Lyme disease in dogs causes joint inflammation, fever, and lethargy, but most dogs that test positive never actually get sick. The bacteria (spread by deer ticks) can live in a dog’s body for months or even years without causing problems. When symptoms do appear, they typically show up two to five months after infection, though they can emerge much later. The good news is that most cases respond well to antibiotics. The bad news is that a small percentage of dogs develop a serious kidney complication that can be life-threatening.
Most Infected Dogs Show No Symptoms
This surprises a lot of dog owners. The majority of dogs that test positive for Lyme disease on a routine blood test will never become visibly ill. Their immune systems keep the bacteria in check, maintaining stable antibody levels without the infection progressing to clinical disease. Your vet might flag a positive test result during an annual checkup, but that positive result alone doesn’t necessarily mean your dog is in trouble.
That said, “positive but healthy” dogs do carry a persistent infection. The bacteria aren’t fully eliminated by the immune system. They’re just being managed. If a dog’s immune function dips or the infection reactivates, symptoms can appear weeks, months, or even longer after the initial tick bite.
Shifting Lameness Is the Hallmark Sign
When Lyme disease does cause symptoms, the most recognizable one is a lameness that moves from leg to leg. Your dog might limp on a front leg one day, seem better, then start favoring a back leg a few days later. This happens because the bacteria trigger inflammation in multiple joints, and which joint is most inflamed can change over time. You’ll often notice swelling near the affected joints, and your dog may flinch or pull away when you touch their legs.
Along with the shifting lameness, dogs typically develop a fever, become lethargic, and lose interest in food. These signs tend to appear in the chronic stage of infection, generally two to five months after the tick bite, though the timeline varies. Because the symptoms can be vague (a tired dog that’s not eating much) and the lameness comes and goes, Lyme disease sometimes gets mistaken for a minor injury or general aging.
How Ticks Transmit the Infection
Lyme disease is caused by a spiral-shaped bacterium called Borrelia burgdorferi, carried by black-legged ticks (also known as deer ticks). A tick picks up the bacteria by feeding on an infected mouse or other small animal, then passes it along during its next blood meal. The tick generally needs to be attached and feeding for at least 24 to 48 hours before transmission occurs, which is why daily tick checks after outdoor activity are one of the most effective prevention strategies. If you find and remove a tick within that first day, the risk drops significantly.
Lyme Nephritis: The Dangerous Complication
The most serious thing Lyme disease can do to a dog is damage the kidneys. Lyme nephritis is a form of kidney disease where the immune response to the bacteria causes inflammation that destroys kidney tissue. It’s not common, but when it happens, it progresses fast and the prognosis is poor. Many dogs with Lyme nephritis decline within days to weeks of diagnosis.
Dogs with this complication lose large amounts of protein through their urine. In Lyme nephritis cases, the ratio of protein to creatinine in the urine is typically above 5 and can climb as high as 15 (normal is well below 1). Signs include vomiting, weight loss, increased thirst and urination, and swelling in the legs or abdomen from fluid retention. Some dogs with Lyme nephritis have survived months to over a year with aggressive treatment, including immune-suppressing medications, but these are the exceptions. Certain breeds, particularly Labrador Retrievers and Golden Retrievers, appear more susceptible.
How Vets Diagnose Lyme Disease
Most veterinary clinics use a rapid blood test that detects antibodies your dog’s body produces in response to the Lyme bacteria. This test can be run during a routine office visit and gives results in minutes. Antibodies become detectable as early as two to three weeks after infection with newer testing methods, while older tests picked them up at four to six weeks.
A positive screening test tells your vet the dog has been exposed, but it doesn’t distinguish between a dog that’s fighting off the infection on its own and one that needs treatment. More detailed testing can measure antibody levels and help determine whether the infection is recent or chronic. These quantified antibody levels also become important for tracking whether treatment is working. After successful treatment, antibody levels gradually decrease over several months.
What Treatment Looks Like
Dogs with active Lyme symptoms are treated with a four-week course of antibiotics. Most dogs start improving within a few days of starting medication, with lameness and fever resolving relatively quickly. The full month of treatment is important even after symptoms disappear, because the bacteria can persist in tissues and a shorter course risks relapse.
After treatment, your vet will likely recommend follow-up blood work to confirm that antibody levels are trending downward, which signals the infection is being cleared. Antibodies don’t disappear overnight. They remain detectable for several months after successful treatment, so a continued positive test result right after finishing antibiotics doesn’t mean it didn’t work.
For dogs that test positive but have no symptoms, treatment decisions are less straightforward. Some vets recommend a course of antibiotics as a precaution, while others prefer monitoring with periodic blood and urine tests to watch for early signs of kidney involvement or rising antibody levels that suggest the infection is becoming more active.
Preventing Lyme Disease
Tick prevention is the most reliable approach. Year-round tick preventatives, whether oral chewables or topical treatments, kill ticks before they’ve been attached long enough to transmit the bacteria. There is also a Lyme vaccine available for dogs, which your vet may recommend if you live in or travel to areas where Lyme-carrying ticks are common, particularly the Northeast, Upper Midwest, and Pacific Coast of the United States.
Beyond products and vaccines, simple habits help. Check your dog for ticks after walks in wooded or grassy areas, paying close attention to ears, armpits, groin, and between toes. Removing a tick promptly, within that first 24-hour window, dramatically reduces the chance of transmission. If you’re pulling ticks off your dog regularly, that’s a sign your current prevention strategy may need an upgrade.