Subclinical Mastitis in Dairy Cows

Introduction to Subclinical Mastitis

• Subclinical mastitis is a bacterial infection of the udder that often goes unnoticed.

• Most cases arise when bacterial exposure at the teat end exceeds the cow's immune defenses.

• Healthy teats are crucial for preventing mastitis infections.

Definition and Characteristics

• Definition: Subclinical mastitis is characterized by normal-appearing milk but presence of excessive inflammatory cells (neutrophils).

• Neutrophils migrate to the udder to combat the infection, resulting in a condition that leads to elevated somatic cell counts.

Somatic Cell Counts

• Somatic cells in healthy udders are usually less than 200,000 cells/mL.

• First lactation cows should ideally have counts below 100,000 cells/mL.

• Bacteriologically negative quarters often show counts under 50,000 cells/mL.

• A count exceeding 200,000 cells/mL indicates mastitis infection.

Importance of Testing

• Individual cow somatic cell counts are essential for identifying infected cows within a herd.

• Regular testing helps in herd management and infection control.

Myths about Somatic Cells

• Somatic cell counts are not affected by:

  • Cow breed

  • Milk yield (unless very low)

  • Stage of lactation (unless late lactation)

  • Nutritional management (unless selenium/Vitamin E deficient)

  • Other diseases (neutrophils target infection in udders specifically)

• Somatic cells are affected by management practices exposing teats to bacteria.

Sources of Bacterial Infection

• Mastitis pathogens include both contagious (from infected udders) and environmental (from the cows' surroundings) bacteria.

• This has been illustrated through studies showing high somatic cell counts and bacteriological analysis of milk samples.

  • Approximately 50% of high somatic cell count samples are culture negative.

  • 50% of detectable cases typically show gram-positive bacteria:

    • Coagulase-negative Staphylococcus

    • Environmental Streptococcus

    • Staphylococcus aureus (about 12%)

    • Streptococcus agalactiae

• In organic herds, cows infected with Staphylococcus aureus or Streptococcus agalactiae should be culled to prevent further infection spread.

Management Recommendations

• Regular testing for subclinical mastitis is crucial for herd health.

• Recommended testing options include:

  • DHIA monthly somatic cell count

  • California mastitis test

  • Somatic cell count test

  • Direct cell counter

• Aim for less than 15% of the herd exceeding 200,000 cells/mL; ideally under 10% for first lactation cows.

Conclusion

• Test routinely to identify and manage subclinical mastitis effectively.

• Determine specific bacterial causes through milk sampling for tailored control measures in collaboration with veterinarians.