Dogs as human Lyme disease sentinels
A paper that will be published in the September edition of Emerging Infectious Diseases (Mead et al 2011) talks about the potential for dogs to act as indicators of Lyme disease activity and risk for people. The use of animals as sentinels for human disease is well established. Sometimes it's because animals are more readily affected. Sometimes it's because the disease is easier to diagnose in animals. Sometimes it's because getting access to samples from animals is easier than from humans.
Because of the distribution of ticks that transmit the causative bacterium (Borrelia burgdorferi) and wildlife that act as the reservoir, the occurrence of Lyme disease is highly variable geographically. Knowing the amount of Lyme disease activity in a specific region is important for understanding the role of this disease in humans, and for implementing preventive measures.
Like people, dogs can get Lyme disease. Dogs are NOT sources of human infection, but since both dogs and people get Lyme disease the same way (from Ixodes ticks), infections in dogs can indicate the potential for infections in people (and vice versa). Since dogs may be more prone to being exposed to ticks, and since ticks are most likely to stay on dogs for the 24 hours or so that is required for ticks to transmit the bacterium, dogs may be more likely to be exposed to Borrelia spp. in endemic regions.
That's the reasoning in the Mead paper which suggests that dogs, because of their potentially greater chance of exposure and tendency to produce a good immune response after exposure, might be good indicators of human Lyme disease risk. To examine this premise, the authors compared data about B. burgdorferi antibody levels in dogs to human infection data. (Note: These are two different things: In dogs, they looked at antibodies, which indicate exposure but not necessarily disease. In people, they looked at disease. It's not inappropriate to compare the two, but you have to be aware of what they were comparing).
Overall, they showed (not surprisingly) that there was a relationship between antibody levels in dogs and Lyme disease in people. Some key findings were:
When the percentage of dogs with antibodies against B. burgdorferi was <1%, there were extremely low rates of disease in people in the area.
- This makes sense since it would indicate that there's little or no transmission occurring in the area. Low levels of B. burgdorferi in dogs or people don't necessarily indicate that Lyme disease is active in the area, since some cases could have been acquired during travel, and false positive results are possible with current testing.
The risk of disease is generally low to non-existent outside the highly Lyme-disease endemic areas: Northeast, mid-Atlantic and upper Midwest regions of the US.
- These are the areas where ticks capable of spreading the bacterium are common and where the bacterium is resident in wildlife, so that's not surprising.
Where 5% of more dogs had B. burgdorferi antibodies in their blood, there was always an above-average incidence of Lyme disease in people in the state, with a lesser association at the county level.
- Again, this makes sense. If most dogs are exposed, more people are going to be exposed, and more people will develop disease.
In 15% of counties where dogs had a >5% rate of antibodies, people did not have above average disease rates. However, in half of them, the incidence of disease in people increased to above average in the following 3 years.
- This is quite interesting and perhaps the most important finding of this study. It suggests that monitoring rates in dogs may predict trends in people.
What does this all mean? Well, a lot of these results would be expected based on what we know about Lyme disease. However, the apparent close linkage between human disease rates and dog antibody rates, and particularly the potential that dog rates could predict human rates, is intriguing and could be useful. By routinely monitoring for antibodies in dogs, areas where Lyme disease might be on the rise or might be emerging in people could be identified, leading to more focused educational efforts directed at both the public and healthcare personnel. Getting the dog data (or at least getting good dog data) is perhaps the problem, since testing would need to be done on a subset of the dog population that's not biased and is of adequate size to say something useful. There are ways to do this, but it takes time and money to do it right. Taking advantage of blood samples collected for heartworm testing is one possible approach, but careful thought needs to go into what could be done and whether it would be better than more intensive surveillance of humans or ticks.
Travel, pets and infectious disease risks
An important concept when dealing with infectious diseases is consideration of the risk that an animal has been, or will be, exposed to a particular microorganism. Some diseases vary greatly geographically, and something that's very important in one region may be rare or non-existent in another. Good veterinarians are aware of disease trends in their area and make informed decisions about vaccination and anti-parasitic treatments based on what's happening in the area. They also know which diseases are common and which are rare or non-existent.
But that only works if the pets stay in their "home" area. Traveling with pets can result in exposure to various infectious diseases they wouldn't normally encounter. If a veterinarian doesn't know a pet travels, they can't make proper recommendations for preventive medicine.
Additionally, travel history can be very important when evaluating a sick animal, since there may be diseases that need to be considered in a traveling pet that wouldn't be an issue with a local pet. However, it's easy to overlook or forget about travel history. Pet owners need to tell their veterinarians about "recent" travel with their pets. What does recent mean? It's hard to say. For some diseases, exposure within the past few days is all that's important. For others, it may be weeks or months. So, if you have a sick pet and have traveled any time in the past year with it, it's good to mention that to your veterinarian. It may have nothing to do with the current illness, but it never hurts to let them know anyway. In some situations, it may be the critical piece of information needed to trigger thinking about a specific disease.
Some examples of diseases that may be travel-related (at least to dogs in most parts of Ontario):
- Blastomycosis, a fungal disease, tends to occur predominantly in specific areas. It's not too common elsewhere, but travel to high-risk areas puts blasto on the list of possibilities in certain cases.
- Around here, there's no indication for heartworm preventive treatment during cold winter months, but that changes if the pet goes to areas where mosquitoes hang around all year.
- Some tickborne diseases have very specific ranges that correspond to their primary hosts and certain vector species (such as birds). In Ontario, ticks are currently quite geographically focused and many dogs have little risk of exposure. Travel to one of the tick hotbed areas changes that, and means that certain tickborne diseases need to be considered.
- Canine influenza currently seems like a non-entity in Ontario. We're still looking for it but haven't found it. It is present in some places in the US, and at times, is a big problem. Travel to a place experiencing a canine flu outbreak would be a good indication to consider canine flu vaccination.
What to do?
- If you travel with your pet, part of your pre-travel checklist should be an appointment with your veterinarian to go over anything that needs to be done, be it vaccination, deworming, flea control, heartworm preventive or anything else. (It's also a good time to make sure there's nothing else going on with your pet, because you don't want a pet health crisis en route.)
- If your pet gets sick and has traveled, make sure your veterinarian knows where you went and when.
- If you travel regularly, even if it's not long distances, it's good to discuss it with your veterinarian to see if anything is required for your pet. Even if you just go a couple of hours away to a cottage regularly during the summer, you may be exposing your pet to something different.
Lyme disease in Australia?
The world is certainly getting "smaller," especially in terms of infectious diseases. One example is the renewed controversy this month over the existence of Lyme disease in Australia. A Sydney man was recently diagnosed with the disease following his death, and now a doctor from Laurieton claims to have "absolute proof" of at least two other Australians with the infection.
Lyme disease is caused by infection with one of three species of Borrelia, previously all known as Borrelia burgdorferi. The disease is transmitted by a few specific species of ticks belonging to the genus Ixodes. It is relatively common in areas of North America (including some parts of Canada) and Europe where these tick species are also found. Ticks become infected by feeding on reservoir hosts, which are typically small mammals. Early signs and symptoms in people following a bite from an infected tick can include a rash, fever, headaches, tiredness and joint pain. The disease can be very difficult to diagnose because the initial signs are quite non-specific, particularly if the person does not report being bitten by a tick.
None of the tick species known to transmit Lyme disease are found in Australia, although there is one species of Ixodes tick there which some believe is a potential candidate for a vector. However, after testing some 12 000 of these ticks, evidence of Borrelia infection has still not been found. Also, none of the known reservoir hosts of Borrelia are said to live in Australia, and no other hosts have been identified.
The Laurieton physician, Dr, Mayne, claims he has "absolute proof" of Lyme disease in at least two of his patients, and says he has about 30 more patients with the disease as well. "Proof" is a very strong word. Not only is the disease hard to diagnose clinically, but there is also no perfect test that can detect infection for certain. Even the DNA test on which Dr. Mayne is hanging his hat can be prone to false-negative and false-positive results. The article also does not state whether or not the infected patients traveled outside the country and could have potentially picked up the disease in a Lyme-endemic area. Further investigation is needed before anyone can claim to have "proof."
So why am I writing about this situation on a zoonotic disease blog, when there hasn't even been any mention of pets, and the disease can't be directly transmitted between people and animals anyway? Because this is a perfect example of a situation in which physicians and veterinarians could potentially work together for the greater good, under the "one health, one medicine" banner. Dogs in particular can also be affected by Lyme disease. If the Australians really want to know if Lyme disease has made it to their shores - or perhaps some other tick-borne disease that mimics Lyme - then they shouldn't just be looking in people. By alerting veterinarians that Lyme disease or a similar condition is cropping up in humans, they can start to look for it in the animal population as well. If they're left unaware, Australian veterinarians may not consider Borrelia as a potential cause of illness in their patients and therefore not test for it. If pets also start testing positive for Lyme disease, then hopefully that would be communicated back to the human medical community to increase testing of suspect cases there as well. If more cases are identified, either human or animal, then further efforts could be taken to identify the tick source and reservoir hosts in Australia.
More information about Lyme disease and ticks in dogs is available in the Worms & Germs archives.
Image: The "classic" bulls-eye rash associated with a tick bite transmitting Lyme disease (source: CDC Public Health Image Library #9874)
Lyme disease from a dog: Don't believe everything you read
InsideToronto.com published an article entitled "Unsuspecting resident contracts Lyme disease from pet dog". However, Lyme disease cannot be transmitted from dogs to people.
Lyme disease is a tickborne disease caused by the bacterium Borrellia burgdorferi. This bacterium is transmitted from wildlife reservoirs to people and pets by ticks that have fed on an infected animal, and then latch onto a person or pet.
The article reports that a woman and her son from Scarborough, Ontario, were diagnosed with Lyme disease after there dog was infected. The owner frequently walked her dog in the Rouge Valley and Morningside Park. After finding a tick on her dog, the owner took the dog to the vet, and it was diagnosed with Lyme disease. She and her son were later diagnosed. The fact that people and pets in the same house got the same disease does not mean that the dog was the source of infection. Lyme disease cannot be transmitted directly between people and animals; ticks must be involved. Additionally, ticks must be attached for approximately 24 hours to efficiently transfer the bacterium. So, if multiple people and a pet in the house got Lyme disease, they were all bitten by ticks, presumably while walking in the woods. (It's also possible that a tick could have been brought into the house by a dog, and then it jumped onto a person). Identification of Lyme disease in a dog does not mean that there is a risk to people from the infected dog, but it does indicate that people may have been exposed in the same manner as the dog, and they should pay attention.
The dog owner in this case is upset that her veterinarian did not warn her about Lyme disease. She wants the College of Veterinarians of Ontario (the provincial licensing body for veterinarians) to "require members to tell people when they may have been exposed to Lyme disease through a pet, as well as what the symptoms are." That's reasonable, to a point. Veterinarians should engage their clients with discussions about zoonotic diseases. If they identify a pet with a zoonotic infection, they should talk about the implications. Similarly, if they identify a pet with an infection to which the owner may have also been exposed (such as Lyme disease), they should mention the risk. However, requiring vets to talk about signs of disease in people starts to cross the line between veterinary and human medicine. Vets should introduce the issue and let physicians take over from there.
People in the Toronto area should not panic based on this report. Lyme disease is quite rare in Ontario, especially in the Toronto area, as the ticks that transmit Lyme disease are not commonly found there.Toronto Public Health reports that an average of nine cases of Lyme disease a year have been diagnosed in people between 1998 and 2007, and that most of these cases were acquired in the US.
Removing ticks
Your veterinarian, physician and/or public health departments should be able to tell you what diseases are of concern in your area. Regardless of where you live, if you and your pet might encounter ticks, you should know how to remove them. Prompt removal is the key. Most tickborne diseases aren't transmitted immediately. Rather, the tick must be attached for a period of time for infection to occur. Therefore, close inspection of your pet (and yourself) for ticks after spending time outside, is important.
The following tick removal tips come from the Centers for Disease Control and Prevention (CDC).
- Remove a tick as soon as you see it.
- Use fine-tipped tweezers to firmly grasp the tick very close to your skin. With a steady motion, pull the tick away from your skin. Then clean your skin with soap and warm water.
- Avoid crushing the tick’s body.
- Do not be alarmed if the tick’s mouthparts remain in the skin. Once the mouthparts are removed from the rest of the tick, it can no longer transmit Lyme disease bacteria.
- If you accidentally crush the tick, clean your skin with soap and warm water or alcohol.
If a large number ticks are present on your pet, you should contact your veterinarian for assistance. Typically, a product that kills ticks will be applied to your pet.
There are various anecdotal recommendations that should be avoided, including burning the tick off with a match and covering the tick with substances such as gasoline, petroleum jelly or nail polish.
Camping with canines - tick tick not!
The warm weather is just about here, and that means the start of camping season. Lots of people love to spend time in the great outdoors during the summer, whether it’s at a summer cottage on the lake, trailer camping in a park with electricity and running water, or roughing it in a tent in the peace and solitude of a more remote wooded location. And many people bring along their faithful companions – their dogs – who enjoy the experience just as much, if not more, than we do.But there are also dangers lurking in the forests – microscopic dangers carried by tiny insects and other bugs. Ticks in particular are problematic. Certain ticks can carry a number of diseases that can make dogs sick, including Lyme disease (caused by Borrelia burgdorferi) and Rocky Mountain Spotted Fever (RMSF)(caused by Rickettsia rickettsii ). Both Lyme disease and RMSF are more common in certain areas where the tick species that carry them are present. You can NOT catch these diseases from your dog, but both you and your dog can be infected by the ticks that carry them. People can also be exposed to these pathogens by accidentally crushing an infected tick while trying to remove it from their dog. Ticks must be removed very carefully to ensure that the entire tick is removed, including the head and mouth pieces, without crushing it. If you're not sure how, contact your veterinarian. Also, the sooner the tick is removed, the less likely it is to transmit certain diseases, so be sure to check your dog thoroughly for ticks when you come back from a walk in the bush.
The Minnesota Department of Health recently reported that the number of cases of (human) Lyme disease increased in that state in 2007. This could be because of spreading tick populations, more people participating in activities in tick-inhabited areas, or increasing awareness and diagnosis of the disease by physicians.
If you and your canine companion will be spending time in some of the wilder and woodier parts of the great outdoors, talk to your veterinarian about what you can do to protect your dog. There are vaccines available for Lyme disease and the bacterial infection leptospirosis (which is also transmissible to people). Flea and tick preventatives are also very important, and many of today’s products are very effective. People should always wear insect repellent when camping or hiking in the woods. Visit the Health Canada website for safety tips on using personal insect repellents. All dogs should be vaccinated for rabies, whether they go camping in the backwoods or they’re house-bound city-slickers.
