How is mycoplasma mastitis transmitted in a dairy herd? The short answer is that there is no short answer. Interestingly, evidence shows that both lactating and non-lactating affected cattle can be the primary cause of mycoplasma mastitis. To make matters more confusing, there is also evidence to support the contrary. Experts say that contradictions will arise, though, especially since one or several strains can cause mastitis within a herd. However, one should note that given the variety in species and strains, it would not be realistic to think that Mycoplasma sp. cause mastitis that behaves in a similar way. Yet, according to most of the data, mycoplasma mastitis is a contagious disease that is ready to colonize a number of body sites, including the respiratory, reproductive, mammary, circulatory and skeletal systems. Furthermore, a single body may be infected in multiple sites with a single or multiple strains. And finally, to further complicate the issue, researchers have also noticed that mycoplasma mastitis outbreaks sometimes occur where other disease outbreaks, such as arthritis and pneumonia, are present in the herd.
So what does this all mean?
“Clearly, transmission of arthritis cannot occur directly from cow to cow through exposure of the infected joints of one animal with the non-infected joints of another,” said M2’s recent report on the subject. “Such multiple infections with arthritis caused by the strain causing mastitis and pneumonias suggest within-animal hematogenous spread.”
Further, researchers say that it is not clear if the point of infection is the mammary gland and whether or not dissemination occurs to the pulmonary and other organ systems or if the pulmonary system is the point of infection and the direction is towards the mammary gland and other organ systems.
What is more certain, though, is that the contagious mastitis pathogens are transmitted from cow to cow at milking time. This is similar to the dissemination of like pathogens, including S. agalactiae and S. aureus. Most of the mastitis from cow to cow in infected herds, however, is still believed to be via the streak canal, as in typical bacterial infections.
The problem with all of this is that in mycoplasma mastitis outbreaks the reservoir is not only the infected mammary quarter, but also the multiple body sites that potentially carry the agent. It is even possible to transmit the disease to newborn calves during pregnancy, as well as to the calves through milk. To further complicate matters, evidence shows that M. bovis can also be transmitted by aerosol.
While it is commonly believed that the majority of mycoplasma transmissions that cause intramammary infections occur during milking time, no case or control studies presented would support that claim. A group of researchers have looked at poorly functioning milking equipment as another possible source of transmission in at least two herds. Based on the necropsy and examination of the histopathological lesions of M. bovis-infected mammary glands, however, the researchers concluded that intrammmary infection was ascending, and therefore likely to have occurred via exposure of the teat orifice to the agent.
While the researchers reiterate the role of poorly functioning milking equipment in the spread of the disease, they also highlight the many other case studies that indicate other routes of transmission that occur outside of milking. It can be concluded, therefore, that in an outbreak of mycoplasma mastitis transmission one case might be due to udder-to-udder contact via fomites at milking time or via nose-to-nose contact through aerosols – or both. Basically, it’s complicated.
Learn more about the prevention and control of Mycoplasma sp. in two week’s time, and happy holidays from M2 Magazine.
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