Bovine mastitis and Staphylococcus aureus: a major problem in dairy
Dr. Taruna Bhati (PhD, Veterinary Microbiology), Assistant Professor, Department of Veterinary Microbiology and Biotechnology, College of Veterinary and Animal Science, RAJUVAS, Bikaner-334 001, Rajasthan, India. ORCID id: 0000-0001-8460-5733
Mastitis is the inflammation of the mammary gland and udder tissues usually caused by an infectious agent such as bacteria but can also result from chemical, thermal or physical injury to the udder. Hence it is a multi-factorial disease affecting all milk yielding animals atleast once in their lifetime thereby decreasing the quality and quantity of milk (change of milk composition and palatabilty), adversely affecting the health of milch animals (esp. cow fertility) as well as of the public who consume such milk and incurring financial loss to the owners. Mastitis is closely related to the virulence of the pathogen, the immune system of the animals and the dairy environment in which the animals are kept.
Bovine mastitis can be classified based on clinical manifestations into clinical mastitis (CM) and subclinical bovine mastitis (SCM). The CM can be easily diagnosed as the affected udder shows inflammation including heat, swelling, discoloration as well as abnormal secretion and the infected cow may exhibit systemic reactions such as fever, loss of appetite and sometimes death (Kibebew, 2017). The SCM cases go unnoticed as there is usually no visible sign of inflammation or infection; but can be diagnosed by either high somatic cell counts (SCC) (predominantly neutrophils) in the milk samples as a result of the host immune response (Harmon, 1994; Kehrli & Shuster, 1994) or positive gelation of the milk samples caused by DNA of the infiltrating somatic cells, as tested by a California mastitis/milk test (CMT) (White et al., 2005). These subclinical infections result when a producer discontinues antibiotic treatment because the milk looks normal but the hard-to-kill mastitis organisms are still alive in the gland and waiting for their opportunity to attack again. The treatment of mastitis often involves repeatedly re-treating a cow that relapses with an infection in the same quarter multiple times.
Staphylococcus aureus is a commensal organism found commonly on skin surfaces, mucosa, and in the environment. The organisms multiply in infected lesions or colonized teat canals and can readily enter the udder and this may be a source of infection if proper hygiene is not maintained in the dairy farm. Being contagious in nature, critical time for spreading of S. aureus among the cows in a dairy farm is during milking through milking equipment such as teat cups, towels and milker’s hands (Benic et al. 2012). So, S. aureus is one of the most prevalent contagious mastitis pathogen worldwide and subclinical mastitis caused by S. aureus is 15-40 times more prevalent than the clinical form. It has been reported that 3 percent of all animals are infected with S. aureus (Schukken et al. 2009). Once established, S. aureus infections do not respond well to antibiotic therapy and infected cows must be segregated or culled from the herd.
The induction of intramammary infection requires production of a variety of cell surface virulence factors, exotoxins and cell surface-associated proteins enabling adherence, colonization, invasion of the mammary cells of the bovine host by the S. aureus, evasion of the immune defence mechanism and survival in the host environment (Momtaz et al. 2010; Memon et al. 2013). This bacterium produces exotoxins like toxic shock syndrome toxin-1 (TSST-1), staphylococcus enterotoxin (SE), and Panton-Valentine leukocidin (PVL). SEs are regarded as the major cause of S. aureus associated food poisoning (Hennekinne et al., 2012). Furthermore, formation of biofilms, highly organized multicellular complexes, is often associated with both epithelial adhesion, evasion of host immune defenses and drug resistance. This bacteria also produce toxins that destroy cell membranes and can directly damage milk producing tissue followed by invading deeply into udder tissues, forming micro-abscesses that are impenetrable by intramammary antimicrobials. Further the bacteria can avoid contact with antibiotics while residing inside leukocytes.
Many strains of S. aureus have acquired antibiotic resistance through the ability to produce an enzyme that inactivates penicillin-based and other antibiotics thereby rendering the treatment ineffective. Prolonged use of antimicrobial agents such as penicillin, tetracycline, oxacillin, erythromycin, cefazolin, clindamycin and tobramycin for the treatment of bovine mastitis may lead to the emergence of antimicrobial resistant bacterial strains such as methicillin-resistant Staphylococcus aureus (MRSA) which is a serious concern not only in animal health but also more importantly to human health. Increasing concerns among the consumers about the antimicrobial residues, antimicrobial resistance, milk quality and animal welfare further demand proper policies for an effective prevention and control of mastitis.
Culture of bulk tank milk is easy, economical, and is an important aid in determining the microbiological cause of mastitis in the herd. A culture of only one bulk tank sample is not a complete guarantee that contagious mastitis will be detected as S. aureus infected cows may shed the organism intermittently hence it is suggested to sample three consecutive bulk tanks. The bacteria and its virulence genes can be easily detected at molecular level through PCR and the pathogenic strains circulating in a dairy environment can be assessed. It is important to identify infections early in order to prevent spread to other animals and increase chances of a successful treatment. Continuous monitoring of SCC (the average SCC above 250,000 is alarming) in a herd is helpful in identifying SCM before allowing it to progress to clinical form or chronic forms.
Treatment will not control mastitis but it may shorten the duration of the infection. Treatment effectiveness decreases as cows become older. New clinical infections should be treated promptly and appropriately, especially in first lactation cows. Tissue damage can be minimized if treated during early stages of infection. Additionally, there may be improved success with dry cow therapy. Many researchers have looked at the efficacy of pirlimycin treatment both in heifers prior to calving and in all animals as an extended therapy treatment during lactation. According to the manufacturer, pirlimycin is one of the most effective compounds against S. aureus because its chemical nature allows it to penetrate mammary tissues. Single-quarter, extended therapy with repeated label doses of pirlimycin has been examined as a means of providing drug levels beyond the expected life of the leukocytes that naturally fight off this infection. This protocol has been widely adopted for new intramammary infections with S. aureus, as it increases cure rates (Wolfe et al., 2010)
The best treatment for S. aureus mastitis is prevention. The most effective control is to prevent new infections by minimizing or eliminating conditions that contribute to the exposure of teat ends through spread of infections from cow to cow and conditions which allow bacteria to contaminate and penetrate the teat canal. In addition, certain nutritional components enhance the animal’s resistance to mastitis. Excellent pre- and post-milking teat sanitation, excellent milking hygiene, wearing gloves, single-use towels, and properly functioning milking equipment are critical to reduce the risk of transmission in the milking parlor. Regular bulk tank culture is a useful screening tool, as well as culturing clinical mastitis cases, particularly those that do not respond to therapy. The war with mastitis can be won through proper vigilance and biosecurity measures.
References
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