Highly contagious, both Staphylococcus aureus and Streptococcus agalactiae are considered major mastitis pathogens that can significantly impact dairy producers economically. For the most part, these pathogens cause subclinical intramammary infections that quite often become chronic, John Middleton of the University of Missouri told attendees at the National Mastitis Council regional meeting in Ghent, Belgium, held Aug. 4-6, 2014. Sometimes, though, they can be associated with a high incidence of clinical disease.

Staphylococcus aureus is present in 43 percent of U.S. herds and in 74 percent of Canadian herds. In some regions of the world, he says, it’s the leading cause of clinical mastitis. Nearly all S. aureus infections come from the teat skin, milking equipment or other fomites but occasionally can come from the environment, Middleton said.

On the other hand, the incidence of Streptococcus agalactiae is fairly low in well-developed countries. In countries where dairy industries are just emerging, however, prevalence can be as high as 60 percent of herds, research suggests.

Sources of infection
In the case of S. aureus, infected mammary glands are usually the source of the infection. However, in some cases, the replacement heifer can also be a significant source, Middleton said. A heifer’s mammary gland can become infected as early as 6 months old, but heifers are most susceptible to infection during the last trimester of gestation, he said.

Another possible source includes colonized flies, especially where fly control either is ineffective or does not exist at all. “In fact, effective fly control,” Middleton stated, “has been shown to reduce risk.”

Finally, herds that purchase replacement heifers may also have a higher prevalence of S. aureus, Middleton stated, especially when those replacement heifers bring with them new strains of the pathogen.

Both S. aureus and S. agalactiae most commonly cause a subclinical intramammary infection (IMI). While S. agalactiae rarely causes clinical disease, S. aureus can be the leading cause of clinical mastitis in some countries, Middleton said.

S. aureus will usually cause only a moderate increase in milk somatic cell count, whereas S. agalactiae is associated with marked elevation in milk somatic cell count. It can be associated with very high bacteria counts, as well.

Prevention and control
When it comes to contagious mastitis, the number one source of infection is without a doubt the cow’s mammary gland. More often than not, transmission occurs during milking. It goes without saying then that to control contagious mastitis, one must eliminate transmission.

“The major focus of contagious mastitis control has been milking time hygiene, including single-use udder towels, having milkers wear gloves and dipping teats in a post-milking germicide to eliminate pathogens that may have colonized the teat skin from contaminated fomites, such as milking unit liners during milking,” Middleton explained.

In order to decrease the potential for transmission, control procedures include segregation, maintenance of milking equipment and re-evaluating milking order by putting uninfected cattle before infected ones.

In the case where particularly aggressive strains are the cause of mastitis, culling chronically infected cattle may be warranted to increase control, Middleton said. Finally, he also suggested producers prescreen replacement heifers and lactating cattle prior to allowing them to join the resident lactating population.

Vaccinating for S. aureus and S. agalactiae
While vaccinations for S. aureus have been studied for years, unfortunately, nothing gets rid of the pathogen entirely, Middleton stated. Currently, there are only two commercially available bacterins available for S. aureus mastitis: Lysigin (Boehringer Ingelheim Vetmedica, Inc.) and Startvac (Hipra). Lysigin is available in the U.S., while Startvac is available for use in Canada and Europe.

Field studies show that cattle vaccinated with Lysigin had lower clinical scores, lower somatic cell counts and fewer cases of chronic mastitis than those left untreated. In another recent study, the efficacy of commercially available Lysigin was compared with two experimental formations of Lysigin as well as unvaccinated controls.

All cattle became infected with S. aureus, Middleton said; however, those vaccinated with commercially available Lysigin, on average, had clinical mastitis for a significantly shorter period of time. Post-challenge, he said, they also had a lower total mastitis score than the controls did.

Similar to results seen for Lysigin, Startvac has been shown to stimulate the production of antibodies and relieve inflammation in the intramammary gland post-challenge, Middleton stated. It does not, however, completely prevent infection. Field trial with Startvac show decreased durations as well as decreased transmissibility of infection, he said.

Finally, little has been done in terms of designing vaccines for S. agalactiae since it can be eradicated through the use of antimicrobials, Middleton said. To his knowledge, there are currently no vaccinations on the market for S. agalactiae specifically for cattle. PD

Melanie Epp is a freelance writer based in Ontario, Canada.