Wildlife Disease and Zoonotics


Date of this Version



Katie A. Portacci, Jason Lombard, Charlse Fossler, Eric Bush, Kammi Johnson, Dianna Mitchell, Todd Weaver, Randy Pritchard, Steve Sweeney, Ryan S. Miller, Robert Harris. Assessment of Pathways for the Introduction and Spread of Mycobacterium bovis in the United States. Technical report edited by Katie Portacci and Jason Lombard. USDA APHIS Centers for Epidemiology and Animal Health, Fort Collins, Colorado. May, 2010.


USDA. 2009. Assessment of Pathways for the Introduction and Spread of Mycobacterium bovis in the United States, 2009 USDA–APHIS–VS–CEAH. Fort Collins, CO


Bovine tuberculosis (TB) was responsible for more losses among U.S. farm animals in the early 20th century than all other infectious diseases combined. The Cooperative State-Federal Tuberculosis Eradication Program (established in 1917 and administered by APHIS, State animal health agencies, and U.S. livestock producers) has nearly eradicated bovine TB from the nation’s livestock population. However, despite the many accomplishments of the program, bovine TB remains a serious and costly disease of livestock in the United States. In 1992, VS conducted an assessment to identify pathways for the introduction and spread of bovine TB, in order to develop the most effective strategies for controlling the disease. Several changes have occurred in the livestock industry since 1992, and new pathways for the spread of bovine TB have been identified. The goal of this assessment is to describe the current pathways for bovine TB introduction and spread. The pathways considered for the 2009 assessment are: 1) Legal and illegal importation of cattle into the United States 2) United States cattle industry practices 3) The U.S. captive cervid industry 4) Wildlife 5) Zoo and other nontraditional species Retrospective epidemiologic analyses of outbreaks in four States (California, Michigan, Minnesota, and New Mexico) identified several risk factors for the introduction and spread of bovine TB. In California and New Mexico, molecular fingerprinting techniques revealed several strains of Mycobacterium bovis (M. bovis), indicating multiple sources of introduction. Risk factors for California and New Mexico included the importation and commingling of Mexican-origin steers, management and biosecurity practices used by calf-raisers for dairy replacement heifers, and a large influx of purchased additions. In States with similar practices and risk factors, both beef and dairy herds are at risk for exposure to M. bovis. Conversely, Michigan and Minnesota each had just one strain of M. bovis, indicating a point source of introduction and local area spread. The same strains were identified in the wildlife of each State, making cattle contact with infected white-tailed deer (especially contact with feed contaminated by deer) an important risk factor for the introduction and spread of bovine TB in those States and other areas in which infected wildlife reside. Human-to-cattle contact has not been fully investigated in the four States analyzed, but cannot be ruled out as a risk factor. It is difficult to determine the original sources of these bovine TB cases because evidence is sparse as to whether these events were new introductions of disease or established cases that were undetected in wildlife or cattle. The importation of Mexican cattle continues to be a risk factor for the introduction of bovine TB into domestic livestock herds, especially when commingling occurs outside of slaughter channels. From 2003-2008, the number of cases of Mexican-origin cattle identified at slaughter has decreased. This is consistent with the decrease in number of animals imported. Each year 1-2 infected animals per 100,000 animals imported from Mexico are identified through slaughter detection or epidemiologic investigations. Management practices play an important role in increasing or decreasing the risks of M. bovis infection and other pathogens in dairy and beef cow-calf operations. The introduction of new cattle to an operation, exposure to wildlife, and commingling with cattle from other operations increase the risk of introducing bovine TB to a herd. Methods for decreasing the risk of introducing bovine TB to an uninfected herd include minimizing the number and sources of incoming cattle, knowledge of the bovine TB status of source herds, testing new additions before exposing the home herd, decreasing cattle and feed contact with free-ranging and captive cervids, and limiting exposure of cattle to animals from other operations with unknown disease status. M. bovis may also be introduced by contact between captive cervids and wildlife. The development of the 1999 TB Eradication Uniform Methods and Rules (UM&R) and the inclusion of captive cervids in Title 9 of the Code of Federal Regulations are regulatory actions taken to decrease the risk of introducing or spreading M. bovis in captive cervids. However, limited surveillance of these animals negates reliable data on the true prevalence of bovine TB in captive cervids. Exotic game ranching is a rapidly growing segment of U.S. animal agriculture. Little is known about the disease status or movement of animals in this industry, which may pose a risk for introducing bovine TB and wildlife diseases to cattle and other animals. The re-emergence of bovine TB in wildlife and cattle highlights the importance of wildlife surveillance. The true prevalence of disease in wildlife species is not known because bovine TB surveillance and control in this population is inconsistent and difficult. Feral swine and supplemental feeding or baiting of free-ranging cervids have been factors in the propagation and persistence of M. bovis. The development of TB vaccines for wildlife may help eradication efforts by decreasing disease transmissions among wildlife. The risk of M. bovis transmission to cattle from zoo animals is low because of limited contact; however, zoo species may pose a risk to humans. Game parks and other areas where animals roam freely or have fence-line contact with cattle or other livestock could contribute to M. bovis transmission. Hay and other fomites are also pathways for bovine TB given the right environmental conditions. Dogs, cats, wild birds, humans, and other species may act as hosts for TB and should be considered during epidemiologic investigations. Although bison are included under the same regulations as cattle, they may not enter routine slaughter channels, thereby limiting the availability of surveillance information. Additional research is needed to determine the true status of bovine TB in the captive bison industry. Eradication of bovine TB continues to be a challenge. The pathways outlined in this assessment are intended to help APHIS-VS identify new measures for controlling and preventing spread of this disease in the United States.