Mycobacterium bovis... don't blame the cows?
I recently wrote about Mycobacterium bovis, the cause of bovine TB and a pathogen that can be transmitted to people and rarely to pets. A reader made the following comment:
"Having come across your very interesting blog, I was questioning/wondering whether your statement regarding Mycobacterium bovis, "whose main natural reservoir is cattle", is in fact actually so any longer; if ever. . In the UK all cattle herds were once declared clear of this disease by testing and culling and the gassing of badgers, until the government protected the badger over here to appease animal rights activists and gain a few extra votes. Now it is rife again. These people somehow believe badgers have 'rights' to life above farmers' cattle.
I often wonder if the 'bovine' association is simply because the bacterium was first isolated in cattle as they were obviously captive and there to be investigated. Could it just as easily have been called Mycobacterium meles? As I understand it, Mycobacterium tuberculosis came first and originated in humans and then developed as Mycobacterium bovis in animals"
Good question. Just because a disease is named after a particular species does not mean that it's the main source or original source of the pathogen responsible. Cowpox virus is a good example - cows aren't actually the reservoir of this virus, rodents are. However, because cows are more closely observed or monitored than rats, it was originally associated with cows and thus named cowpox.
We are more likely to detect diseases in humans first, followed by domestic animals, followed by wildlife. Severe acute respiratory syndrome (SARS) is a good example of this. This disease was first found in people. It was then linked to civets and raccoon dogs. However, civets and raccoon dogs aren't the true reservoirs, nor where the disease originated. It appears that the reservoir is in fact bats. Bats are a lot harder to investigate than captive animals, so even though they are now the presumed reservoir, it took a while to figure that out.
Back to Mycobacterium bovis... It's impossible to say for certain where it originated. Since it is thought to have evolved from Mycobacterium tuberculosis, the reservoir of which is people, it makes sense that the evolution of M. bovis would involve a domestic animal species instead of a wildlife species. Cattle have much closer contact with people than do wildlife such as badgers, opossums and deer. So, since M. bovis has historically been most strongly associated with cattle, and cattle live in close contact with humans, I woudn't be surprised if they are the true orgin in this case. However, since M. bovis can infect a very wide range of species, we can never really know.
"The other TB" Mycobacterium bovis
Tuberculosis (TB) is an incredibly important disease caused by Mycobacterium tuberculosis. It's a huge problem internationally, and the problem is getting worse in many areas. Another cause of "tubercular" (or tuberculosis-like) disease is Mycobacterium bovis, a related microorganism whose main natural reservoir is cattle.
Mycobacterium bovis is cause of bovine TB. It can also infect people (usually through drinking or eating unpasteurized dairy products) and pets. Pets can be exposed by a few different routes, including eating contaminated dairy products, eating infected animals (e.g. snacking on carcasses of wildlife like deer that have died of the disease), and perhaps from direct exposure to wildlife carrying the organism. Mycobacterium bovis is an important problem in some areas, typically because of its presence in a wildlife reservoir like deer or the European badger (a major problem in the UK).
Mycobacterium bovis can cause serious disease in pets. It often causes non-specific signs that makes it hard to diagnose until disease is very advanced (and unfortunately likely beyond the point of successful treatment). Some groups recommend prompt euthanasia of infected pets without considering treatment because of the potential for infection of people. The risk of pet-human transmission is completely unclear, but it's such an important disease that some people think any risk is unnecessary and unjustifiable. So, the key is avoiding infection in the first place (for both people and pets). This is of particular concern in regions where M. bovis is present in wildlife and cattle. In areas where it is not known to be present, there should be little to worry about.
Here are some simple steps that can help you reduce the risk of your pet becoming exposed to M. bovis:
- Keep cats indoors.
- Don't allow dogs to roam free outdoors.
- Don't allow animals to have access to unpasteurized dairy products or dead animals.
Pretty basic, isn't it?
Tuberculosis in a dog in Ontario
A report in the latest newsletter from the University of Guelph Animal Health Laboratory describes a case of tuberculosis (TB) in a pet dog. The dog was a seven-year-old Bichon Frise that had an abdominal mass, low-grade fever, nasal discharge and pneumonia. Tuberculosis was diagnosed by testing a sample of the mass that was obtained during an exploratory surgery. The dog was euthanized because of the poor prognosis and because of concerns about transmission to people in the household.
Tuberculosis has historically been one of the most important infectious diseases in people and has re-emerged as a huge problem in human medicine, particularly because of the development of highly drug-resistant strains. The disease is caused by the organism Mycobacterium tuberculosis. It can be spread through the air over short distances by minute droplets when someone with active TB coughs, sneezes, speaks or sings. Other individuals become infected by breathing in the bacterium.
Tuberculosis is primarily a human disease. It has been reported in various animal species, but only rarely. Dogs are considered relatively resistant to TB, even so there are several reports in the scientific literature of TB in dogs. The source of the dog's infection in this case was not discussed. Presumably, the dog was infected by close contact with an infected person.
One of the reasons for euthanasia of the dog in this case was the risk to household members. At this point, we have little information about the risk that infected animals pose to their human contacts. This makes providing evidence-based advice difficult. Many people may err on the side of caution by euthanizing the animal to prevent transmission. Important aspects that need to be considered include whether the owner can afford to attempt treatment of the animal (with no clear evidence of what works and longterm treatment being required), whether the disease is potentially treatable (i.e. what are the chances the animal will recover if it is treated), and the status of other household members with respect to TB disease or exposure. Of course, these consideration are all in addition to that of the animal's condition and quality of life, which may warrant euthanasia regardless - tuberculosis can be a devastating disease, and it is often not detected until it is quite advanced.
There's no correct answer. Our poor understanding of this disease in dogs and the significant health risks of TB in people unfortunately make euthanasia a reasonable decision.
Transmission of tuberculosis between a man and his parrot
A recent article in the journal Avian Pathology describes a case of Mycobacterium tuberculosis, the cause of tuberculosis (TB), in a pet bird (African Grey parrot) and its owner. Mycobacterium tuberculosis mainly causes disease in people, but can sometimes be found in other animal species, including birds. In this case, the bird was presented to a veterinarian because it had a decreased appetite and nodules under its tongue. The bird was wild-caught in Africa 11 years earlier. The owner was treated for TB two-and-a-half years earlier. Apparently, the owner usually fed the bird pre-chewed food (don't ask me why), and the vets suspected TB because of this close exposure. Because of the severity of the disease, the bird was euthanized and TB was confirmed by culture and PCR.
Often, we get to a point like this where both an animal and person have been diagnosed with the same disease, and can't go any further in terms of determining how each of them got the disease, and if it was transmitted between them. Fortunately, the Mycobacterium tuberculosis isolate from the owner had been saved, and they were able to compare it with the strain found in the bird. They were same. This strongly supports the theory that TB was transmitted between the owner and the bird. However, that's as far as we can go with confidence. The authors hypothesized (reasonably) that since the owner was diagnosed first, and since TB is mainly a human issue and is rarely found in birds, that the person acquired TB from some source then infected the bird. Additionally, two other reports of TB in birds also stated that the owners pre-chewed the birds' food.
TB in birds (and pets in general) is rare, and people shouldn't panic about it. However, it is apparent that transmission between species can occur. Transmission from an infected person to a pet is more likely than the other way around, but both are certainly possible. It's a good reminder that people with TB who are considered infectious should take precautions around their pets, just like they do around other people.
...and pre-chewing food for your bird is probably not a good idea, either.
Transmission of tuberculosis to pets
Some people consider tuberculosis to be a disease of historical interest, but TB is alive and well, and spreading. TB is caused by Mycobacterium tuberculosis, a microorganism that can cause severe lung disease. TB rates are climbing rapidly in some countries and the emergence of highly drug resistant strains is a major concern. Anytime infectious diseases become more common in people in the general population, there is the potential for exposure of household pets. Recently, transmission of TB from humans to different animal species, including a dog, was reported. This follows earlier sporadic reports of TB in dogs. Finding TB in pets, albeit a small number, raises concerns about the potential role of pets in transmission of TB back to people.
Diagnosis of TB in pets is not particularly surprising, since it has been clearly shown by a few other infectious diseases that disease trends in people can sometime be reflected in their pets. It's hard to say if this is an under-recognized problem, an emerging problem or a very rare event that gets a lot of publicity. Really, the only way to tell will be to see what happens over time. Currently, TB is very rare in pets so it is important to keep possible risks and concerns in perspective. There is no reason for pet owners with TB to give up their pets. People with active TB should consider their pets as part of the family, and interact with them as they have been instructed to with people (i.e. if they are supposed to avoid close contact with people they should do the same with their pets).
There is currently no indication that infected pets can transmit TB back to people, and the risks are probably very low. TB is most easily spread through coughing, and pets with TB rarely develop respiratory disease with coughing. That, combined with the rarity of TB in pets, means that the risk of acquiring TB is much, much higher from other people than from pets.
This is just one more example of the "one medicine" concept, whereby we need to break down the barriers between 'animal diseases' and 'human diseases' (because the bugs are clearly doing so).
