Mastitis cases caused by yeast and mold are typically sporadic and are usually not a significant issue in a herd. Yeast form white or cream-colored colonies that can easily be confused with coagulase-negative staphylococci. Gram staining will differentiate yeast from CNS; yeast are two to three times larger than CNS and typically exhibit budding. Mold colonies can have a fluffy texture and exhibit several different colors.

Coagulase negative staphylococci (CNS) and Staphylococcus hyicus are mastitis-causing pathogens that originate from skin flora. CNS infections are usually mild and subclinical, and result in a two- to three-fold increase in somatic cell count. Proper milking procedures including the use of efficacious post-milking teat disinfectants and milking clean, dry teats will help to reduce the number of new infections.

Corynebacterium bovis is a contagious mastitis-causing pathogen. It is mildly pathogenic, and will usually cause only a mild increase in SCC and a slight reduction in milk production. Efficacious post-milking disinfectants will dramatically reduce the infection rate from C. bovis, and dry cow therapy is very effective in eliminating C. bovis.

Enterobacter spp. are environmental mastitis-causing pathogens. The spread of Enterobacter spp. occur primarily through environmental contact, particularly when teats come in contact with manure or contaminated bedding. The primary way to prevent infection by Enterobacter spp. is by keeping cows clean and dry to decrease exposure, as well as proper teat sanitation prior to milking.

Mycoplasma spp. are a contagious mastitis-causing pathogen. Infected cows typically have mastitis in multiple quarters and exhibit a dramatic decrease in milk production. A Mycoplasma spp. mastitis outbreak can be preceded by a respiratory disease event in bovines of different ages on the farm.

Pasteurella spp. are contagious pathogens that are seldom reported as a cause of bovine mastitis. Mastitis caused by Pasteurella spp. usually appears as a thick, creamy-yellow, viscous secretion, sometimes with a foul odor. The spread of Pasteurella spp. is not fully understood, but most likely is cow to cow. Preventing teat injuries and removing infected cows are the main ways to prevent and control this pathogen. Pasteurella spp. do not respond well to antibiotic therapy and can result in death; thus, prevention is key.

Proteus spp. are uncommon environmental mastitis pathogens that have been known to cause outbreaks. Little is known about how Proteus spp. infect the mammary gland, however procedures effective in controlling coliform mastitis pathogens should apply to Proteus spp. as well. Proteus spp. typically cause chronic infections that do not respond well to antibiotic therapy.

Prototheca spp. are mastitis-causing pathogens that are classified as achlorophyllic algae. Mastitis infections from Prototheca spp. can occur at any time but will most likely occur when cows have access to wet areas such as stagnant ponds. Because Prototheca spp. are contagious, once a cow becomes infected, other cows are at risk during milking. Prototheca spp. can cause chronic infections that do not respond to current antimicrobial therapies.

Pseudomonas spp. are environmental mastitis-causing pathogens. Pseudomonas spp. has been isolated from milking parlor drop hoses, and is known to cause mastitis through the use of water during milking. Prevention and control of Pseudomonas spp. is based on reducing the exposure of cattle to infected water sources. Pseudomonas spp. does not respond well to antibiotic therapy, thus prevention is key.

Authored by Turner Swartz, former Ph.D. student, Dairy Science, Virginia Tech, and Christina S. Petersson-Wolfe, Associate Professor & Extension Dairy Scientist, School of Animal Science, Virginia Tech. First published April 2016, last reviewed January 2026.

Mastitis cases caused by yeast and mold are typically sporadic and are usually not a significant issue in a herd. Yeast form white or cream-colored colonies that can easily be confused with coagulase-negative staphylococci. Gram staining will differentiate yeast from CNS; yeast are two to three times larger than CNS and typically exhibit budding. Mold colonies can have a fluffy texture and exhibit several different colors.