As dairy herds expand and animals are moved from area to area, the risk of spreading Mycoplasma infections in herds is very real. Mycoplasma are the smallest free-living organisms, that have a widespread distribution and may be present in mammals, reptiles, plants, and humans. Mastitis is a common disease that may result from a Mycoplasma infection, although the disease may manifest itself in other ways, including inner ear and joint infections in young calves.
The organism has minimal metabolic capabilities and can only replicate in close proximity to cells or in highly enriched culture media. Under proper conditions (moderate temperatures and moisture) many strains can remain infective while outside the host, free in the environment, for weeks to months. Most species are highly species specific and frequently are very tissue specific. The organism can be a co-factor in disease or be the sole pathogen in diseases of the joint, respiratory tract, urogenital tract, or mammary gland. Disease caused by mycoplasma may be clinical and severe, although the disease signs associated are more frequently very subtle or subclinical. Clinical signs of mycoplasma infection are rarely definitive, and diagnosis can be difficult. In young animals, the disease is most frequently observed as pneumonia or joint/middle ear disease.
Treatment of mycoplasma infections can be very prolonged and difficult, and due to the lack of a cell membrane the vast majority of antibiotics are ineffective for therapy. Even if the organism appears in the laboratory to be susceptible to the antibiotic selected, the organism is often sequestered in tissue or organ spaces where it can "hide" from antibiotics, making elimination of the organism a challenge. Many animals once infected either develop chronic disease with micro-abscesses or articular damage and most often become carriers for life. Such carriers can be essentially normal for months or years but may actively shed the organism in the future when stressed. During these subclinical periods of infection the organism can be difficult or impossible to isolate via routine culture techniques. The lack of cell membrane tends to stimulate at best a modest immune response, so serological testing can be very difficult to interpret as well.
Mastitis due to mycoplasma can be very difficult to prevent, control, and eradicate. Mycoplasma are frequent residents of the respiratory tract of normal animals. When stressed these organisms can grow rapidly, and may be spread in many secretions. Bedding and the environment can become contaminated from nasal, ocular, or vaginal secretions. The organism can also travel hematogenously (via the bloodstream.) Therefore, the udder can potentially be contaminated through the circulatory system, via contamination of the bedding, from milking equipment, or from contaminated caretakers' hands.
This scenario is often played out during herd expansions. Producers purchase apparently normal healthy heifers. When these heifers are added to the larger herd, especially if air quality or nutrition place excessive stress on the heifer, it is not unusual for subclinical animals to begin to shed organisms into the environment and develop clinical signs. Cows of all stages of lactation are susceptible to infection. However, animals under physiological stress, such as during the transition phase, or at peak lactation seem to have the greatest susceptibility.
Classic signs of mycoplasma mastitis are failure of response to treatment, multiple quarter infections, udder edema often without fever, mild to severe milk yield depressions, and udder secretions that can range from tannish, serous with a few clots, to thick, clotted material. The organism is a contagious mastitis pathogen and once one animal is shedding the organism in her milk, it is easily spread from cow to cow. Astute milkers can be very helpful in identifying early infections if properly trained. However, if the milker crews are not keyed into good contagious mastitis pathogen control or careful screening for abnormal milk prior to unit attachment, the organism can spread through a herd very quickly. It is important that milker crews be trained to consistently follow established protocols for contagious pathogens. (E.g., clean latex gloves, use individual towels, careful and complete post milk dipping with an effective germicide, and segregate and/or milk last known-infected animals).
Under the best of conditions mycoplasma are difficult to treat with a successful outcome. Most antibiotics approved for use in dairy animals are β lactams (penicillin-class antibiotics) which are ineffective against mycoplasma. Much cost and lost time is consumed when producers attempt to treat mastitis caused by mycoplasma. Known infected animals should be segregated immediately and as economic conditions dictate perhaps designated for early culling.
Mycoplasma vaccines are commercially available, but these tend to be species-specific and offer little cross-protection. Mycoplasma bovis is the most prevalent species isolated in diagnostic laboratories (>50%). However, there are approximately eleven other species of mycoplasma that have been isolated in cattle. Acholeplasma can be isolated at times, but the pathogenicity of this mycoplasma strain is controversial. While strains other than M. bovis are less prevalent, to ensure a reasonable level of protection post-immunization, it is important that the vaccine strain be similar to the strain isolated on the farm. In general even wild-type species infections do not stimulate a robust immune response. For this reason veterinarians may be somewhat disappointed in the efficacy of mycoplasma vaccination in infected herds. Immunization appears to be more effective if properly administered to heifers prior to their first infection.
Mycoplasma are shed very intermittently and therefore a single culture can be misleading. The organism is also very fragile, and therefore if collection, storage and shipment conditions are less than ideal it can be isolate mycoplasma using traditional culture techniques. Thus, although a positive culture is a very valuable finding, a negative culture in an animal showing signs of mycoplasma should be treated with suspicion, and suggests the need to repeat the culture. PCR technology is available to detect mycoplasma. This technique is very sensitive and specific and shows great promise as both a screening tool for testing bulk tank milk samples, and in cattle where traditional cultures are not diagnostic.
For additional information on mycoplasma in herds, or assistance with diagnostics please contact Dr. Ernest Hovingh @ 814-863-2160
The following is a good review article on Mycoplasma:
- Gonzalez, RN and DJ Wilson, Mycoplasma mastitis in dairy herds, Vet Clin N Amer: Food Animal 19 (2003) 199-221.