Mycoplasma pneumonia is caused by the organism Mycoplasma bovis (M. bovis). It also causes otitis media, arthritis and mastitis. It is often difficult to diagnose because animals may exhibit different signs when infected. However, it is a major cause of otitis media, so the chances of calves being infected with M. bovis is fairly high if they are exhibiting signs of ear infection, like a tilted head or droopy ear. This organism is actively being studied because there are many unknown factors about how it causes disease, how it spreads within the animal and how it spreads between animals.
M. bovis is highly contagious and can be spread by respiratory aerosols, respiratory secretions, nose to nose contact, feed, water, bedding material, feeding equipment and workers. If M. bovis becomes established in a dairy herd, close to 100 percent of calves will become infected. However, they may not develop clinical disease or shows signs of being ill. In populations of calves that are healthy and not experiencing environmental stress, it is possible to culture M. bovis from the lungs of up to 7 percent of the calves. The highest incidence of disease is usually in calves that are housed in group pens and are suffering from environmental stress, especially cold-stressed calves.
This organism has many properties that allow it to invade tissue, establish an infection and cause clinical disease. It has the ability to readily attach to the mucous membranes of the respiratory tract. This is the reason that pneumonia in dairy calves is usually the most common disease caused by M. bovis. It can also spread from the lungs through the bloodstream to other parts of the body, such as the joints. Animals with subclinical infections act as a reservoir for new infections and can shed the organism into the environment for years without developing clinical disease. This shedding may be constant or intermittent. Animals that develop clinical signs of disease will shed extremely high numbers of Mycoplasma into the environment.
Modes of infection
Animals that are subclinical carriers allow this organism to remain established in the dairy by continually shedding Mycoplasma into the environment. Animals that have developed chronic respiratory disease will be shedding high numbers of organisms, increasing the incidence of respiratory disease, especially in crowded pens. This shedding of Mycoplasma makes it almost impossible to eliminate this organism from the premises.
Newborn calves can easily become infected in the maternity pen through contaminated bedding, poor quality ventilation and cows that are shedding the organism. M. bovis has been cultured directly from the air in calf barns and is an important source of infection to young calves. In the past, Mycoplasma was thought to be a fairly fragile organism that did not survive for long periods of time outside of the body because of its lack of a cell wall. However, recent studies have shown that Mycoplasma will survive for extended periods of time in the environment, especially if the environment is cool and humid. One published study showed that M. bovis survived in recycled sand bedding for more than six months.
Mycoplasma organisms have many virulence factors that allow them to easily establish an infection and cause disease in an animal. As mentioned earlier, it has the ability to adhere to the mucous membranes of the upper respiratory tract, invade the tissue and multiply rapidly. In order for the immune system to mount a response against an invading pathogen, it has to recognize certain antigens, usually on the surface of the pathogen. Mycoplasma has the ability to change or modify these antigens so that the immune system has to start all over again to produce antibodies against these new antigens. Mycoplasma also can produce “biofilms” on tissue surfaces that protect it from the animal’s immune system. As it grows, Mycoplasma can produce metabolites that are toxic to the tissues it has infected.
M. bovis is commonly cultured from lungs of animals that are infected with other pneumonia-causing pathogens. Mycoplasma was isolated from 82 percent of the pneumonia cases caused by Mannheimia haemolytica. It has also been isolated from pneumonia cases caused by Histophilus somni and Pasteurella multocida. A common theory is that Mycoplasma organisms invade the lung tissue, cause inflammation in the lungs, which in turn predisposes the animals to infections by the previously mentioned pathogens that rapidly cause serious disease.
When M. bovis establishes an infection in an animal, an antibody response alone usually will not cure the infection. Since M. bovis can alter the immune response, it has a strong tendency to develop into a chronic infection. These animals require multiple treatments and are usually permanently poor-doers in the group. A high percentage of these animals have to be culled from the herd.
Early treatment critical
It is extremely important that new cases of Mycoplasma pneumonia be identified and treated as early as possible. A high percentage of antibiotics used in dairies work by inhibiting cell-wall synthesis and will not work against Mycoplasma since it does not have a cell wall. If Mycoplasma is diagnosed, it is important to develop an effective treatment protocol using specific antibiotics that are effective against M. bovis.
Research indicates oxytetracycline, spectinomycin, florfenicol and tulathromycin can effectively treat primary M. bovis disease. Remember that any extra-label use of antibiotics requires a valid veterinarian-client-patient relationship.
Record temperatures of calves upon signs of decreased appetite, nasal discharge, runny eyes, droopy ears or increase in respiration rate. Immediate treatment with the proper antibiotic is essential in order to eliminate the infection before the animal becomes chronically infected. On farms where there is a very high incidence of pneumonia, it may be worth checking the temperatures on all calves in a specific age group where the greatest incidence of pneumonia is occurring. It is common to see an increase in body temperature before any clinical signs of pneumonia are observed, and treatment at that time will greatly increase the chance of recovery from Mycoplasma infections.
It is important to continue treatment until the animal is fully recovered from the infection. Protocols should include the appropriate antibiotic, the length of the treatment, route of injection and required withholding times for meat following treatment.
Prevention and control of pneumonia caused by M. bovis is mainly focused around reducing exposure of the animal to this organism. After birth, the calf should be removed from the maternity area as soon as possible and placed in an individual pen that has been cleaned after the preceding calf. Using individual calf hutches that prevent nose to nose contact between calves will significantly reduce exposure. Prevent fenceline contact between pens if possible. This is especially important in hospital pens, which should never have fenceline contact with pens of non-infected calves.
Stocking density of the group pens also plays a major role in spreading the organism. Ideally, there should be at least 28 square feet of space for each calf in the pen. Over-crowded pens will result in more direct contact and increase the density of the Mycoplasma organisms in the air, greatly increasing the chances of the calves becoming infected.
Proper ventilation of calf barns has been shown to play an essential role in the prevention and spread of pneumonia in calves. Dr. Ken Nordlund at the University of Wisconsin has done a tremendous amount of work on ventilation systems for calf barns. This has had a major influence on the industry and greatly decreased the incidence of calf pneumonia. Good ventilation will significantly reduce the concentration of pneumonia pathogens in the air and reduce exposure.
Larger dairy operations and calf operations with the ability to fill a calf barn rapidly, such as in one to two weeks, have decreased the incidence of pneumonia by practicing an “all in, all out” style of management. In this case, the calves all stay in that particular barn until they are all weaned and ready to be moved out into the first group pens. This prevents continual exposure by bringing new animals into an established group of calves.
Extremely good sanitation of feeding utensils, buckets, stomach tubes, bottles, calf hutches, etc. will obviously reduce the exposure of calf to pneumonia pathogens. When treating sick calves, workers should wear gloves and change them between calves. Calves should be treated after feeding so pathogens are not transmitted to healthy calves by feeding after handling the sick calves. In large dairies and calf ranches, it is advisable to have a separate team of people, not involved in feeding, assigned to treat sick animals.
Since M. bovis also causes mastitis in dairy cows, unpasteurized colostrum or milk from cows infected with M. bovis should not be given to calves. Colostrum should be pasteurized at 60° C for 60 minutes. There are excellent commercially available pasteurizers available for the pasteurization of colostrums that are able to maintain the correct temperature, circulate the colostrum to ensure even heating and then cool the colostrum down after pasteurization. High-temperature short-time pasteurizers are more common for large volumes of milk fed to calves following colostrum delivery.
Dairies that are known to have Mycoplasma mastitis should identify the infected cows and ideally remove them from the herd. It is difficult to make progress with reducing the incidence of Mycoplasma pneumonia in calves if there is a significant problem with Mycoplasma mastitis in adult animals, even with pasteurization.
A good biosecurity program should be in place to help prevent the spread of disease from outside sources as well as within the dairy. Serological tests are available now that can determine if an animal is positive for M. bovis. Several dairies that have utilized this test have come to realize that the majority of the animals that they are bringing onto the premises have been exposed to M. bovis, thus emphasizing the importance of biosecurity in concert with other preventative practices.
It is also a good practice to ear-notch test calves to identify and remove any persistently infected with bovine viral diarrhea (BVD-PI). There are also tests that can be run on bulk-tank milk that are sensitive enough to detect one BVD-PI animal out of 3,000. BVD virus is immunosuppressive and it is very common to see an increased incidence in respiratory disease in herds where BVD-PI animals exist.
Nutritional and environmental factors in immunity
Good nutrition is absolutely essential in order for the animal’s immune system to function properly. Unfortunately, young calves are often fed milk at a level that does not allow them to gain weight according to their genetic potential. This deficiency in nutrient intake also prevents the immune system from receiving the necessary nutrients to mount an effective immune response against invading pathogens. Calves should receive at least 20 percent of their birthweight per day in milk in order to reach the necessary nutrient intake to grow and also maintain a healthy immune response.
We commonly see an increase in respiratory disease in calves during and after weaning. Calves are often cut back on milk suddenly and then expected to receive enough energy and protein from low-protein calf grower rations along with poor-quality forages. A weaning program needs to allow the calf to make this transition smoothly, while consuming enough nutrients to maintain a steady growth rate.
Cold stress is one of the most important factors that increases the incidence of Mycoplasma pneumonia in dairy calves. Deep straw bedding in the calf hutches is one of the best ways to reduce cold stress on the calf. The straw bedding should be deep enough that the legs of the calf are not visible when the calf is lying down. Also, the use of calf jackets during the first 30 days of life will help the calf to maintain its body temperature and reduce body heat loss in cold weather. The opening of the calf hutch should be positioned to avoid severe drafts inside the hutch.
Wet straw bedding in the hutch can lead to ammonia production from bacterial growth, which irritates the respiratory tract of the calf and predisposes it to respiratory disease. This is also true in group pens, with wet sawdust, shavings and straw providing an excellent environment for bacterial growth and ammonia production.
Mycoplasma pneumonia is a very common problem on dairies and calf ranches. Facility design, ventilation, hygiene, nutrition, stress management, early disease detection and treatment, and biosecurity are all important aspects of a well-designed program to reduce the incidence of Mycoplasma pneumonia in dairy calves. This will result in healthier animals, increased rates of gain and increased milk production once these replacement heifers enter the lactating herd.