History of Bovine Tuberculosis
By Colleen Bruning-Fann, DVM, MS, dipl. ACVPM, USDA-APHIS, 7/2/1998
In the early 1900's, the federal government instituted an eradication program for bovine TB. This program includes testing of livestock on farms and monitoring of animals sent to slaughter or transported across state lines. As a result of this program, bovine TB has been close to being eradicated from cattle in the United States.
Michigan was declared free of bovine TB in 1979. The 1994 discovery of bovine TB infection in the wild white-tailed deer population of Northeastern lower Michigan and subsequently in nearby cattle has resulted in the change of Michigan's Bovine TB Accredited-Free State status to modified accredited. Click here for a Chronology of Bovine TB in Michigan since 1975. The tuberculosis eradication program officially began in the United States in 1917. At that time it was estimated that 1 out of every 20 cattle slaughtered had bovine tuberculosis. That resulted in extensive economic losses to the farmers and was a detriment to trade. The effect of bovine tuberculosis on human health was also a concern. It was also thought that approximately 20% of humans with tuberculosis were infected with Mycobacterium bovis the causative agent of bovine tuberculosis. Most human cases were attributed to the consumption of unpasteurized milk.
The decision to attempt eradication of bovine tuberculosis was reached after a great deal of discussion and debate. There were many people who didn't think this goal could be accomplished. It is now apparent that the benefit to both animal and human health is tremendous.
Substantial progress on the eradication of bovine tuberculosis was quickly made. In 1917, the reactor rate of cattle that were tested for tuberculosis via the tuberculin skin test was 4.9%. By 1930, only 1.8% of tested cattle were reactors, a 64% reduction. A similar trend was seen in cattle at slaughter with a 95% decrease in the number of cattle with lesions of tuberculosis seen by 1940.
As tuberculosis eradication progressed, it became apparent that area testing was becoming increasingly inefficient as a method to locate diseased animals. In 1959 the emphasis was switched from area testing to the tracing of lesioned animals found by meat inspectors at slaughter. An epidemiological investigation located the herd of origin for the lesioned animal and any other herds that may have been exposed to this animal. All implicated herds were tuberculin skin tested. To eliminate any possibility of tuberculosis remaining in a herd, it is highly recommended that all animals in an infected herd be depopulated.
The epidemiological investigation of animals found infected at slaughter is still the main method that is used to locate tuberculous herds. To encourage the collection and submission of tissue samples at slaughter, meat inspection personnel are given a cash award if the submitted tissue is found to be compatible with tuberculosis. To encourage the submission of complete animal identification which aids in tracing the animal, a second larger award is made when the herd of origin is located.
Great progress has been made in the effort to eradicate bovine tuberculosis in the United States. By 1990's, only a few cattle herds each year were found to have tuberculosis but a new problem arose. Following the discovery of tuberculosis in an elk exported from the United States, Canada banned the importation of cervid species from the United States in December, 1990. Tuberculosis was confirmed in a cervid herd (elk) in the United States in 1991. With the loss of cervid exports to Canada and the discovery of bovine tuberculosis in a domestic cervid herd, actions were taken to address the situation. A plan to eradicate tuberculosis in cervids was developed. In May of 1994, the Tuberculosis Eradication in Cervidae; Uniform Methods and Rules was published. With the inclusion of cervid species in the effort to eradicate bovine tuberculosis considerable progress was again made.
Tuberculosis was eradicated from the island of Molokai, Hawaii in 1986 through depopulation of all the cattle on the island. In 1980 a survey of wild animals was conducted. Tuberculosis was found in feral hogs and axis deer. At that time the affected area was opened for hunting with considerable hunter success. A second survey of wildlife was conducted in 1982 with only 1 hog found with tuberculosis. In 1985, a third survey was conducted with no evidence of tuberculosis found in wildlife. In 1997 another cattle herd found was found infected with tuberculosis on Molokai. A survey of wildlife has been initiated in 1998.
However, a major impediment to the eradication of tuberculosis occurred when, following a hunter-killed deer discovered to have tuberculosis in 1994, tuberculosis was confirmed in free-ranging (wild) deer in the northeastern lower peninsula of Michigan in 1995. Since then, a number of actions have been taken to ascertain the extent of spread of tuberculosis. Infected wild deer have been found in 5 counties (Alpena, Alcona, Montmorency, Oscoda, and Presque Isle). An on-going survey of other wildlife has not found tuberculosis in wild elk, opossum, or bobcats thus far. Five coyotes and 2 raccoons have been found infected. Due to the potential for exposure to tuberculosis, testing of all cattle and goats over 12 months of age in the affected 5 counties has begun. At this time, tuberculosis has been confirmed in 1 herd of cattle in Alpena County. This herd has been depopulated.
Although great progress has been made in the eradication of tuberculosis from the United States, the discovery of a wildlife reserve poses a unique and difficult impediment to this effort. It is thought that supplemental feeding of wild deer serves to congregate deer and therefore contributes to the spread of tuberculosis. Supplemental feeding has been banned and baiting (the practice of hunting deer over feed) has been limited with the intention of reducing the spread of tuberculosis between deer and eventually eliminating this wildlife reservoir.