have been used almost exclusively in dairy cattle. Public health concerns express a need to reduce the use of both preventive and curative antimicrobials in food animal production.

Alternatives to antimicrobial therapy to control mastitis, such as immunotherapy, have been an active area of investigation. Immunotherapy consists of the manipulation of the host’s immune system. For example, production of specific cytokines could be stimulated in order to strengthen natural host defenses against infections. Enhancing a diminished mammary gland defense system may provide an effective barrier against new intramammary infections during increased susceptibility periods.

Several studies have shown promising results in using immunotherapy treatments. In a study from Ohio State University researchers found that addition of a specific cytokine, Interleukin 2, to a regular dry cow antibiotic targeting Staphylococcus aureus, enhanced the effectiveness of the treatment. However, abortion was observed in 7.5 percent of the cows that received this particular cytokine. Further research is required to identify specific cytokines that can induce similar levels of benefit without side effects.

Researchers of USDA in Iowa stimulated the immune system of bovine mammary glands by infusing an extract of the yeast Saccaromyces cerevisae during the dry period. The results showed an increase in the activity of immune cells, and suggested that this immune stimulation was similar to that of vaccination, making immune cells in the gland prepared to respond immediately in case of a new infection. This treatment could minimize the risk of a new IMI during the periparturient period.

The use of immune-stimulating substances such as cytokines and Saccaromyces cerevisae extracts are some examples of immunotherapy that could be a potential alternative to the current dry cow therapy. Research is still in development, but soon methods of immunotherapy may be available for the dairy industry and will be safe for animals and humans. Always remember to seek the advice of your veterinarian before using any new product at your dairy. PD

— Excerpts from Texas Dairy Matters Newsletter, Fall 2009

Daniela Resende Bruno