What Are the Signs of Staphylococcus aureus Infections in Dairy Cows?

Mastitis is a persistent issue for dairy farmers worldwide, and one of the most notorious pathogens responsible for this disease is Staphylococcus aureus. Known for its ability to cause chronic and difficult-to-treat infections, S. aureus can have devastating consequences on milk production, cow health, and farm profitability if not properly managed. In this guide, we will explore the signs of S. aureus infections in dairy cows, how they spread, and what farmers can do to mitigate their impact.

Understanding Staphylococcus aureus and Its Impact

Staphylococcus aureus is a gram-positive bacterium that has adapted remarkably well to the dairy cow’s udder environment. It is highly contagious and typically spreads during the milking process through infected milk, contaminated equipment, or human contact. Once it enters the udder through the teat canal, it can establish long-lasting infections that are often difficult to treat.

One of the reasons S. aureus is so problematic is its ability to form protective biofilms within the udder. These biofilms act as a shield, making the bacteria resistant to the cow’s immune response and even certain antibiotics. As a result, S. aureus infections can become chronic, persisting in the herd for months or even years, leading to repeated flare-ups of mastitis and ongoing milk losses.

Signs of Staphylococcus aureus Mastitis in Dairy Cows

The clinical presentation of S. aureus mastitis can vary significantly depending on the stage and severity of the infection. Farmers must be vigilant and aware of both subclinical and clinical signs to ensure timely intervention.

1. Subclinical Mastitis
   In many cases, S. aureus causes subclinical mastitis, meaning there are no visible signs of infection. However, this does not mean the cow is healthy. Subclinical infections are often identified through milk testing, particularly by measuring the somatic cell count (SCC), which is typically elevated when the cow’s immune system is fighting an infection.
   While cows may appear to be producing milk normally, subclinical mastitis can result in a decrease in milk quality. Infected cows may produce milk with:
   • Higher SCC levels, indicate inflammation and can affect milk quality.
   • Decreased butterfat and protein content, leading to lower quality milk that is less desirable for processors.
   • Higher bacterial counts may result in spoilage or rejection by milk processors.
2. Subclinical infections are especially insidious because they are easy to overlook, allowing the disease to spread quietly throughout the herd.
3. Clinical Mastitis
   Clinical mastitis caused by S. aureus presents with more obvious symptoms and is generally divided into acute, chronic, and gangrenous forms. Each presents unique challenges and requires different management strategies.
   • Acute Mastitis: In cases of acute S. aureus mastitis, cows will show clear signs of udder inflammation, including:
     • Swelling, heat, and tenderness in the affected quarter.
     • Changes in milk appearance, such as the presence of clots, flakes, or pus, may occur. The milk may also become watery or discoloured.
     • The cow may show discomfort, particularly during milking, and may reduce her overall milk output.
   • While these symptoms are concerning, acute infections may respond to treatment if caught early, though this depends on how far the infection has progressed and the cow’s overall health.
   • Chronic Mastitis: Unfortunately, chronic infections are much more common with S. aureus. These cows may experience recurrent episodes of mastitis, with periods of apparent recovery followed by new flare-ups. Signs of chronic mastitis include:
     • Persistent high SCC, even after treatment.
     • Decreased milk yield over time as the infection damages the milk-producing tissue in the udder.
     • Hard lumps or fibrosis in the udder due to tissue scarring. These lumps are typically non-painful but indicate irreversible damage to the affected quarter.
   • Chronic mastitis is often difficult to treat because S. aureus can hide in scar tissue or biofilms, making it resistant to antibiotics. In these cases, affected cows may become permanent carriers of the bacteria, posing a constant risk to the rest of the herd.
   • Gangrenous Mastitis: The most severe form of S. aureus mastitis is gangrenous mastitis, which is relatively rare but extremely serious. This condition occurs when the bacteria produce toxins that cause tissue death in the udder. Symptoms include:
     • Sudden, severe swelling in one or more quarters, often with a bluish or black appearance.
     • Coldness of the udder, as blood flow to the affected area is compromised.
     • The cow may show signs of systemic illness, such as fever, depression, and a sharp drop in milk production.
   • Gangrenous mastitis is a veterinary emergency, and cows affected by this form of the disease often require aggressive treatment or, in severe cases, euthanasia to prevent suffering.

How Staphylococcus aureus Spreads in a Herd

The primary way S. aureus spreads is through cow-to-cow transmission, particularly during the milking process. This can happen when:

• Infected cows shed bacteria into the milk, contaminating the milking equipment.
• Milking machines or staff hands are not properly sanitised between cows, allowing bacteria to spread from one udder to the next.
• Cows with open teat ends or damaged skin are more susceptible to infection, making proper milking techniques and post-milking teat disinfection crucial.

Once a cow is infected, she can continue to shed bacteria into the milk for long periods, even if she shows no signs of mastitis. Therefore, it is essential to identify and manage infected cows effectively to prevent the spread of S. aureus within the herd.

Diagnosis of Staphylococcus aureus Infections

Timely diagnosis is key to managing S. aureus infections. Diagnostic methods include:

• Somatic Cell Count (SCC): Routine SCC testing helps detect subclinical infections early. Elevated SCC in individual cows or bulk tank samples can indicate the presence of mastitis, even before clinical signs appear.
• Bacterial Culture: A milk culture test can confirm the presence of S. aureus bacteria. This test is important not only for identifying the pathogen but also for determining which antibiotics may be effective in treating the infection.
• PCR Testing: This is a faster and more sensitive diagnostic tool that detects the DNA of S. aureus bacteria in milk samples, even when they are present in low numbers.

Management and Control of Staphylococcus aureus Infections

Controlling S. aureus mastitis requires a multifaceted approach:

1. Milking Hygiene: The cornerstone of S. aureus control is maintaining excellent milking hygiene. This includes sanitising teats before and after milking, using properly cleaned and maintained equipment, and milking infected cows last to avoid spreading the bacteria.
2. Culling Chronic Cases: Cows with chronic or recurrent S. aureus infections may need to be culled to prevent them from acting as a reservoir of infection for the rest of the herd.
3. Selective Dry Cow Therapy: Administering long-acting antibiotics at dry-off can help eliminate lingering infections and prevent new cases of mastitis during the dry period.
4. Vaccination: In some cases, vaccines against S. aureus are available, which can reduce the incidence and severity of infections. However, vaccination should be used in conjunction with 

Staphylococcus aureus infections can severely impact your herd’s health and milk production if not properly managed. Early detection and targeted interventions are key to preventing the spread of this pathogen. At FarmLab Diagnostics, we offer comprehensive milk testing services, including SCC testing, bacterial culture, and PCR diagnostics, to help you identify and control S. aureus mastitis in your herd. Contact us today to learn how our services can protect your cows and your livelihood.