Article Index "Texas Livestock Health Officials Tracking Down Tuberculosis" Article Index

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By: "Carla Everett"
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A beef cattle herd in south Texas has been found to be infected with cattle tuberculosis (TB), and Texas livestock health officials are seeking the source of the infection. The disease, caused by Mycobacterium bovis, can produce internal lesions in animals. Cattle TB can be spread within a herd when an infected animal coughs, releasing bacteria-laden mucus onto feed that is consumed, or into the air that is inhaled by nearby cows.

Grazing animals--such as cattle, sheep, goats, exotic and domestic deer, and horses--ingest anthrax bacteria when they consume contaminated grass. By the time an animal displays signs of disease, including staggering, trembling, convulsions, or bleeding from body openings, death usually follows.

"The investigation began early this summer, when a federal veterinarian, conducting a routine exam in a slaughter facility, detected lesions in a carcass that were compatible with those of TB," said Dr. Linda Logan, Texas state veterinarian and executive director of the Texas Animal Health Commission (TAHC), the state's livestock health regulatory agency. U.S. Department of Agriculture's Food Safety and Inspection Services (USDA-FSIS) inspectors are present in all federally inspected slaughter facilities to examine carcasses for indication of disease. The inspectors condemn carcasses that are lesioned.

"Tissue samples from the carcass were tested at the National Veterinary Services Laboratory (NVSL) in Ames, Iowa, where a definitive diagnosis of cattle TB was made," she said. "We traced the animal back to its original herd in south Texas. The herd was placed under quarantine, and all of the adult animals were skin-tested for the disease."

Dr. Dan Baca, the TAHC's TB epidemiologist, explained that, of the 26 animals tested, 10 had positive results. In late August, the 10 animals were slaughtered, and the carcasses were examined thoroughly for internal lesions indicative of the disease. Seven animals had lesions, and tissue samples were collected and forwarded to NVSL for confirmation tests. The carcasses were incinerated, preventing their entry into food channels. The remaining 16 animals in the herd have also been destroyed.

"Fortunately, herds surrounding the infected herd have tested negative," said Dr. Baca. "The owner of the infected beef herd has been extremely cooperative and has maintained excellent records of sales and purchases, enabling TAHC veterinarians to trace animal movement. Our staff has tracked seventy-nine head of cattle that have been sold out of the infected herd."

As of early September, Dr. Baca said nearly half of the 79 exposed animals have been slaughtered and examined. One of these animals, a heifer, had lesions that were indicative of TB. The remaining "traced out" animals also will be slaughtered and examined. To ensure infection has not been missed, he explained that herds in which these "traced out" animals have spent time also will also be skin-tested.

"The lesioned heifer (an offspring of the infected cow detected at slaughter) had been commingled with 208 steers and heifers. These 208 animals, currently under TAHC quarantine, may be depopulated to avoid any potential disease problems," said Dr. Baca. "Furthermore, we'll be testing surrounding herds to make sure there wasn't disease spread. Also, we'll track any movement of animals out of the herd from the time the lesioned heifer was introduced. The epidemiological investigation will broaden each time we find a lesioned animal, in order to make sure we find all potentially infected animals."

"Cattle tuberculosis was a human health threat in the early l900s, when infection rates among herds was high and before pasteurization--or heat treatment--to kill bacteria in milk and cheese," said Dr. Terry Conger, TAHC's state epidemiologist.

Dr. Terry Conger said intensive efforts will be continued to determine the source of the TB infection in the beef herd. He explained that the rancher had purchased animals from several herds during the past five years. Three of those source herds have tested negative. He said the remaining herds will be tested before the end of September.

"Depopulating an infected herd is the only sure way to eradicate cattle tuberculosis, a disease that 80 years ago affected nearly five percent of the nation's cattle herds," said Dr. Conger. "In November 2000, all of the U.S.--with the exception of Texas' El Paso and Hudspeth Counties, and the state of Michigan--was recognized by the USDA as tuberculosis-free. If we find that the TB infection has not spread beyond the animals we are tracing from the infected herd, the USDA may allow Texas to maintain its accredited-free TB designation."

He said the U.S. Department of Agriculture's Animal and Plant Health Inspection Services, Veterinary Services (USDA-APHIS-VS) has agreed to provide indemnity--or purchase funds--for depopulating animals involved in this TB case, including the 79 head of cattle that had been moved out of the infected herd prior to disclosure of infection.

Keeping a TB-free designation is particularly important for Texas, because it allows ranchers to move cattle across state lines without having them tested for tuberculosis. Dr. Conger said testing can be inconvenient and costly, because it requires that animals be held for 72 hours from the time an accredited veterinarian injects tuberculin into the skin under the animal's tail until the site is examined for swelling that indicates the animal has had exposure to the TB bacteria. A slaughter examination, followed up with bacterial culture and tissues tests, are necessary to confirm infection in animals that react to the skin test.

In the "restricted zone" of El Paso and Hudspeth Counties, ongoing low levels of TB infection in 10 dairies along the Rio Grande have been detected during the past 15 years. Despite periodic testing and the slaughter of infected animals, all herds in that region have not remained free of disease for more than a few years at a time, commented Dr. Conger.

Currently, one dairy is under TB quarantine in this zone. Nearly $43 million in federal funds have been appropriated to buy out the dairies, and the Texas Department of Health, by a mew state law, will not issue new dairy permits in this TB restricted zone. Because they originate in a high-risk area for TB, beef and dairy animals moved from the "restricted zone" into the rest of Texas, or the rest of the country must comply with strict testing and identification regulations.

Dr. Conger explained that, in Michigan's northeastern quadrant of its lower peninsula, 17 TB-infected cattle herds have been detected since 1998. Michigan livestock health authorities believe infected white-tailed deer have spread the disease to the herds, and have instituted feeding bans to prohibit close commingling of deer, which can encourage the spread of disease.

"The TAHC staff will be working closely with producers, private veterinary practitioners and the USDA to finalize the testing of animals moved from the herd," promised Dr. Logan. "Furthermore, the TAHC staff will continue to investigate how the herd became infected. Until we have looked into every possible source, we have not completed our job for Texas producers."

About the author: Carla Everett is an Information Officer with the Texas Animal Health Commission.

Agricultural Research Service

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