# High-Mortality Outbreak of Staphylococcus aureus Mastitis Associated with Poor Milking Practices in a Goat Dairy

## TL;DR

A severe outbreak of Staphylococcus aureus mastitis in a goat herd led to high mortality, but improved milking practices and control measures successfully reduced the infection.

## Contribution

The study demonstrates how poor milking practices contribute to S. aureus mastitis outbreaks and how targeted interventions can control them.

## Key findings

- Approximately 30% of the herd developed fatal gangrenous mastitis caused by Staphylococcus aureus.
- Poor milking hygiene and equipment dysfunction were key factors in the spread of the infection.
- Improved hygiene and vaccination reduced S. aureus to undetectable levels in subsequent seasons.

## Abstract

Mastitis is a painful and economically significant udder infection that reduces milk production and can result in death in severe cases. During a kidding season, a commercial dairy goat herd experienced a severe outbreak of mastitis caused by Staphylococcus aureus, a contagious bacterium transmitted primarily during milking. Approximately one-third of the herd developed gangrenous mastitis, resulting in mortality. A substantial proportion of apparently healthy goats had S. aureus recovered from their milk, contributing to bacterial dissemination. Poor milking hygiene and milking equipment maintenance were identified as key risk factors for transmission. Goats with infections scored higher on an on-farm screening test, demonstrating its utility for early detection and management. Control measures, including improved milking hygiene, equipment maintenance, segregation of infected animals, and vaccination of replacement does, successfully reduced S. aureus to undetectable levels in the subsequent two kidding seasons, although environmental Staphylococci persisted. This case highlights the role of subclinical carriers in mastitis outbreaks and illustrates practical interventions that enhance animal welfare, milk quality, and farm sustainability.

An outbreak of mastitis caused by Staphylococcus aureus occurred in a commercial dairy goat herd during kidding season, resulting in fatal gangrenous mastitis in approximately 30% of the herd. S. aureus was recovered from milk, mammary tissue, and other organs in does subjected to necropsy. The S. aureus milk culture-positive rate among does in the hospital pen was 58.3%, while whole-herd milk cultures of clinically normal mature does identified S. aureus in 15.0% with an additional 15.0% positive for coagulase-negative Staphylococcus (CNS), yielding a total culture-positive rate of 30.0%. The prevalence of CNS in subclinical animals was consistent with previous reports from U.S. dairy goats; in contrast, S. aureus isolation rates substantially exceeded previously reported prevalences. Poor milking hygiene and milking machine dysfunction were identified as major factors contributing to the spread of the S. aureus from goat to goat. California Mastitis Test (CMT) scores were significantly higher in culture-positive does compared with culture-negative animals (p < 0.05), demonstrating the value of CMT as a practical on-farm tool for early treatment decision making. Interventions focused on addressing milking hygiene and milking machine maintenance, as well as segregation and vaccination of replacement females. S. aureus dropped to undetectable in the next two kidding seasons, whereas the CNS culture rates remained unchanged, suggesting other factors may be contributing to CNS infection. This case highlights the role of subclinical intramammary infection and milking practice factors in transmission and control of contagious mastitis pathogens like S. aureus.