Mastitis, milk quality and dairy food safety are all very much interrelated. However, we generally do not associate mastitis with dairy food safety. That’s probably because we have always focused on the relationship of mastitis and milk quality. However, it’s time to change our way of thinking.

Mastitis remains the most common and most expensive disease affecting dairy cows throughout the world. Mastitis results when bacteria invade the udder, multiply and produce harmful substances that result in inflammation. The National Mastitis Council estimates that mastitis reduces the income of U.S. dairy producers by $2 billion each year.  

Mastitis reduces milk yield and negatively impacts milk composition and milk product quality. The magnitude of reduced milk yield and changes in milk composition is influenced by the severity of the inflammatory response, which in turn is influenced by the mastitis pathogen causing the infection. One characteristic feature of mammary gland inflammation is an elevation in the number of somatic cells in milk. Milk from uninfected mammary glands contains less than 100,000 somatic cells per milliliter. Milk somatic cell counts (SCC) greater than 200,000 per milliliter suggest that an inflammatory response has occurred, that a quarter is infected or is recovering from an infection. In addition, milk with SCC greater than 200,000 per milliliter clearly indicate that milk quality has been reduced.

Reason for concern
The potential for antibiotic residues in milk from dairy cows treated for mastitis is an important human health concern. Many people are allergic to, or can develop allergic type responses to milk and dairy products containing the smallest amount of antibiotics. Consequently, milk sold for human consumption should not contain any antibiotics.

Recent research from the University of Wisconsin showed that dairy herds with an elevated SCC were more likely to have bulk tank milk antibiotic residue violations than herds with lower SCC. The highest rate of antibiotic residue violations was seen in herds with average SCC values for the year of more than 700,000 somatic cells per milliliter of milk. High herd SCC indicates a significant mastitis problem. Herds with a high incidence of mastitis are more likely to treat more cows with antibiotics, thus increasing the potential occurrence of antibiotic residues in milk. Other risk factors associated with antibiotic residues in milk include: variation of antibiotic dosage, duration of treatment, use of multiple drugs, and mistakes regarding withholding periods.

In addition to antibiotic residues, development of antibiotic resistance is another human health concern. The public perception — right or wrong — is that the use of antibiotics in animal agriculture may be partly responsible for the emergence of drug-resistant bacteria, which, in turn, may decrease effectiveness of similar antibiotics used in human medicine.

Two examples of antibiotic resistant bacterial pathogens are Salmonella Typhimurium DT 104 and methicillin-resistant Staphylococcus aureus, both of which are major human health concerns. The potential for development of antimicrobial-resistant bacteria, and antibiotics that are no longer effective against animal and human bacterial pathogens, have lead to an increased awareness and concern over the use of antimicrobials in food- producing animals.

Take control of mastitis
Mastitis, milk quality and dairy food safety are indeed all interrelated. An increase in the incidence of mastitis in a herd will generally result in increased use of antibiotics, which, in turn, increases the potential for antibiotic residues in milk and the potential for increased bacterial resistance to antibiotics. This clearly illustrates the importance of effective mastitis prevention and control programs for your herd.

A safe, wholesome, abundant and nutritious milk supply should be the goal of every dairy producer in the world. Effective mastitis control strategies, including prudent use of antibiotics, will help dairy producers achieve these important goals.  

Stephen Oliver is a professor in the Department of Animal Science and Co-Director of the Food Safety Center of Excellence at the University of Tennessee, Knoxville.