An overlook at tuberculosis in dairy livestock, including causes, symptoms, transmission, history and prevention.
My wife and I were traveling a seasonal mountain road along the ridge that separates Massachusetts and Connecticut from New York State when we came across a rich, golden foliage. The trees that held these beautiful leaves were second growth American Chestnut. Although they once covered our hills and ridges, they died in the 1920s due to a blight; the present growth is from still live roots of these apparently long-dead trees. They will grow to about 20 feet, and then, like their parents, they will die back with only the roots surviving.
Although some foresters tell us it will never happen, those of us who knew the value of its edible nut and durable wood of many uses keep hoping that one of these new growth trees will be blight-resistant and reproduce—that the species will flourish again. We know that other living organisms develop resistant strains, such as bacteria becoming resistant to antibiotics, or the Brahma cattle becoming resistant to certain tropical diseases. Conversely, animals (including humans) lose resistance to certain diseases if they are not exposed to it for several generations. When they are exposed, they may die from a disease which would have caused only a mild illness in their grandparents.
There are many examples in recent history of diseases once rampant in animals and humans that have practically disappeared and then suddenly reappeared. One disease that has suddenly reappeared after years of being virtually unknown is tuberculosis, once commonly referred to as "TB."
With all the publicity about its resurgence, we have recently received many questions about tuberculosis. Since there is no known immunization against the disease, the best way to protect ourselves and our livestock from it is to learn all we can about what causes it, what its symptoms are, and, most important, how it is spread.
Tuberculosis, which affects most warm-blooded animals, was thought to be almost nonexistent in the United States and Canada 25 years ago. Any veterinarian of my generation with knowledge and experience of TB could write enough to fill this magazine, which would no doubt bore most of you. Instead, I offer a little background and a discussion of what you, as a livestock owner, need to know in order to protect yourself and your animals from the resurgence of this disease.
There are several strains of TB, caused by a bacteria of the genus Mycobacterium tuberculosis, the human form. The most common strains in barnyard animals are M bovis and Mavium, which are found in cattle and birds, respectively. The human strain and the bovine strain are the most closely related. Literature often refers to the organism as being "acid fast," which means that when the material containing the organism is stained with a red dye and washed with a bleaching acid, the organism retains the red color. This makes it more visible under a microscope.
Many of us think of tuberculosis in the human as being a slow, debilitating condition. However, stress from poor nutrition, exposure to cold and wet, or other diseases can all cause it to be a rapid killer. The human form is seldom found in animals other than non-human primates, but the bovine form readily affects most warm-blooded vertebrates. Birds seldom have any form except the avian, but this form has been found in most barnyard animals, as well as dogs and cats. Exotic pets, in particular, may be carriers of the disease in any one of its three main forms.
Symptoms of TB may go completely unnoticed in your animal until some form of stress—poor nutrition, old age, missing teeth, or some other disease—takes over. I have seen cattle that were generalized cases at postmortem that had been fat, slick, and producing well just prior to slaughter.
The autopsy lesions of cattle with early cases of TB may only be swollen mandibular lymph glands. Later, a caseous solid pneumonia is apt to be found. In so-called generalized cases, many organs are involved. Regardless of the lesion's size, there will be a scraping sound (as if the lesion contained sand) when a vet cuts into it with a knife.
In humans, the bovine form of the disease often settles in the bones, or the glands of the throat and neck. Literature describes large numbers of human cases that became hunch-backed because the disease causes collapsed vertebrae. I can remember as a boy seeing people with scars on their neck and throat where infected glands had been removed.
Early in this century—prior to the pasteurization of milk—there were so many cattle infected with TB that many humans contracted the bovine form of the disease from drinking milk. Tuberculin testing of cattle began in the 1920s as a public health measure. State and federal indemnities were paid to the owners who had reactor cattle removed from their herds.
These reactor cattle were branded with a "T" on their right cheek, and a special reactor tag was placed in their left ear. They were then shipped to slaughter houses, where veterinarians of the Bureau of Animal Industry did postmortem inspection as they were slaughtered. Generalized cases, in which more than one organ was involved, were "tanked." That is, the whole carcass was destroyed by heat and chemicals in order to keep it out of the food chain.
If only one small lesion was found, that part was removed, and then the rest of the carcass had to be cooked in order to pass for human food (such as in the making of bologna).
During the 1930's, all herd of cattle in all states were tested, reactors removed, re-tested until clean, and then tested yearly. By 1950, few herds in the United States had cattle reacting to the test, and they were fast being cleaned up. Canada had been following a similar test and slaughter plan, and by the mid-1950s, tuberculosis was nearly eliminated. Tuberculosis-free cattle, pasteurization of milk, and better human hygiene nearly eliminated the disease in humans as well. However, other countries were years behind in eliminating tuberculosis in their cattle, and remained as a reservoir of the disease.
In the 1980s with cutbacks in money for the testing of cattle, importation of species that were not TB-tested (such as fallow deer and llamas), an increase in the size of untested herds of wild buffalo, and a general lack of knowledge and respect for the disease among livestock owners. Human TB also increased because of AIDS, the drug culture, homeless people, and a general breakdown in human hygiene.
Since tuberculosis first affects the lungs of most victims, whether human or animal, it is spread most often by saliva and nasal discharge, such as coughing. Common mangers, drinking tubs, and poorly ventilated stables can spread the disease to every animal in the barn if even one carrier animal is introduced to a herd.
In April of 1948, when I tested a barn of 80 mature dairy cattle, 63 reacted and were sent to slaughter. One cow, a carrier, had been introduced to the herd the previous November. Although the cow was fat and slick, and producing well, she was a generalized case of postmortem.
This barn was well ventilated, but had common mangers that were cleaned by sweeping. It is presumed that the sweeping carried the TB organism from the slobber of the infected cow to the rest of the herd. On a follow-up test, calves that were receiving milk from this herd reacted to the TB test. Despite the fact that some people drank raw milk from this herd, fortunately there were no human cases.
Remember that tuberculosis is spread by handling pets. Don't let animals lick your face, eat at the table with you, or eat from your dish.
Although this is a veterinary column, I would be remiss in not mentioning how the disease is spread among humans. Although modern, human tuberculosis is primarily a disease of cities, even those of us lucky enough to live in rural areas have contact with people in public places.
Because of TB awareness from the early 1920s up until forty years ago, there were certain taboos: spitting in public, the use of common drinking vessels, and sharing food or drinking from the same glass, dish, or utensils. These are the main means of spreading TB. Saliva and nasal discharge from a human or animal infected with tuberculosis is loaded with tubercle bacilli.
Children should be taught the basic rules of hygiene early in life. The next time that your six-year-old runs and hides when a visiting aunt (or other relative) wants to plant a big juicy kiss, stick up for the kid. Also, if you must kiss a baby, place a dry kiss on the back of the neck. The baby will like it better and you won't be spreading your germs to the infant.
Also, remember that TB is spread in handling pets, particularly exotic ones. Letting an animal lick your face, eat at the table with you, or eat out of your food dishes is playing with fire. Non-human primates can be particularly dangerous in this respect.
Whether you own one cow or a thousand, if your herd has not been TB tested regularly, according to your state or province schedule, make every effort to have it done. Give the testing veterinarian every bit of cooperation.
Testing is done by injecting a tiny amount of tuberculin into the skin of one tail fold, and the opposite vulvar lip, or, in the male, under the anus. Then, 72 hours later, the veterinarian must feel the injection site to be sure there is no swelling, which would indicate a reaction. Running a herd of heifers through a 12-foot gate, five at a time, while the veterinarian tries to check the injection site is not cooperation. If a reactor is missed, the next test may find your whole herd is infected.
If you drink the raw milk from one cow or goat, you have to make sure she is TB free. If you buy a cow or goat for family milk, insist on a negative TB test, and have her re-tested at least once a year just to be safe. If you are like me and feel that properly cared for raw milk simply tastes better than home pasteurized milk, ask your local veterinarian what other types of tests your cows or goats should have done. Even if you pasteurize your home milk supply, it should come from cattle and/or goats that are disease-free.
Lastly, goats, domestic deer, and llamas that have had contact with the cattle, goats, or sheep that you use for milk should be tuberculin tested. Once these animals are clean—if no exposure is encountered (such as contact with new animals or having your animals travel to shows)—testing every three years is probably often enough. However, it's a good idea to check with your local veterinarian. Most important, get that first test done!
Whether you want to learn how to grow and raise your own food, build your own root cellar, or create a green dream home, come out and learn everything you need to know — and then some!LEARN MORE