Broadly speaking, mastitis in New Zealand is separated into two groups: environmental and contagious. While the majority of cases throughout the season are from environmental sources, contagious mastitis bugs are also present and can become a major issue if not identified early on. Contagious mastitis is spread from cow to cow, often during the milking process where bacteria can sit on milker’s hands or inside the liners. The major contagious pathogen in New Zealand dairy herds is Staphylococcus aureus.

Clinical signs and disease process:

Clinically, Staph aureus mastitis occurs in many forms, ranging from the suddenly collapsed cow with black mastitis, to a chronic, low grade subclinical infection. Chronic infections are the most important cause for financial losses via reduced milk production, high somatic cell count (SCC), treatment costs, and/or culling. Chronically infected cows also pose a major risk to other cows in the herd.

New infections with Staph aureus occur when uninfected quarters are exposed to infected milk. The bacteria multiply in milk ducts, leading to inflammation, blockage, and atrophy of the milk producing tissue. They can form fibrotic tissue and tiny abscesses, which are difficult to penetrate with antibiotic treatment. For this reason, Staph aureus infections tend to have a poor cure rate, and early identification and treatment is important.

Detection:

Culture has been the standard way to diagnose Staph aureus infection, however, an infected cow will only shed Staph aureus intermittently, with a peak in shedding pattern every 6.5 days. Because of this, a single culture has only a 74.5% probability of detecting Staph. This number will jump to 94 and 98% when culturing 2 and 3 samples, respectively.

Staph aureus screening can also be done during your herd test, using a test called PCR (polymerase chain reaction). Basically, the test is looking for the genetic material of the pathogen but has similar limitations as milk culture due to the intermittent shedding from an infected gland. When positive, there will be no doubt about the presence of infection. However, a single negative PCR result does not rule-out presence of “non-shedding” Staph. Approximately 70% of Staph positive cows will be identified by a single herd test.

StaphGold™ is an antibody test detecting the immune response to the Staph infection. The test has a sensitivity (true positive cows) of over 90% and the specificity (true negative cows) is very high (95%). Because this test looks for antibodies to Staph, rather than the bug itself, it is not affected by intermittent shedding.

Control and prevention:

Once a cow is identified positive, it is important that she is separated and milked last to avoid transmission to her herd mates. It is also important that all operators in the shed wear gloves. If treating, the cow should not be returned to the main milking mob until it is certain she has cleared the infection. Due to its poor cure rate and the risk to other cows in the herd, culling is often the best option for chronically infected cows.