|
Mastitis in Cattle: Overview |  |
| Almost any bacterial or mycotic organism that can opportunistically invade tissue and cause infection can cause mastitis. However, most infections are caused by various species of streptococci, staphylococci, and gram-negative rods, especially lactose-fermenting organisms of enteric origin, commonly termed coliforms. From an epidemiologic standpoint, the source of infection may be regarded as contagious or environmental. Except for
Mycoplasma
spp
, which may spread from cow to cow through aerosol transmission and invade the udder subsequent to bacteremia, contagious pathogens are spread during milking by milkers’ hands or the liners of the milking unit. Species that utilize this mode of transmission include
Staphylococcus
aureus
,
Streptococcus
agalactiae
, and
Corynebacterium
bovis
. Most other species are opportunistic invaders from the cow’s environment, although some other streptococci and staphylococci may also have a contagious component. The bedding used for housing cattle is the primary source of environmental pathogens, but contaminated teat dips, intramammary infusions, water hoses used for udder preparation during milking, water ponds or mud holes, skin lesions, teat trauma, and flies have all been incriminated as sources of infection. |
| Intramammary infections are often described as subclinical or clinical mastitis.
Subclinical mastitis is the presence of an infection without apparent signs of local inflammation or systemic involvement. Although transient episodes of abnormal milk or udder inflammation may appear, these infections are for the most part asymptomatic and, if the infection persists for at least 2 mo, are termed chronic. Once established, many of these infections persist for entire lactations or the life of the cow. Detection is best done by examination of milk for
somatic cell counts (predominantly neutrophils) using either the California Mastitis Test or automated methods provided by dairy herd improvement organizations. Somatic cell counts are positively correlated with the presence of infection. Although variable (especially if determined on a single analysis), cows with a somatic cell count of ≥ 280,000 cells/mL (≥ a linear score of 5) have a >80% chance of being infected. Likewise, the higher the somatic cell count in a herd bulk tank,
the higher the prevalence of infection in the herd. Causative agents must be identified by bacterial culture of milk. |
|
Clinical mastitis is an inflammatory response to infection causing visibly abnormal milk (eg, color, fibrin clots). As the extent of the inflammation increases, changes in the udder (swelling, heat, pain, redness) may also be apparent. Clinical cases that include local signs only are referred to as mild or moderate. If the inflammatory response includes systemic involvement (fever, anorexia, shock), the case is termed severe. If the onset is very rapid, as often occurs
with severe clinical cases, it is termed an acute case of severe mastitis. More severely affected cows tend to have more serous secretions in the affected quarter. Although any number of quarters can be infected simultaneously in subclinical mastitis, typically only one quarter at a time will display clinical mastitis. However, it is not uncommon for clinical episodes caused by
Mycoplasma
to affect multiple quarters. Gangrenous mastitis can also occur, particularly when subclinical, chronic infections of
S
aureus
become severe at times of immunosuppression (eg, at parturition). As with subclinical mastitis, culture of milk samples collected from affected quarters is the only reliable method to determine the etiology of clinical cases. |
