MYCOPLASMA IN dairy herds is a topical issue. Much of the media coverage has related to farm biosecurity, management options and the decision to eradicate. This article discusses mycoplasma in relation to animal health: how does Mycoplasma cause disease in our herds?
What is Mycoplasma?
The mycoplasmas are a type of bacteria that, unlike other bacteria, do not have a cell wall. There are many different species of mycoplasma but the species of interest in this article is Mycoplasma bovis. This mycoplasma is adapted to cattle and does not impose a human health risk.
How does Mycoplasma bovis spread?
M. bovis is very effective at evading the immune system. This results in some infected animals becoming carriers of this disease without showing any clinical signs for many months or years.
Intermittent shedding of M.bovis by these carrier animals is associated with stress, for example at transportation, co-mingling, entry into the herd after calving and cold stress.
The introduction of an asymptomatic carrier animal is considered the primary route by which M.bovis-free herds become infected.
Transmission can be delayed making it difficult to identify the source of infection and outbreaks can occur in seemingly ‘closed herds’.
Once a herd becomes infected, M.bovis can be readily transmitted at milking, with udder-to-udder spread being the major route of infection.
It can also be transmitted by aerosols, nasal secretions, nose-to-nose contact or indirectly via feed, water and housing.
In young calves, ingestion of infected milk is a major route of infection. Aerosol transmission is also likely as once disease is established in a calf rearing facility it is very difficult to eradicate, suggesting ongoing transmission from older to incoming calves.
Calves exposed to M.bovis-infected milk may become asymptomatic carriers themselves, thus continuing the cycle of infection when they calve into the herd as heifers.
Can Mycoplasma bovis be treated?
One of the features of mycoplasmas is their resistance to many types of antibiotics.
Penicillins and other antibiotics target specific molecules in the bacterial cell wall, resulting in cell wall destruction and death of the bacteria.
However, as M. bovis does not possess a cell wall this mode of action is ineffective, meaning that once an animal is infected with M.bovis there is little chance of a cure.
Treatment of cows with mycoplasma-mastitis is not recommended as these animals often remain intermittent shedders and should be regarded as permanently infected.
Treatment of calves with tulathromycin (Draxxin®, Zoetis) has shown some success in experimentally infected calves.
In outbreak situations it has shown variable results, most likely due to rapid disease progression and possibly delayed timing of treatment.
The likelihood of recovered animals becoming subclinical carriers is unknown.
What are the signs of M. bovis infection?
There are multiple manifestations of M. bovis, the common ones being described below. They may feature as the sole presenting sign or can occur in combination with each other.
Many mycoplasma mastitis infections are subclinical, meaning an infected cow may not show any abnormal signs, with an ‘average’ somatic cell count and milk production.
Cows of any age or stage of lactation can become infected, including heifers and dry cows. Clinical mastitis is non-specific but typically involves multiple quarters and a drastic reduction in milk production.
Signs of sickness in the cow herself may be mild. Affected quarters may be swollen, usually non-painful and the changes to milk range from mildly abnormal to gritty or thick discharge and may be brownish in colour.
A classic feature is a history of recurring mastitis due to resistance to treatment with antibiotics. Return to production is usually slow between cases.
M. bovis pneumonia or respiratory disease can occur in both dairy and beef cattle at any age.
Signs can include fever, difficulty in breathing, coughing, nasal discharge, loss of appetite and poor weight gain. Mycoplasma pneumonia is often poorly responsive to treatment.
Otitis media occurs when the infection spreads to the middle ear. This may be the primary sign of infection in calves. In early cases, calves remain alert but as the disease progresses they develop a fever and loss of appetite.
Signs include drooping of the affected ear and signs of ear pain such as head shaking, scratching or rubbing the ears.
One or both ears can be affected. If the ear drum ruptures, a tilt of the head to the affected side may be evident.
Severely affected animals may also show signs of circling, drifting and meningitis. Drooping of the upper eyelid(s) may lead to dry eye in one or both eyes.
Septic arthritis and swollen joints
Septic arthritis can occur sporadically but also in outbreaks in calves. Animals usually present with sudden onset non-weight bearing lameness with joint swelling, pain and heat.
Multiple joints such as stifle, hip, hock and elbow are frequently affected and calves often have a fever and loss of appetite.. Poor response to treatment is a common feature of mycoplasma arthritis.
Diagnosis of M. bovis is challenging due to the asymptomatic signs and intermittent shedding of the pathogen.
A herd screen PCR test can be performed on bulk milk as part of routine biosecurity measures. Early identification of the disease is critical if any form of control is to be achieved.
Speak with your veterinarian about screening tests or if you are concerned about recurring cases of mastitis, respiratory disease, joint ill or otitis media.
• Gemma Chuck is an adviser with Apiam Animal Health.