Why Pathogen Identification Matters for Mastitis Control
Pathogen identification is an essential tool for implementing an effective mastitis control program. The disease can present as subclinical, having elevated Somatic Cell Count (SCC) but no outward signs in the udder or milk, or as a clinical infection, characterized by inflammation of the udder, flaky/serous milk or in severe cases anorexia, fever and sometimes death.
In 2012, North Dakota State University estimated that mastitis costs the US dairy industry $2 billion annually, approximately 10% of the total value of farm milk sales. This revenue loss is in the form of discarded and lower quality milk, as well as time, materials and labor for treatment and control.
In order to reduce losses associated with mastitis, an effective control program at farm level should target preventative measures towards the most predominant pathogen(s) responsible for mastitis cases. There are more than 100 different organisms (Schroeder, 2012) that can be responsible for mastitis and their prevalence depends on region, season, and farm setup among other factors.
Pathogens can be separated into several categories based on characteristics such as transmission, manifestation of disease and response to treatment. Pathogen identification is therefore an essential step to develop a targeted control program for mastitis.
Major and Minor Pathogens
The presentation of mastitis as clinical or subclinical depends on the interaction between the causative pathogen and the cow’s immune response. Infection by major pathogens tends to produce a higher elevation in SCC, with a more severe response and longer duration of infection.
Major pathogens include Staphylococcus aureus, Streptococcus spp, E. coli and Klebsiella spp. Clinical mastitis cases often return bacterial cultures of major pathogens. The cow’s immune system responds more aggressively to infection by these pathogens, resulting in significant decreases in milk production as the body’s energy and metabolic resources are diverted to fighting infection.
Minor mastitis causing pathogens generally produce a less severe response compared to major pathogens, therefore are responsible primarily for subclinical mastitis. Pathogens classified as minor include Corynebacterium bovis, and Coagulase Negative Staphylococci. In udders with a history of clinical infections, minor pathogens can cause SCC levels to rise above those normally produced by major pathogens. There is some evidence that the presence of minor pathogens may have a protective effect against major pathogens by outcompeting for resources and stimulation of leukocytes to build immunity against the more aggressive organisms (Hurley, 2009).
Contagious & Environmental Pathogens
Contagious pathogens are spread from cow to cow, with prevalence increasing with parity and lactation stage. Contagious pathogens include Staphylococcus aureus, Streptococcus agalactiae & dysgalactiae, Corynebacterium bovis and Mycoplasma. Persistent infections by contagious pathogens can damage milk secretory cells, resulting in long term detrimental effects on yield and quality.
Due to the lower cure rates for contagious pathogens, especially Staphylococcus aureus, culling chronically infected cows may be a more economical long term treatment to eliminate the risk of transmission to uninfected cows rather than repeated antibiotic treatment. Other contagious pathogens such as Streptococcus spp have better responses to antibiotic treatment and control measures.
Environmental pathogens produce new cases of mastitis independent of infections in herd mates, transmitted from contaminated soil/bedding, water or feces. Organisms include E. coli, Klebsiella spp, Streptococcus uberis, bovis & dysgalactiae and Actinomyces pyrogenes.
Compared to contagious pathogens, infection by environmental pathogens is shorter in duration. Environmental pathogens are largely resistant to germicides and antibiotics, with the highest risk period being early lactation when the immune system is suppressed. Herds with low SCC tend to have the majority of clinical cases caused by environmental pathogens (Politis, 2012).
Gram Positive & Gram Negative
Gram positive and negative bacteria differ in that Gram negative bacteria have an impenetrable cell wall. This means that natural antibodies of the immune system are ineffective against them and they develop resistance to antibiotics faster than Gram positive bacteria, making them more virulent and more harmful. Gram positive infections tend to be chronic; long subclinical phases before flare-ups of clinical symptoms.
Gram positive bacteria include Staphylococcus aureus, Mycoplasma, Streptococcus dysgalactia & uberis and Actinomyces pyrogenes. Infection by these pathogens is characterized by slow recovery time (Pantoja & Ruegg, 2013). Gram negative pathogens include E.coli and Klebsiella spp and infections caused by these are characterized by very short subclinical phases.
Knowing the causative pathogen of mastitis outbreaks allows effective control measures and treatment to be taken. Clinical cases of mastitis caused by major pathogens may result in more milk being discarded due to visible signs of the disease in the milk, such as flakes. While the visible effects on milk of subclinical mastitis are not as severe, infection by minor pathogens may require that a cow’s milk is withheld from the bulk tank for a longer period of time due to prolonged high SCC.
Being aware of whether the responsible pathogen is environmental or contagious will be the most important factor in developing an effective control plan. Environmental pathogens respond well to dry treatment, teat dipping and general barn/cow hygiene measures such as udder clipping. Most pathogens in this category respond at least moderately well to antibiotic treatment. Contagious pathogens tend to be chronic and may require more severe control measures such as segregation or culling of infected cows to prevent the spread of disease.
The Gram status of a pathogen will determine the most effective treatment route to clear a mastitis infection. Gram negative bacterial infections are rarely cleared up spontaneously by the immune system, and may be resistant to antibiotics, making culling or segregation more likely to be an effective measure. Gram positive pathogens tend to produce chronic infections, but most species are more likely to be cleared with proper antibiotic treatment.
While an individual infection may not fit perfectly into a particular category, knowledge of pathogen identification will enable early treatment based on the measures most likely to be effective.
Table 1 summarizes the characteristics discussed above for several common mastitis causing pathogens.
For definitive identification, samples should be properly cultured and then veterinary treatment advice should be sought. During the time the samples are being cultured (approximately three days), commencing early treatment based on likely pathogen will shorten infection duration and intensity, translating into reduced treatment costs and production losses.
Despite the current large annual cost of mastitis to the dairy industry, it is estimated that an effective control program can save approximately $112 per cow/year (Schroeder, 2012). Knowing which pathogens are predominantly responsible for cases of mastitis in an individual herd is a crucial step to controlling the disease.
Whether a pathogen is major or minor is a large determinant of whether the disease will present as clinical or subclinical, while knowing the pathogen’s primary form of transmission and Gram status allows for anticipating high-risk periods, implementing control and evaluating treatment options. Knowledge about the causative pathogen(s) enables a targeted approach to focus time, money and labor where it will be most effective.
Are you having difficulties finding a solution to reduce your milk SCC levels? We offer instant, on-farm milk testing to help you take control of your business and implement effective milk quality control. Contact us, or visit our Dairy Health Check page for more information.
Hurley, W. L. (2009). Types of Mastitis/Modes of Transmission. Retrieved from University of Illinois: http://ansci.illinois.edu/static/ansc438/Mastitis/types.html
Pantoja, P. R. (2013). Understanding and using somatic cell counts to improve milk quality. Irish Journal of Agricultural and Food Research, 101-117.
Politis, I. (2012). Reevaluation of vitamin E supplementation of dairy cows: bioavailability, animal health and milk quality. Animal, 1427-1434.
Schroeder, J. (2012, July). Mastitis Control Programs. Retrieved from North Dakota State University.
About the Author
Anna Schwanke is an undergraduate student at the University of Guelph, Ontario. She is responsible for researching and writing about a wide variety of topics related to dairy cow welfare and management for Dairy Quality Inc. The 10 years she spent living in Australia, as well as her love of travelling, give her a firsthand viewpoint of issues facing the international dairy community. She plans to graduate from the University’s College of Physical & Engineering Science in 2019 and pursue a career in the Life Sciences or Agriculture industry.
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