Current Status of the Bovine Tuberculosis Eradication Program in the United States – April 1, 2003

 

 

Robert M. Meyer DVM, MS, Regional Epidemiologist, USDA APHIS Veterinary Services

 

 

Forty-eight states plus Puerto Rico and the U.S. Virgin Islands are considered free of bovine tuberculosis as per national eradication program standards outlined in the Uniform Methods and Rules for Bovine Tuberculosis Eradication.  The State of Texas is currently classified as Modified Accredited Advanced as a result of finding two infected cattle herds in the past two years. The State of Michigan is considered a Modified Accredited state due to tuberculosis infection detected in 29 cattle herds since 1998.  These infections in Michigan herds are mostly a result of repeated exposures to free-ranging, infected whitetail deer located in northern lower Michigan.

 

California and New Mexico most likely will experience a downgrade in their TB-free state status this year as a result of detecting new infections in their dairy herd populations over the past year.  The State of Michigan has also submitted an application to USDA’s Animal and Plant Health Inspection Service (APHIS) for split-state status that would classify 11 counties of the state as Modified Accredited, and the remainder of the State as Modified Accredited Advanced.  This application is currently under review by USDA APHIS.

 

As of October 1, 2002, 10 cattle herds are considered as being affected with bovine TB.  Five of these herds have been in Michigan, 3 dairy herds in California, and 2 dairy herds in New Mexico.  Five of the 10 herds have been depopulated to date.

 

The 5 infected herds in Michigan consist of 2 dairy herds and 3 beef herds.  One of the infected beef herds was detected in Fiscal Year 2002, but depopulated this year.  The other 2 beef herds are small herds consisting of about 50 head of cattle each, and have also been depopulated.  Both infected dairy herds consist of 150 milking cows or less, and have chosen to repeatedly test and remove reactors as their method of clearing the herd quarantine.  Five of 29 cattle herds detected with bovine TB in Michigan since 1994 have been dairy herds. Twenty-four have been beef herds.

 

DNA fingerprinting of M. bovis isolates from the Michigan herds suggests that TB infection continues to be associated with infected, free-ranging whitetail deer and possibly other wildlife hosts.  As a result, researchers with USDA APHIS Wildlife Services, the Michigan Department of Natural Resources, and Michigan State University are looking at various methods to reduce the wildlife-livestock interface and minimize exposures between animals.  Such methods include the use of various barriers including dogs, fencing, and human effigies to deter deer from farms and protect feed storage areas.  

 

Three large dairy herds have been detected with bovine TB in California over the past year.  The first herd was found as a result of tracing a tuberculous cow detected during slaughter inspection in May, 2002.  The herd was located in Tulare County, CA - an area of central California containing the largest dairy cattle milking population in the country.  A total of 6,482 cattle were depopulated from this herd this fiscal year requiring federal indemnity monies of over $11 million.  The source for this infection is still under investigation. Hundreds of potential source herd investigations have been developed to date.  In addition, an extensive listing of cattle sales from this herd has also been developed.  Fortunately there has been no evidence of spread of infection from this herd by way of sales of exposed animals to other owners. 

 

The second dairy herd detected in California was found as a result of adjacent herd testing associated with the first infected dairy herd. This herd of approximately 5,000 cattle was recently depopulated at a federal indemnity cost of about $6 million.  Epidemiology as to the source for this infection is in progress, and heifer sales from this herd have been extensive. As a result, a significant number of herds in California receiving potentially exposed cattle will need to be evaluated over the next several months or years.  

 

The third infected dairy herd in California was detected by tracing a “downer” cow found with TB lesions at slaughter to its source herd.  Initial testing of 480 adult cattle in the herd revealed 47 reactors, many of which had extensive TB lesions upon post mortem examination.  Several of the TB lesioned reactors traced to states in the mid-west and eastern United States. Investigations are now in progress.  USDA APHIS hopes to identify funds to depopulate this herd in the future if possible. 

 

Two newly infected dairies have been identified in eastern New Mexico over the past few months.  Both dairies were detected as a result of tracing tuberculous cattle found during post mortem inspection at slaughter.  One of the dairies consists of over 8,000 cattle divided between three premises.  Over 7,000 potentially exposed cattle have been shown to have left the dairy over the past few years, and epidemiological investigations related to tracing these cattle are now in progress in New Mexico, Texas, Oklahoma, Kansas, Arizona, and Idaho.  The second infected dairy in New Mexico consists of approximately 1,800 milking cows and is located in the same eastern New Mexico milk shed as the first dairy.  Both dairies are still considering management options for dealing with the disease in their herds. 

 

No newly infected captive cervid herds have been identified with bovine TB in Fiscal Year 2003 to date. 

 

Slaughter surveillance for detecting new cases of bovine TB continues as the primary method of locating infected herds.  Since October 1, 2002, 30 cases of bovine TB have been found during inspection in U.S. slaughter plants.  These cases have been confirmed to be bovine tuberculosis on the basis of PCR testing or bacteriological culture.   If the current rate of finding new cases continues, USDA APHIS projects that as many as 50-60 cases could be found this fiscal year.  102 cases were detected in 2002, and 71 cases were found in 2001.

 

Twenty-eight of the 30 cases have been identified in immature, feeder type cattle, and 2 cases have come from adult dairy cattle two years of age or older.  Investigation and testing of the most likely herd of origin for one of the adult cases found at slaughter in California revealed 47 tuberculin test reactors in a small dairy in California.  Infection was confirmed in many of the reactors.  The second adult cattle case was found in a Holstein cow at an Arizona slaughter plant.  Tracing and testing of possible source herds for this cow resulted in confirmation of the disease in a large 1,800 cow dairy in eastern New Mexico.  Both adult cattle cases had sufficient identification collected at slaughter to allow officials to successfully trace them.

 

Results of investigating 28 cases of TB in immature, fed cattle to date indicate that 17 cases were of Mexican origin. Three cases have been investigated and are untraceable due to insufficient identification.  Eight cases are still under investigation as to their origin.  Of the 20 cases in immature cattle investigated to date, 85% (17 of 20 cases) trace to Mexico. 

 

Identification that allows tracing of diseased cattle to their herds of origin is critical in completing the task of TB eradication.  Of the 30 cases of bovine TB detected at slaughter so far this fiscal year, 14 cases have been identified with official Mexican origin tags.  Two cases had official U.S. identification tags that allowed the infected herds of origin to be identified.   Eight cases were identified with feedlot plastic tags, and 6 cases had no identification of any type.  These 6 cases without ID are of concern from an epidemiologic perspective since we will most likely be unable to trace them to a definitive source.

 

Good support and efforts in TB surveillance continues by our state and federal meat inspection agencies.  4,842 suspicious granulomas were submitted to the laboratory from all types and ages of cattle in Fiscal Year 2002 as compared to 2,991 for the year previous.  And, more specifically, submissions of suspicious tissues from adult cattle (>2 yrs of age) have increased over six times in the past two years from 436 submissions in Fiscal Year 2000 to 3,147 in Fiscal Year 2002.  Recent improvements to the TB Performance Awards program which rewards federal meat inspection personnel for their efforts in sending samples was largely responsible for the increase.

 

The Comprehensive Strategic Plan for the Eradication of Bovine Tuberculosis adopted in October, 2000 originally set a program goal of December 31, 2003 to eradicate bovine tuberculosis from the domestic livestock population of the United States.  Clearly, it is unlikely that this goal will be attained by that date.  TB in Michigan wildlife, probable exposures from Mexican feeder cattle, the need to continue improving surveillance, insufficient funding to depopulate large herds of exposed cattle, and an insufficient number of experienced personnel to accomplish essential program functions are all issues that have been identified in the past as needing resolution.  Without doubt these issues are complex and will challenge us to find creative solutions. Perhaps the time has come to once again bring together all federal, state and industry stakeholders to re-examine our national program, its goals, policies, and methodologies, and make those recommendations and adjustments that will allow the great progress we have achieved in the eradication of bovine tuberculosis from this country to continue.

 

Note:  The author greatly appreciates and acknowledges Dr. Joe Vantiem, USDA APHIS Veterinary Services, and Dr. Dorothy York and Dr. Melanie Swartz, California Department of Food and Agriculture for their contributions to this report.