This review is written in response to an announcement I first heard on January 7, 2005. The radio report declared that there had been a confirmed case of bovine tuberculosis in a human male hunter who had become infected from a deer he had shot. The source of his infection was attributed to direct transmission as a result of the hunter cutting himself while dressing the deer. This announcement was accompanied by an interview with a health official, who stated that such cases were rare, and raw milk is a risk to consumers because it could be infected.
I subsequently obtained the original news release from the Michigan Department of Community Health, dated January 6, 2005.1 In this release the comment, “Bovine TB infection is generally transmitted to humans through consuming unpasteurized milk or milk products obtained from infected cattle,” is attributed to the acting-chief Medical Executive of Michigan at that time, Dr. Dean Sienko. The story was picked up by many newspapers (from Anchorage to South Florida) and distributed by the Associated Press. Almost all of the articles contained a comment about unpasteurized milk as the source of contagion to humans.
Further details in the story indicate that the hunter had gone to his doctor because the cut was not healing. Culture of the wound revealed the particular strain of bovine tuberculosis that has been the focus of a surveillance program in the upper counties of the Michigan lower peninsula. Alcona County, where the hunter shot his deer in October of 2004, is in the heart of that surveillance terrain. Much of the delay in reporting this infection was due to the fact that culturing tuberculosis bacteria requires weeks, and determining the unique strain requires additional weeks.
The news release states that the hunter was on an extended program of antibiotic treatment to rid him of the infection.
Even though this is an extremely complex topic, I am convinced that there is no evidence to support the contention that families in Michigan who drink unpasteurized milk should be afraid of becoming infected with bovine tuberculosis.
There are two forms of tuberculosis that cause significant disease in mammals. Human tuberculosis, a sometimes acute, but much more commonly, a chronic lung infection is caused by the bacterium, Mycobacterium tuberculosis. It is primarily a disease of humans, and is said to be transmitted via bacteria contained in water droplets, usually from the cough of a person with active tuberculosis. It primarily infects the lungs and associated lymph nodes, but can spread throughout the body and infect the liver, spleen and other organs.
Bovine tuberculosis is a very similar disease that infects cattle, as well as deer, goats, elk and many other animals. This infection is caused by a related bacterium, Mycobacterium bovis. Similar to the human disease, infected animals rarely appear ill. Transmission among animals is more complicated than in humans. The bacteria may be transmitted by inhaling cough droplets from an infected animal. However, shared feeding sites are considered a key factor. Unlike most other bacteria, mycobacteria grow very slowly. And one of the most effective ways of killing these bacteria is via direct exposure to sunlight.
In warm-blooded animals, including humans, disease resulting from infections with these bacteria is similar. However in both, the disease is extremely complex.2,3 There is reason to believe that many individuals who are exposed to the tuberculosis bacteria do not become infected, or they successfully kill off all of the bacteria. How often this occurs is difficult to determine, because (as will be explained) the only practical test for tuberculosis does not distinguish infection from exposure.
Those animals who become infected, including people, typically mount a defensive response by walling off the bacteria, forming nodules. This walling-off does not kill the bacteria, it merely isolates them. In healthy individuals (animals or humans) the nodules usually succeed in keeping the bacteria isolated but may very slowly enlarge. The primary location of the infection and nodules is related to the mode of infection. Most infections result from inhalation and the nodules form in the lung. When the bacteria are transmitted in mucus directly to the mouth or nose, the nodules form in the tonsils and lymph nodes of the neck. When the bacteria are ingested, infection occurs in the intestinal lymph nodes (the acid and digestive enzymatic environment of the stomach does not kill these bacteria). When the bacteria enter a wound or cut in the skin, the infection results at that location. Infection from inhalation may result from very few bacteria, whereas infections transmitted by the other modes are successful only when very large numbers of bacteria are introduced.
Individuals successful in keeping the bacteria isolated in nodules appear healthy and are not contagious. If, for some unrelated reason, individuals become debilitated, the effectiveness of the isolation within nodules can fail. This can result in proliferation and spread of the bacteria. In those very few individuals in whom the bacteria escape the nodules, the disease may become contagious and “active.” Individuals whose lung nodules lose their effectiveness in isolating the bacteria and release proliferating bacteria frequently have a cough which contain the bacteria. Once the disease becomes active, proliferating bacteria may enter the bloodstream and lymphatic system and infect other portions of the body. Rarely, when nodules are in the intestine, and if the bacteria break free, the bacteria can be excreted in feces.
Treatment with antibiotics requires months, not because the bacteria are resistive, but because it is difficult to get the antibiotics into the nodules. In a practical sense, the tubercle bacillus acts more like a parasite. Under natural conditions it does not grow outside the body of the infected animal. These diseases are treatable, although the cost and duration of treatment argues against treating animals.
In general, the human form of tuberculosis is confined to humans. Although extraordinarily rare, there is one contemporary case of documented transmission of human tuberculosis to a cow.4 In contrast, bovine tuberculosis has been found in virtually all warm-blooded animals. Interestingly, there have been a large number of captive elephant deaths in the US from tuberculosis. All of the studied cases have cultured the human form, Mycobacterium tuberculosis.5 Bovine tuberculosis in humans is extremely uncommon even in regions where the infection in cattle is widespread.
An important point to keep in mind is that only a few of the humans or animals with any form of tuberculosis appear ill, and most of those infected are not contagious.
Also contributing to the complexity surrounding tuberculosis is the fact that the most commonly used test for the disease (in humans and animals), a skin test, is not specific for active disease. This skin test may become “reactive” in any individual with past exposure to the bacteria, regardless of whether the individual succeeded in killing the bacteria, has disease confined to nodules or has active, contagious disease. And in all large testing programs there is a small but real number of individuals whose skin test is reactive but who have no evidence of exposure to tuberculosis, nor any evidence of the disease. (This is why the term “reactive” is used rather than “positive.”) Adding to this confusion is the fact that the commonly used skin test does not distinguish between infections from the bovine or human forms of the bacteria.
Bovine tuberculosis has been specifically and conclusively diagnosed in humans. It is decidedly uncommon in a world in which human tuberculosis is common. The spread of bovine tuberculosis in humans is clouded by historic misinformation and imperfect science. In The Untold Story of Milk, Ron Schmid does a thorough job of debunking the huge store of medical dogma on this subject.6 During the 1800s, when tuberculosis was widespread in the US, the complexity of the disease was unknown. A few people had intestinal tuberculosis presumably from ingesting, rather than inhaling, the bacteria. Since it was known that many dairy cows were infected with tuberculosis it was presumed—and reinforced by those pushing for pasteurization—that milk was the vehicle of contagion. When it was found that cows had a distinct form of tuberculosis, the dogma expanded, generalizing that all human infections with the bovine form of the bacteria were transmitted through milk (even though the vast majority had lung infections caused by inhalation not ingestion).
In his book, Ron Schmid further details the lack of any association of human infection with bovine tuberculosis within communities that regularly consumed raw milk.
At the present time, when culturing of infections is more likely to be specific, it is known that most of the intestinal tuberculosis in humans is from the human bacteria (M. tuberculosis), medically explained by individuals swallowing their own bacteria-laden lung secretions coughed up from lung nodules that had become active. In a current review of tuberculosis in the United Kingdom for the period 1990-2003, there was an average of 7,000 cases of human tuberculosis reported each year. Between 0.5-1.5 percent of those cases which had confirmed cultures were caused by M. bovis, and these were found mostly to be either reactivation of old lesions or infections contracted in other countries which lacked aggressive animal control measures. In this 13-year study, only one case of human M. bovis was determined to be acquired from an animal source in the United Kingdom.7
Most contemporary experts agree that the most likely mode of bovine tuberculosis transmission to farmers is through inhaling cough droplets or through direct contact with material contaminated with nose and mouth secretions from an infected herd of cattle. This would occur only when some of the animals had active tuberculosis. And the farmer would typically develop lung nodules. When the disease in cattle was widespread in this country, bovine tuberculosis was recognized as a vocational hazard in people working in meat-processing plants. This was presumably from contact with the lung secretions of slaughtered cattle with active disease, or when tuberculous nodules were cut open during processing. A significant clue to the mode of transmission is the correlation with the location of the initial infection nodules. As an example, in Michigan, since the location of most of the lesions in the infected wild deer is in the lymph nodes of the mouth and neck, transmission is presumably by direct contact with secretions of infected deer at communal feeding locations (mainly bait feeding caches set out by hunters). The apparent contradiction of this principle in humans with human tuberculosis—that nearly all intestinal infections are not the result of ingesting food contaminated with the bacteria—is explained because these intestinal infections are not the primary site of infection but are the result of swallowing pulmonary secretions when the primary pulmonary tuberculosis nodules break down and spill bacteria into the mucus from the lungs. And these people have conspicuous lung nodules from their primary infection.
Growth of the tuberculosis bacteria is extremely slow, even under ideal conditions. They are unlikely to proliferate outside of a living animal. And growth in milk is considered very unlikely. With our current understanding of the complexity of this disease, it has been shown in several rigorous studies conducted in countries where eradication programs are not in effect, that in large dairy herds with widespread infection, a few samples of milk do contain the bovine tubercle bacillus.8,9,10 This has been documented even when the milk was carefully handled to eliminate the possibility of contamination outside of the cow.
In one study in Tanzania,11 two out of 805 samples obtained from infected cows were confirmed to contain the bovine bacteria. In a 2003 study in Lahore, Pakistan, researchers were able to identify bovine tuberculosis bacteria in the milk from four of 16 cows with confirmed bovine tuberculosis.12 It is critical to note that all of the cows tested in the Lahore study were specifically selected because they were visibly ill, showing “low milk yield, emaciation, and anorexia, intermittent diarrhea, not responding to anthelmintic treatment, irregular febrile episodes, and stubborn recurring mastitis.”
It is also reported that cow’s milk can become secondarily contaminated from ulcerating tuberculous lesions on the teats of cows, as well as from milk-handlers who have active tuberculosis. These are incredibly rare circumstances. Milk-handlers in all regions of the world are much more likely to have human tuberculosis than bovine tuberculosis.
Critical analysis would conclude that the transmission of bovine tuberculosis from milk to a human would only occur under a set of extremely uncommon circumstances:
All these circumstances are extremely unlikely, and to my knowledge not one of these theoretical cases has ever been documented. And finally, in all of these cases, the humans who ingested the infected or contaminated milk would develop tuberculosis in the mouth or neck lymph nodes, or in the intestine, not in the lungs.
I am unaware of any rigorous study that has shown that the infection of bovine tuberculosis in people in any region of the world can be traced to ingestion of infected milk. Furthermore, in regions where herd infection is widespread, the few reports of human infections with bovine tuberculosis do not describe lesions that would be associated with ingestion of milk. In a contemporary analysis in the United Kingdom, thorough clinical examinations were performed on 138 people who had close contact with herds infected with bovine tuberculosis or who drank raw milk from those herds. No cases of bovine tuberculosis were found in these people.13
Starting in 1917 there was a massive federal and state program to eliminate tuberculosis from all cattle in the United States. In 1979, Michigan gained, as had all other states, status as officially free of tuberculosis in cattle.
In 1994, a hunter noticed that the deer he killed in Michigan had lung nodules, and it was confirmed that this was the first case of tuberculosis within a wild herd of deer in North America. This launched a surveillance and testing program in Michigan’s deer herds. Then, in 1998, a case of an infected cow was reported in Alpena County, Michigan. The two cases were of the same specific strain of bovine tuberculosis. Consequently federal and state agencies have coordinated efforts in the Michigan Bovine Tuberculosis Eradication Project.14 And Michigan lost its Accredited Free status.
To date the Michigan eradication effort has been primarily a surveillance program. From 2000 to 2003, essentially all herds of dairy cows, cattle, goats and domestic herds of deer, elk and bison in Michigan were tested for tuberculosis. More than 138,000 free-ranging deer were also tested. All of the positive tests (confirmed by culture or microscopic examination of tissues) have occurred in a total of 33 wild and domestic herds in thirteen counties in the northern portion of the Lower Peninsula of the state. All of these infections that have been cultured have that same specific strain of bovine tuberculosis. This specific strain has not been identified in any other location. Following rigorous federal rules, testing continues in these counties, while random surveillance is being done throughout the rest of Michigan. Most of the animals that tested positive, both wild and domestic, have been in Alpena and Alcona counties. The infected deer and domestic cattle that have been examined consistently have infections in the lymph nodes of the neck region. Because of this characteristic and observations of animal behavior, it has been speculated that the bacteria are transmitted from nose to nose among congregating deer while eating (often at baiting sites used by hunters to attract deer). Unfortunately, additional cases of infected cattle have been discovered in 2006 suggesting that the current strategy is not adequately controlling reinfection. No infected cattle have entered the food system, nor has any milk from infected cows been distributed.
By law, all forms of human or animal tuberculosis must be reported to health agencies. Since the start of the Michigan eradication project in 1997 there have been 2,284 cases of tuberculosis reported in humans in Michigan. Of these, 10 were found to be infections of the bovine form. And only two of these were found to be infected with the particular strain of Mycobacterium bovis that has been detected in all of the infected deer and cattle identified in the Michigan surveillance program. One of these is the young hunter in Alcona County described in the January 7, 2005 reports mentioned above. The other was an elderly resident of Alpena County who in 2002 was discovered to have tuberculosis bacteria in bronchial washings obtained during a medical workup. He died within a short time from unrelated causes and a tuberculous nodule was found in his lung at autopsy. Subsequently the bacteria were determined to be M. bovis and of the identical strain to that found in the deer and cattle.
This individual, born in the US, spent most of his life in southeastern Michigan, and had returned to the area where he had been raised on a farm when he retired. He was a deer hunter. Forty years before his death he had been exposed to a family member who had a diagnosis of tuberculosis. Since his infection was in the lung, the mode of transmission would have been airborne. If the relative had infectious tuberculosis, this could have been the source of his initial infection. It has been speculated that he became infected from contact with tuberculous cattle as a child. The source of infection of the other eight people is unknown. They are reported to be older individuals and some were born outside of the U.S.
In several countries where bovine tuberculosis control programs are considered effective, authorities have attempted to trace the origin of the extremely rare cases of human bovine tuberculosis. Mostly these cases are in older people and are attributed either to slaughterhouse exposure or to childhood infections that occurred in countries where infected herds were common. In some areas of the United Kingdom, herd infection with bovine tuberculosis has recently been increasing; some believe that endemic infection in badgers is responsible. A brother and sister from a farm family that had infected cattle were found to have active clinical bovine tuberculosis typed the same as those cattle. Interestingly, both had been vaccinated to prevent tuberculosis. Neither had ever consumed unpasteurized milk or milk products. But the boy had occasional direct contact with the infected cattle several years previously. Both siblings and the infected cattle all had pulmonary disease. Clinically it was concluded that the boy had acquired infection by inhaling saliva droplets from the infected cattle, and the sister had acquired her infection from active pulmonary disease in her brother. Interestingly, none of the other family members became infected.15
Almost every public health and agriculture official commenting on tuberculosis states that pasteurization has saved us from bovine tuberculosis.
In reality, the almost complete eradication of bovine tuberculosis in the United States is the result of herd management. The persistent use of herd testing and slaughter of suspected animals has been remarkably effective.
This success shifted from that policy of eradication to a mode of surveillance relying on visual inspection of cattle in meat processing plants. Inspectors have incentives to identify and submit abnormal lesions for laboratory examination. Since 2001, six states in the United States have reported cattle infected with M. bovis.16 In February, 2007 the Colorado Department of Agriculture and the USDA announced that an imported bull had been detected with bovine tuberculosis. This and all of the initial infections have been detected by surveillance of animals in slaughterhouses. Inspectors spot suspicious lesions and samples are tested in federal laboratories.17 Under the USDA’s Tuberculosis Eradication Program, states are assigned one of five status levels. As of February 2007, all but three states have Accredited Free status. Michigan, New Mexico and Minnesota have the USDA’s Modified Accredited Advanced (MAA) status.18 Over the last six years bovine tuberculosis has been identified in cattle in Arizona, California, Michigan, Minnesota, New Mexico and Texas. Infected herds were identified and “depopulated.” (Depopulation is the term used to indicate that the whole herd was killed. Slaughtering individual animals, such as those testing “reactive” is not utilized since the testing does not necessarily detect all recently infected animals.19) In Arizona the infected herd was depopulated and testing did not identify spread. And by the USDA’s standards, Arizona maintains Accredited Free status. In California, cattle in three dairy herds were found to be infected and testing confirmed the spread to other associated herds. In April, 2005, following depopulation of all infected herds, California regained its Accredited Free status.20 In Texas, two cattle herds were found to be infected. Four dairy herds were subsequently discovered on testing also to be infected. Herds were depopulated. In October 2006 Texas regained Accredited Free status.21 In New Mexico two counties (Curry and Roosevelt) have a status of Modified Accredited Advanced. Two cattle herds were infected, although there have been no additional active cases of bovine TB.22,23 Minnesota was placed on MAA when an infected cow was identified at a slaughterhouse in Wisconsin and traced to a herd in Minnesota.24 Minnesota is in a full eradication program. It is theorized that the initial infections in these states have all originated in animals imported from other countries, one of the unanticipated effects of NAFTA. In reaction to the recently identified cases of infected cattle, an increasing number of states are instituting requirements for tuberculosis testing of all mature cattle entering from other states or countries.
In Michigan it is generally agreed that the current infection in cattle has spread from endemic bovine tuberculosis in the wild deer herds in one particular region. As part of the Michigan Bovine Tuberculosis Eradication Program, historical records were searched to determine how the deer might have been originally infected.25 During the periods when data were available, Michigan had a higher rate of livestock infections than any other state. And infections were highest in the area that has the current endemic deer population. The rates of reactor cattle were 32 percent and 20 percent in the two central counties of that endemic region. In the 1950s, as the national eradication program was progressing, approximately 30 percent of all tuberculosis reactors in the United States were in Michigan. Authorities speculate that the deer became infected from the livestock during the time when the infection rate was high. Testing and slaughtering eradicated the tuberculosis in the livestock, but since it was not known at that time that the deer had become infected, they were not tested. The fact that the bovine tuberculosis in the elderly man was of the current specific strain was seen as confirmation of the hypothesis that he must have become infected in his youth in the 1930s. Although not discussed in the reference, his primary lesion was in the lung which would mean that his infection was transmitted through airborne bacteria or by direct contact with secretions from an animal with active disease. Not surprisingly, the authors overlooked this fact and simply concluded, “Hunting deer with tuberculosis is less likely to result in infection than consumption of unpasteurized milk from cows with tuberculosis.” There is absolutely no scientific support for this statement.
Michigan currently has a “split-state status” with portions Accredited Free, most of the state as Modified Accredited Advanced, and the endemically infected region as Modified Accredited.
All of the reviews and contemporary national risk assessments conclude that careful management of cattle has been, and continues to be, the most effective means of controlling bovine tuberculosis in herds and preventing its spread to humans. This includes maintaining closed herds, insuring that all of the cattle are free of tuberculosis, checking that all animals added to the herd are free of tuberculosis, determining whether the farmers are free of tuberculosis or are adequately treated, and maintaining practices of good hygiene. Farmers in regions in which wild animals have bovine tuberculosis should manage their cows to avoid physical contact with infected animals. In these regions testing of both cattle and people must be continuous. High numbers of animals in close proximity increases the potential spread of tuberculosis. This includes spread from animal to animal, animal to human, and human to animal.
It is quite common for public health officials when issuing reports or news releases to state with emphasis that it is well documented that Mycobacterium bovis can be found in milk from infected cows; however, objective review of the research literature contradicts this allegation. Even in countries where bovine tuberculosis is uncontrolled, where infected dairy herds are common, and infections in a few humans are well known, researchers have repeatedly failed to find any bovine tuberculosis organisms in the fresh milk. Even studies to determine the cause of mastitis in infected cows repeatedly fail to find M. bovis. The few studies cited above were only able to isolate the bacterium from a limited number of infected cows when the cows were overtly sick.
There have been no cases of bovine tuberculosis in hundreds of families who consume unprocessed milk in the affected counties in Michigan. For families who have made a choice to drink fresh unprocessed milk from their own cow or from a local farm, the very minor risk of becoming infected with bovine tuberculosis is best controlled by ensuring that the cows are disease free.
In all geographic areas in which there is no bovine tuberculosis in the local wild animals, periodic testing of all cattle imported from outside the US as well as meat-processing plant surveillance should be adequate. In those geographic areas in which there is infection in the wild animals, all dairy cows, both individual or in herds, should be enrolled in a program to test for tuberculosis on a regular basis. Farmers should be vigilant for any unexpected decrease in milk production or other evidence of declining health in their herd. They should be suspicious of recurring, or unexplained mastitis. The US Department of Agriculture has published an updated set of comprehensive regulations for bovine tuberculosis eradication which became effective January 1, 2005.26 When farmers share unprocessed milk with others, I would suggest that the results of all testing should be posted conspicuously.
Since those of us who consume fresh unprocessed milk look for high nutrient quality, we are concerned about the health of the lactating animals. Regarding bovine tuberculosis, our public health focus should be to ensure that the dairy herds are free of disease. Even though not required by any governmental standard, all animals brought into a dairy herd should be tested for tuberculosis unless the herd of origin is known to be tuberculosis free.
The subject of bovine tuberculosis is a complex topic, clouded by the spin of various special interest groups and remarkably persistent widespread unfounded medical dogma. My aim has been to present a credible and objective review that would be understandable to those of us who drink fresh unprocessed milk from our local farmers. I am satisfied that there is no evidence to support the contention that people in Michigan who drink unpasteurized milk should be afraid of becoming infected with bovine tuberculosis.
The full 2005 Activities Report and Conference Proceedings of the Michigan Bovine Tuberculosis Eradication Project can be viewed at: http://www.michigan.gov/documents/MDA_2005_BTB_Report2_148142_7.pdf
The full text of the USDA’s latest rules: Bovine Tuberculosis Eradication Uniform Methods and Rules, effective January 1, 2005 can be viewed at https://www.aphis.usda.gov/animal_health/animal_diseases/tuberculosis/downloads/tb-umr.pdf
The current status of the United States Bovine Eradication Programs can be viewed at: http://www.aphis.usda.gov/animal_health/animal_dis_spec/downloads/eradication_status.pdf
This article appeared in the Summer 2007 edition of Wise Traditions, the quarterly journal of the Weston A. Price Foundation.