Contagious Mastitis caused by Staphylococcus aureus Flashcards
What kind of intramammary infection does S. aureus cause?
Difficult and impossible to eliminate, chronic, subclinical, difficult to diagnose, resistant towards many drugs and cause gangrenous mastitis
In addition to decreased productivity, what other tissue changes can S. aureus cause?
Permanent parenchymal damage with fibrosis and microabscess formation
Why do you think S. aureus causing mastitis is difficult to treat?
As S. aureus commonly acquires antibiotic resistance, this will lead to incomplete killing of the bacterium which may give rise to L-forms
What are L-forms?
Cell wall-deficient variants that may not grow on standard culture media and produce following exposure of S. aureus to beta-lactam drugs
How to improve bacterial culture detection?
Freezing and thawing of milk samples as intracellular organisms are released from phagocytes
Describe the gangrenous changes that occur in peracute S. aureus mastits
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
How does S. aureus cause gangrenous mastitis?
The alpha-toxin of S. aureus is dermonecrotic & vasoconstrictive
What clinical signs can be seen in heifers freshening with chronic S. aureus mastitis?
Ricelike clots or pus from infected quarter(s), firm, oedematous & warm
What are the consequences of chronic S. aureus mastitis?
Palpable fibrosis, subclinical and remain as a major reservoir of S. aureus and a major risk to uninfected herdmates
How to diagnose S. aureus mastitis?
Creamy or puslike secretion, history of recurrent bouts of mastitis. Definitive diagnosis requires culture
Why does SCC is not as reliable as individual quarter cultures in the diagnosis of S. aureus mastitis?
Up to 50% of the cows in a herd need to be infected by S. aureus before bulk tank SCC become alarmingly elevated
What forms of treatment for S. aureus mastitis?
Lactation therapy for acute mastitis flareups & dry cow therapy
Explain why the true cure rate with lactation therapy is less than 40%
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)
What factors that can determine the bacteriological cure rates?
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
How to treat gangrenous S. aureus mastitis?
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
What are the best treatment options to treat clinical, nongangrenous S. aureus mastitis?
- To combine systemic (penicillin) & local therapy (up to 5 days)
What are the steps that should be taken if local therapy is indicated?
- Perform culture and susceptibility testing
- Use antibiotics for local treatment at least 4x at 12h-intervals or as directed
- Infected cows should be milked last or at least have mandatory backflushing or dipping the teat cups and milking machine claws into disinfectants
- Teat dip before and after milking
- Do not mix the treated milk into bulk tank
Explain why dry cow therapy is reported to be much more successful at curing than lactation therapy
- Less dilution of drug
- Longer duration of the drug within the udder
- Fewer opportunities for bacteria to form biofilms and L-forms during dry period
How to perform dry therapy?
Assess the antibiotic sensitivity of the predominant S. aureus strains in the herd & all quarters in infected herds should be treated at dry off
What are the objectives of dry therapy?
- To reduce the existing intramammary infection
2. To reduce the incidence of new intramammary infection
If lactation therapy and dry cow therapy fails, what can you advise the farmers?
Dry off of affected quarters and culling of the cows especially those who are chronically infected
What other non-antibiotic options to treat S. aureus mastitis?
- Bacteriocins (natural bactericidal proteins) to treat Streptococcus and S. aureus mastitis and for use in teat dips
- Lysostaphin (isolated from Staphylococcus simulans)
- Nisin (isolated from Lactococcus lactis)
- Combination cytokine & antibiotic therapy to improve bactericidal efficacy of certain antibiotics
- Immunomodulators such as beta-1,3 glucan (but unproven efficacy)
