Instructions

Read the chapters on Staphylococci, Streptococci, and Mastitis to prepare for the questions in Case Studies A–D. Also review the chapter on Enterobacteriales to strengthen your understanding of Gram-negative bacterial infections. The mastitis practical will help you learn the diagnostic tests used for mastitis detection.

Case A – An outbreak of mastitis in a dairy herd

A dairy farm near Gympie milks 200 Jersey cows. About six months after introducing replacement cows, 15 lactating cows developed acute and subacute clinical mastitis. Both clinical and subclinical mastitis cases resulted in elevated somatic cell counts (SCC) and reduced milk production. Milk from affected quarters of some cows yielded Staphylococcus aureus.

Questions

1. What is the clinical significance of finding S. aureus in milk samples from affected cows?
2. Explain why mastitis caused by S. aureus leads to reduced milk yield (describe the pathogenesis).
3. What methods can be used to identify infected cows in the herd?

Case B – Subclinical mastitis in a Dairy herd

A dairy farm near Milla-Milla on the Atherton Tablelands noticed a significant increase in bulk tank somatic cell counts (SCC) over three months, resulting in financial penalties for reduced milk quality. The overall milk production of the 150 lactating cows also dropped. Culture of the bulk tank milk sample revealed the presence of Streptococcus agalactiae. Streptococcus agalactiae is a contagious mastitis pathogen.

Questions

1. What is the clinical significance of detecting S. agalactiae in the bulk tank milk sample?
2. What interventions are required to control this outbreak and prevent future cases (including treatment, cow management, and preventive measures)?

Case C- Peracute mastitis in Dairy goats

Following a period of heavy rainfall, a goat dairy farm near Cairns reported eight nanny goats with high fever and udders that were hard, swollen, cold, and bluish in color. Milk from affected halves was watery and blood-tinged. Culture of milk samples yielded Staphylococcus aureus and Mannheimia haemolytica. These clinical signs indicate peracute, gangrenous mastitis, which is often fatal and requires urgent intervention.

Questions

1. Explain the clinical significance of isolating S. aureus and M. haemolytica from milk samples in this outbreak.
2. Discuss the risk factors and circumstances that could lead to peracute mastitis in goats under these conditions.

Case D

A Holstein dairy farm in the Darling Downs region near Toowoomba has reported an unusually high number of cows developing peracute and acute clinical mastitis over the past few weeks. Most affected cows are in late lactation and in their third or later lactation. Milk cultured from affected quarters yielded Escherichia coli.

Questions

1. Explain the clinical significance of isolating E. coli from milk samples in this outbreak.
2. Based on the pathogenesis of coliform mastitis, outline how you would manage cows with peracute and acute mastitis.

END OF CASE STUDY