If My Dog Has Been Treated For Lyme Disease, Why Does He Still Test Positive?

Clearing up common misconceptions about Lyme Disease.


Common Misconceptions About Lyme Disease:

1. If a dog tests positive for Lyme disease, it means she has it.

2. Dogs are much more likely than people to become ill from Lyme disease.

3. A dog infected with Lyme disease will tend to start showing symptoms within a month, and sometimes within the first 18 hours.

4. Dogs can transmit Lyme disease to people.

5. Lyme disease strikes only in warm weather.

A lot of times, a dog will test positive for Lyme disease, and her owners get nervous that their pet is sick and needs treatment, but the dog is fine. How can that be?

It’s because testing “positive” for Lyme simply means that antibodies to the illness have been detected in the dog’s blood. It does not mean that the animal’s body is actually being affected by the disease. How can that be?

Consider, first off, that very few dogs infected with Lyme disease ever exhibit clinical signs. Says Tufts veterinarian and infectious disease expert Michael Stone, DVM, “having antibodies against the Lyme organism yet no signs of illness approaches 90 percent in unprotected dogs in Lyme, Connecticut [where Lyme disease was first identified], yet less than 5 percent of owners report their dog to have any sign of illness.” It sort of comes down to how you define Lyme disease, he comments. “If you mean exposure and asymptomatic seroconversion [testing positive on a blood test but without becoming sick], infection is very common. In fact, Lyme disease is one of the most common tick-transmitted diseases in dogs. But if you mean actual signs of illness associated with Lyme disease, it’s much less common.” Indeed, people who test positive for Lyme disease are much more likely to actually fall ill from it than dogs who test positive. The bottom line: most dogs exposed to the offending bacteria never get sick.

There’s another way a dog can test positive for Lyme disease without being sick — if she has already been treated for the illness but still has antibodies to it in her blood. It happens all the time. Antibody levels can decline very slowly in some dogs. In fact, they are usually detectable in the blood for months to years even if treatment is completely successful and the dog no longer has any symptoms of the disease. People often think their dog must still be sick or has been re-infected by another tick and want another course of medical treatment when none is necessary. Of course, if antibody levels go down and then go up again after a dog has been treated, re-infection has occurred and the dog may need medical management once again. But there has to be that drop followed by a new rise. The presence of antibodies by themselves is not clinically significant.

The fact that a dog can test positive for Lyme disease yet not actually have it in a way that matters has led to a lot of nervous owners, and sometimes, even veterinarians who have over-treated. To clear up the misconceptions, here’s what you need to know.

What is Lyme Disease? How Do Dogs Catch It?

Lyme disease is transmitted via ticks, most commonly a type of tick known as the deer tick. But it is not the tick itself that is responsible. A tick will become infected with offending bacteria from the Borrelia bugdoferi group when it feeds on infected mice or other rodents and then transmits the bacteria to an animal such as a dog (or person) when it bites the “host” in order to take blood for nourishment. Out comes the blood from the dog and into the tick’s body (which makes it blow up from a tiny parasite to the size of a raisin) and, in the process, in go the bacteria from tick to your pet. (When the skin is penetrated, bacteria are passed, literally, into the new host from the tick.)

Infection takes place anywhere from 18 to 36 hours after the tick attaches itself to the dog. But that doesn’t mean the dog becomes ill right away. In fact, if your pet is going to show any signs of illness — and remember, most pets don’t — it’s going to take some two to five months.

How Will You Know if It’s Lyme?

Class signs of Lyme disease in a dog include loss of appetite, listlessness, and limping. Sometimes a dog will have what is commonly referred to as shifting leg lameness. A dog will become lame on one leg (it’s inflammation of joints caused by the offending bacteria that leads to the lameness), and the lameness will resolve on its own, only to show up days or even weeks later on either the same leg or a different limb. The dog may also exhibit a stiff gait or an arched back, and, usually, fever. In rare cases Lyme disease in dogs has been associated with kidney failure — a life-threatening complication.

Bear in mind that you might not be thinking about tick bites when signs start to appear. Your dog may have been bitten by a tick in the dead of summer yet not start to feel ill till after Christmas since there’s a long lag between infection and signs of sickness. Of course, she may also start to feel sick during the warm-weather months. Ticks can bite at any time of year. Yes, tick season extends from spring through early fall, but all you need is a day or two of weather approaching or exceeding the freezing mark during the dead of winter, and a tick will be able to do its thing. Thus, a tick can bite a dog during ski season in February, but the dog won’t start to have any signs until July 4th. Those signs are not from the tick that just bit her.

Bear in mind, too, that Lyme disease can happen to a dog anywhere in the country. “Absolutely” it is more prevalent in certain areas, says Dr. Stone. “It is most common in the Northeast, around Wisconsin, and in some areas of California,” he notes. But there is no locale that is absent of ticks that carry the bacteria. Canine Lyme disease has occurred across the U.S. (and in Europe, too).

Lyme Disease Diagnosis

Because testing positive for Lyme disease does not in itself mean a dog is sick, confirming a diagnosis is more tricky than just doing some blood work, particularly since some of the signs of the condition — loss of appetite, lethargy, even limping — are not terribly specific and can be signals of any number of conditions. The veterinarian has to consider the blood work in combination with the signs and observe a quick response to treatment. If you treat for Lyme disease and see no improvement in symptoms within a few days, then Lyme disease is not what’s making the dog sick.

Lyme Disease Treatment for Dogs

Fortunately, treatment of most cases of canine Lyme disease is very simple: a three- to four-week course of antibiotics. Even if the dog has had signs of disease for quite some time, she should respond very quickly, within the first 48 hours of starting antibiotic therapy, and recovery will in the great majority of cases be complete. There will generally be no residual effects of the illness. (Some people have said their dogs experience long-term joint pain, but the evidence is anecdotal only. We suspect such dogs had pre-existing arthritis and for that reason appeared to respond only partially to antibiotics. Lyme disease itself is treatable, quickly and completely.)

The one Lyme-related situation for which administration of antibiotics won’t work is if the infection is associated with kidney damage (sometimes referred to as “Lyme nephritis”). It isn’t certain that the bacteria responsible for Lyme disease are what cause the kidney syndrome. It may be a different pathogen transmitted with tick bites. But in very rare cases, a dog will die from kidney complications having to do with a tick borne infection. Dr. Stone at Tufts is one of the veterinarians conducting research into the problem and how to treat it successfully. Specifically, he is investigating a different bacteria associated with tick bites, Babesia microti, as the possible cause of “Lyme nephritis.” According to Dr. Stone, “the association of kidney failure and Lyme disease is incompletely understood, and much more work needs to be done.”

Lyme Prevention Beats Any Cure

“Tick prevention is my main recommendation,” Dr. Stone says, and “is recommended for every dog in an endemic area,” meaning the Northeast, the west coast, and the upper Midwest. It involves using an effective tick preventative (topical, oral, or collar) and also checking for ticks regularly, especially if you walk your dog in the woods or other area thick with underbrush or tall grass. Ticks will use the leaves of underbrush or unmowed grass as spots from which to fall onto a host animal as she walks by.

When you get home from such areas, inspect your dog visually — ticks are large enough to see crawling around with the naked eye (although very hard if your dog is black or has long hair). If you see one, crush it between two hard surfaces. That will protect you as well as your dog. Your pet can’t give you Lyme disease — it’s not a zoonotic ailment that passes from one species to another — but the same tick that can make your dog sick can give you the disease.


Also, every single day if you walk your dog in wooded areas, especially in warmer weather, feel all around your dog, including under the neck, for any new bumps. A tick that has bitten your dog and is still attached must be pulled out straight from your pet’s hair with tweezers. Pet stores even sell little plastic tweezer-like devices that do the lifting out very nicely. If you pull to the side or don’t use enough force, you could leave the tick head, which will cause a small area of swelling while the head is being dissolved by the dog’s immune system. Getting the attached tick out in fewer than 24 hours (fewer than 18 is better) means the bacteria that cause Lyme disease won’t have been transmitted.

Many veterinarians will also recommend the vaccine against Lyme disease (there is none for people) as yet another preventive measure. It must be given once a year and may provide extra protection for your pet. However, Dr. Stone stresses, “vaccination against Lyme disease in no way precludes the need for effective tick medicine. There are other diseases, some as or more serious than Lyme, that are passed by ticks.”

Note: Vaccination against Lyme disease will induce circulating antibodies and thereby cause some tests for the illness to read as falsely positive. But the results of a test called the C6 test are accurate even in the face of vaccination.

Okay, go back and re-answer the questions at the beginning of this article. If you answer “true” for all five, you’re a quick study.



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