Lyme Disease Should We Worry?

We recently received a call from an owner panicked about Lyme disease after reading a column in the Oregonian.  It’s unfortunate that the quoted sources don’t seem understand the disease ecology of Lyme very well and have alarmed our pet-owning public unnecessarily.   Those quoted in the article cite information from the Companion Animal Parasite Council, an organization which is sponsored by the very manufacturers of Lyme vaccine and tick preventatives, as well as by companies which sell diagnostic tests for Lyme*.  We are fortunate to have companies that provide important drugs and services to our patients but medicine is filled with conflicts of interest that can sometimes muddy the waters.

Lyme disease is caused by Borrelia burgdorferi.  It’s a disease with a fascinating ecology.  The bacteria can be carried by Ixodes ticks (pacificus in the West and scapularis in the East).  Ixodes pacificus is the “Western black legged tick”.  Tick nymphs aren’t born with Borrelia, they get it through feeding on the first host–which is a small rodent or lizard.

Here is where the ecology gets really interesting:  On the East Coast, the preferred and available first host for the tick is the White-footed deer mouse, a rodent who is loaded with Borrelia.  On the West Coast, we don’t have that mouse and the tick’s preferred host is the Western Fence Lizard, which not only doesn’t carry the Borrelia but actually has toxins in its bloodstream which are toxic to the bacteria.  On the East Coast, Lyme correlates with the amount of rainfall and subsequent acorn harvest since this determines the number of White Footed Deer mice, but here it does not.

On the West Coast, there are a few rodents who can carry Borrelia but they are very uncommon.  A fairly recent study looked at where Ixodes pacificus lives in Oregon and how many ticks are infected with Borrelia. Ixodes lives west of the Cascades and in the Columbia Gorge out to the Dalles.  We aren’t very “ticky” in Portland but the eastern Gorge can be a tick-fest.  Luckily, none of the ticks found in Portland carried Lyme and only 4% of the ticks in the Gorge were found to be positive.

Lyme disease in humans is reportable and the CDC lists the statistics.  It is a very uncommon disease in Oregon (and the entire Pacific West) due to the ecology. The incidence of Lyme per 100,000 people in Connecticut was 58.7. In Oregon the incidence was 0.3 (many of whom contracted the disease outside the state).

We aren’t suggesting that Lyme disease doesn’t happen but it is a very rare disease in Oregon.  When researchers have tried to infect dogs with Lyme, 95% were resistant to clinical infection.  The test for Lyme is not terrific since it only measures whether a dog has been exposed to the organism, not whether they are actively infected.  In parts of the country with a lot of Lyme, 70-90% of normal dogs test positive and the American College of Veterinary Internal Medicine consensus guidelines suggest that testing asymptomatic dogs may lead to over-diagnosis.

The vaccine is a bit of a quandary since it seems to work in experimental infection but has no field trials—it has never been demonstrated to reduce the number of Lyme infections in owned dogs.  Since only 5/100 dogs experimentally exposed to Lyme become ill and of those, only one to two develop serious disease, you would have to vaccinate at least 100 dogs to protect 5 and that’s assuming 100% effectiveness of the vaccine.  The majority of internists who developed the consensus guidelines don’t recommend the vaccine.

So what should you do about Lyme?

  • Don’t panic about a rare disease.
  • Use good tick control when visiting a “ticky” place.  There are a number of safe products which reduce the number of living ticks able to feed on your dog.  Products containing amitraz, fipronil or pyrethroids can help but none are 100%.
  • After your outdoor adventures, brush your dog out and remove any ticks before they latched on.  Lyme transmission requires the tick to stay latched on for 36-48 hours
  • If you do find a tick, pull it out with tweezers or one of a number of tick “spoons” which can be purchased.  If the tick breaks and the head is left in the skin, it’s not     normally a huge problem but a small secondary skin infection can develop.  These are usually very easily treated.We’re fortunate to be fairly “bug free” in Oregon though that may change with warming temperatures.  The exception to that rule is the flea festival which Portland dogs and cats can suffer from, even in the winter months.  Most topical flea products are also tick preventatives so if you need help reading the product labels and picking a safe and effective product, let us know.

    If you like medical ecology as much as we do, you may find this website fascinating.


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