Positive Lyme test in dogs doesn't always mean Lyme disease

A new Worms & Germs Blog post from Scott Weese revisits one of small-animal practice’s most persistent tick-borne disease questions: what a routine positive Borrelia burgdorferi antibody test actually means in a healthy dog. His central point is straightforward but important: in many cases, the result reflects exposure to infected ticks, not “Lyme disease” in the clinical sense, and treating every positive screen as active disease can push practices toward unnecessary antibiotics and muddy conversations with pet parents. (wormsandgermsblog.com)

That debate isn’t new, but it remains highly relevant as canine seroprevalence rises or expands in some regions. CAPC notes that asymptomatic dogs in endemic areas are often, or even usually, seropositive, which complicates interpretation of routine screening. The ACVIM consensus update similarly described screening healthy dogs as controversial, even while recommending screening in endemic or travel-exposed populations and proteinuria assessment in dogs that test positive. In other words, the profession has largely moved toward using serology as a risk and monitoring signal, not a stand-alone diagnosis. (capcvet.org)

The diagnostic nuance matters because serology measures antibodies, not necessarily current illness. CAPC says serology is the mainstay for confirming a clinical impression of Lyme disease, but emphasizes that diagnosis usually depends on compatible signs, such as arthritis or renal disease, together with positive serology. C6-based assays are often preferred because they identify antibodies from natural infection rather than prior vaccination, whereas whole-cell tests can be confounded by vaccine-induced antibodies and may require confirmatory testing. Human CDC guidance echoes the same general principle of antibody persistence: once mounted, antibodies can remain detectable for months or years after infection has resolved. (capcvet.org)

What should clinicians do with the healthy positive dog? CAPC recommends looking for coinfecting tick-borne pathogens, checking for abnormalities on hematology or urinalysis, and instituting a tick-control program. It specifically says antibiotic treatment is not recommended for seropositive dogs in the absence of clinical or pathologic abnormalities. ACVIM goes further in explaining why: most seropositive dogs will never develop Lyme borreliosis, the magnitude of a Quant C6 titer does not predict illness, and there’s no evidence-based support for using that value alone to decide on treatment in a nonclinical, nonproteinuric dog. (capcvet.org)

Industry and clinical commentary around this issue appears to be converging on the same message. A recent Today’s Veterinary Practice article on seropositive screening results in healthy dogs emphasized structured follow-up and nuanced client communication rather than reflex treatment. In a separate clinical discussion, Weese has also stressed that B. burgdorferi seropositivity in a clinically normal dog warrants consideration, but that C6 levels are not predictive of disease. Those reactions reflect a broader shift toward antimicrobial stewardship and away from the older habit of treating the test result instead of the patient. (todaysveterinarypractice.com)

Why it matters: For veterinary professionals, the practical challenge is balancing vigilance with restraint. Most infected dogs remain healthy; AAHA says no more than 10% develop polyarthritis, while an estimated 1% to 5% may develop Lyme nephritis, a severe complication with often poor outcomes despite therapy. That means the positive screening test is not trivial, but its value is less about triggering doxycycline and more about prompting a thoughtful next step: assess for proteinuria, review travel and exposure history, consider coinfections, tighten tick prevention, and document a monitoring plan the pet parent can understand. (aaha.org)

There’s also a public health dimension. CAPC and AAHA both note that canine seroprevalence tracks with human Lyme risk, making dogs useful sentinels for emerging exposure zones. Recent CDC-published work from northwestern North Carolina, for example, found substantial increases in canine B. burgdorferi seroprevalence and incident seropositivity from 2017 through 2021, underscoring how quickly local risk patterns can shift. For practices in emerging regions, that strengthens the case for routine screening and prevention discussions, even if it doesn’t change the basic rule that exposure alone is not the same as disease. (capcvet.org)

What to watch: The next phase of this conversation will likely center on more standardized care pathways for asymptomatic seropositive dogs, especially as Lyme risk expands geographically and practices look for clearer protocols on urinalysis, UPC testing, coinfection workups, vaccination decisions, and follow-up intervals. (capcvet.org)