Contagious Mastitis caused by Staphylococcus aureus

Question 1

What kind of intramammary infection does S. aureus cause?

Answer 1

Difficult and impossible to eliminate, chronic, subclinical, difficult to diagnose, resistant towards many drugs and cause gangrenous mastitis

Question 2

In addition to decreased productivity, what other tissue changes can S. aureus cause?

Answer 2

Permanent parenchymal damage with fibrosis and microabscess formation

Question 3

Why do you think S. aureus causing mastitis is difficult to treat?

Answer 3

As S. aureus commonly acquires antibiotic resistance, this will lead to incomplete killing of the bacterium which may give rise to L-forms

Question 4

What are L-forms?

Answer 4

Cell wall-deficient variants that may not grow on standard culture media and produce following exposure of S. aureus to beta-lactam drugs

Question 5

How to improve bacterial culture detection?

Answer 5

Freezing and thawing of milk samples as intracellular organisms are released from phagocytes

Question 6

Describe the gangrenous changes that occur in peracute S. aureus mastits

Answer 6

Most frequently occur in postpartum cows with signs of toxaemia worsen as the swollen, firm quarter changes colour from pink to red to purple to blue within a few hours, cold udder. Affected cattle have tachycardia, depression and inappetence

Question 7

How does S. aureus cause gangrenous mastitis?

Answer 7

The alpha-toxin of S. aureus is dermonecrotic & vasoconstrictive

Question 8

What clinical signs can be seen in heifers freshening with chronic S. aureus mastitis?

Answer 8

Ricelike clots or pus from infected quarter(s), firm, oedematous & warm

Question 9

What are the consequences of chronic S. aureus mastitis?

Answer 9

Palpable fibrosis, subclinical and remain as a major reservoir of S. aureus and a major risk to uninfected herdmates

Question 10

How to diagnose S. aureus mastitis?

Answer 10

Creamy or puslike secretion, history of recurrent bouts of mastitis. Definitive diagnosis requires culture

Question 11

Why does SCC is not as reliable as individual quarter cultures in the diagnosis of S. aureus mastitis?

Answer 11

Up to 50% of the cows in a herd need to be infected by S. aureus before bulk tank SCC become alarmingly elevated

Question 12

What forms of treatment for S. aureus mastitis?

Answer 12

Lactation therapy for acute mastitis flareups & dry cow therapy

Question 13

Explain why the true cure rate with lactation therapy is less than 40%

Answer 13

The use of wrong drug, reduced dosage, compromised length of treatment, resistant of strains of organism, decreased activity of drug due to pH, biofilm inflammatory debris, high SCC in mammary gland environment as well as inability of the drug to reach the organism (intracellular location, presence of abscesses & fibrotic lesions)

Question 14

What factors that can determine the bacteriological cure rates?

Answer 14

Time of infections (newly acquired infection can achieve as high as 70%), age of affected cows, stage of lactation (earlier the better), number of affected quarters (higher for front quarters, SCC & treatment duration

Question 15

How to treat gangrenous S. aureus mastitis?

Answer 15

Systemic antibiotics (extra-label doses of ceftiofur or tetracycline), fluids and flunixin. If the teat is purple, remove with an emasculator/emasculatome to allow drainage

Question 16

What are the best treatment options to treat clinical, nongangrenous S. aureus mastitis?

Answer 16

1. To combine systemic (penicillin) & local therapy (up to 5 days)

Question 17

What are the steps that should be taken if local therapy is indicated?

Answer 17

1. Perform culture and susceptibility testing
2. Use antibiotics for local treatment at least 4x at 12h-intervals or as directed
3. Infected cows should be milked last or at least have mandatory backflushing or dipping the teat cups and milking machine claws into disinfectants
4. Teat dip before and after milking
5. Do not mix the treated milk into bulk tank

Question 18

Explain why dry cow therapy is reported to be much more successful at curing than lactation therapy

Answer 18

1. Less dilution of drug
2. Longer duration of the drug within the udder
3. Fewer opportunities for bacteria to form biofilms and L-forms during dry period

Question 19

How to perform dry therapy?

Answer 19

Assess the antibiotic sensitivity of the predominant S. aureus strains in the herd & all quarters in infected herds should be treated at dry off

Question 20

What are the objectives of dry therapy?

Answer 20

1. To reduce the existing intramammary infection

2. To reduce the incidence of new intramammary infection

Question 21

If lactation therapy and dry cow therapy fails, what can you advise the farmers?

Answer 21

Dry off of affected quarters and culling of the cows especially those who are chronically infected

Question 22

What other non-antibiotic options to treat S. aureus mastitis?

Answer 22

1. Bacteriocins (natural bactericidal proteins) to treat Streptococcus and S. aureus mastitis and for use in teat dips
2. Lysostaphin (isolated from Staphylococcus simulans)
3. Nisin (isolated from Lactococcus lactis)
4. Combination cytokine & antibiotic therapy to improve bactericidal efficacy of certain antibiotics
5. Immunomodulators such as beta-1,3 glucan (but unproven efficacy)