Mycoplasma in Adult Dairy Cattle Can Initially Show Many Different Clinical Signs

Dr. David Wilson, DVM, PhD
USU Extension Veterinarian

Mycoplasmal infection with Mycoplasma spp., usually M. bovis, is an important disease complex of dairy cattle. This unusual organism can only be detected with special culture methods under special laboratory conditions; typical laboratory methods such as culture of milk or tissues will not detect it. As part of the 2002 National Animal Health Monitoring System (NAHMS) Dairy Survey, it was reported from selected sampling of herds in California, Colorado, Idaho, New Mexico, Texas, and Washington that the herd prevalence of mycoplasma was 9.4%. Utah has never been included in any surveillance study for Mycoplasma spp., but these surrounding states show the highest percentage of mycoplasma-infected dairy herds in the U.S. by far. In addition to mastitis, mycoplasma affects all ages of cattle, and can cause arthritis, pneumonia, metritis, agalactia, bloodstream infection, and death of cattle. The history often includes negative culture results from milk or other samples using standard microbiological methods and a frustrating inability to diagnose the problem when signs first appear, sometimes for months. Mycoplasma infections cause some of the greatest losses of milk production among all mastitis pathogens, including among cows without clinical mastitis. In New York and Pennsylvania, Mycoplasma spp. mastitis was associated with approximately 3,500 pounds of milk loss per 305-ME lactation relative to uninfected herdmates.

There can also be substantial losses from culled or dead cows. The first presenting signs in dairy herds often include lameness, respiratory disease including pneumonia of calves or adult cattle, and clinical mastitis of multiple quarters, or first in one quarter and then in other quarters, often poorly responsive to therapy. There is often a history of animals purchased 3 to 6 months earlier, and of some cows (not necessarily purchased cows) with major milk production loss, changing from high production cows to nearly agalactic cows. Lameness associated with mycoplasmal infections is usually observed in fetlock joints, or especially in hock joints (swelling, if present, is observed on the cranial side of the hock joint, not the lateral side). Mycoplasma can first show many different types of clinical signs in a herd, including the first signs being mainly a major loss of milk production in some cows for no apparent clinical reason.

Prevalence within herds that are infected with Mycoplasma spp. mastitis varies considerably. The mean prevalence within infected herds seen in the Northeast U.S. was 5%, ranging from only one cow in herds of over 500 cows to approximately 33% in two herds. At present the percentage of mycoplasma-infected cows in Utah, both at the herd and cows-within infected-herds levels, is unknown.

Many dairy herds have mycoplasmal infections in calves, with no cases ever found among cows within the herd when they become adults. Mycoplasma infection in dairy calves is a different disease complex that cannot be covered in this space but it does not necessarily dictate that the adult herd will be affected with mycoplasmal disease.

It has been reported that mycoplasmal mastitis outbreaks can be initiated by intra-herd transmission from asymptomatic carriers. A few herds have had mycoplasmal mastitis, pneumonia or arthritis outbreaks reoccur more than a year after the disease was believed eliminated from the herd, including 100% of the herd having cultured negative for Mycoplasma spp. in milk and remaining closed to purchased animals. However, these rare outbreaks are much less common than those that follow the purchase of animals into dairy herds. Nevertheless, they indicate that diagnostic testing for Mycoplasma spp. should be performed when clinical signs of the mycoplasmal infections appear, even in herds that have long been closed to outside animals. The greatest financial loss results from culling, death or lost milk production of affected cows. While SCC can be increased and herd mean milk production can be decreased during mycoplasmal outbreaks, they often do not change substantially, therefore bulk tank SCC and mean milk production are generally not suitable for ruling out Mycoplasma spp. infections in dairy herds. This indicates that surveillance for Mycoplasma spp. utilizing milk from bulk tanks and/or lactating cows should be routinely performed to screen for mycoplasmal infections within dairy herds, as discussed in our November, 2006 Newsletter. If you have further questions, please contact me at (435) 797-1899 (M-W) and (435) 797-7120 (Th-F) or David.Wilson@usu.edu. ©