The usual suspects:
Arcanobacterium pyogenes creates abscesses, which are unlikely to be penetrated and resolved by any antibiotic given by any route. Staphylococcus aureus creates “microabscesses” which make therapy difficult, and it may also survive inside phagocytic cells and thus be inaccessible to antimicrobial drugs.
Number of Quarters Infected
This should always be considered when deciding whether or not to treat mastitis. Expenses rise and prognosis usually worsens when more than one quarter is affected.
Drugs with good lipid solubility are more likely to achieve therapeutic concentrations in milk. Even intrammammary therapy will not penetrate well into abscessed areas of the gland or into milk ducts blocked by the products of inflammation or necrosis.
Mastitis-causing Staphyloccocal bacteria other than Staphylococcus aureus are known as the coagulase-negative Staphylococci. They include S. epidermis, S. xylosis, S. warneri and S. intermedius. They are normal skin flora of cows and humans. Mastitis caused by these organisms is generally mild, with no noticeable inflammation of the udder and only mild changes in the milk (clots and flakes) and elevations in Somatic Cell Count. They may cause chronic subclinical infections. These infections are typically contracted during the dry period and persist into early lactation. It is common for this type of mastitis to resolve spontaneously or following antimicrobial treatment.
Arcanobacterium pyogenes causes sporadic cases of suppurative mastitis and is frequently found in combination with other pathogens in milk culture samples. Arcanobacterium pyogenes infection is most common in the dry period and in wet environments. Biting flies may carry the organism and potentiate infection by damaging the skin of the teat. A. pyogenes mastitis is found more commonly in pastured cattle than housed cattle. Mastitis due to A. pyogenes is usually moderate to severe, with milk eventually becoming thick and purulent with a foul odor. It is generally confined to one quarter, which becomes quite swollen, hard, and painful. Systemic signs of fever, anorexia and depression may also be present. If the cow survives the initial onset of mastitis, the quarter will eventually abscess, production will be lost in one or all quarters, and the affected quarter is likely to burst open and drain from abscessed areas. This pathogen is slow growing in culture, and freezing milk samples will reduce pathogen yield from the culture. Arcanobacterium. pyogenes mastitis is not responsive to treatment. Fly control and care to minimize teat injury may help reduce the incidence of A. pyogenes mastitis.
Pseudomonas aeruginosa mastitis is most commonly caused by contamination of intramammary therapy products, teat dips, or wash water used to wash udders before milking (the danger posed by such contamination may be compounded by milking without ensuring that teats are completely dry first). Clinically, P. aeruginosa infection may be peracute and severe due to endotoxin production, or it may be subacute or chronic with intermittent flare-ups. Chronic infections may suddenly become acute and severe. It is not uncommon for the function of the gland to be lost, and therapy is generally unsuccessful. Herd outbreaks of P. aeruginosa mastitis usually originate from some source of contamination.
Nocardia species (most commonly N. asteroides) can cause an acute or subacute granulomatous mastitis. Systemic signs such as fever may be present. Chronically infected cows may lose production and body weight over time. There is progressive tissue damage to the udder, which may be felt as hard masses, particularly in the lower part of the udder. Nocardia is commonly found in the soil or on udder skin, and it can survive in some teat dips and udder infusions. This organism may take 72 hours to grow in culture, and it is not responsive to treatment.
Bacilli are widespread in the environment, particularly in the soil. B. cereus is associated with teat injury or teat surgery. B cereus mastitis may also result from the feeding of contaminated brewer’s grains. Mastitis due to Bacillus is typically chronic, but it may become acute and cause potentially fatal hemorrhagic mastitis with gangrene and toxemia. Bacilli are also a common milk culture contaminant.
Serratia mastitis usually exhibits mild clinical signs and it is frequently self-limiting. Serratia is a common contaminant of milk culture samples. Serratia mastitis may be a result of contaminated teat dips.
C. bovis is considered primarily an inhabitant of the streak canal, but it may also cause mastitis and elevate Somatic Cell Counts. C. bovis infection may be protective against Staphylococcus aureus infection. Isolation of C. bovis from a mastitic cow may suggest insufficient teat dipping practices on the farm.
S. dublin infection is typically chronic and does not respond well to treatment. S. typhimurium mastitis is usually secondary to more severe systemic disease.
Isolation of yeast from a clinical mastitis case suggests contamination of treatments or teat dips. Mastitis due to yeast infection is extremely likely to resolve spontaneously, although it may take weeks to do so. Treatment with antibiotics will delay resolution of yeast mastitis.
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