Merlin was bright, alert, and clinically normal. He was afebrile, and physical examination was unremarkable. His gait was normal, and no joint pain or effusion was noted on palpation.

B burgdorferi seropositivity was identified as part of multiplex testing for heartworm disease. Although B burgdorferi seropositivity was an incidental finding in this patient, the positive result required consideration because of the potential for clinically inapparent disease. Merlin had been evaluated yearly for B burgdorferi antibodies, so the result was consistent with exposure during the past year. Because the owners did not report any signs consistent with Lyme disease (eg, shifting leg lameness) and physical examination was unremarkable, the main clinical consideration was Lyme nephritis, an uncommon but serious condition. A free-catch urine sample was collected to assess for potential Lyme nephritis. Routine urinalysis was unremarkable (Table 1), and urine protein:creatinine ratio was 0.1 (reference range, <0.5).

No further testing directed at Lyme disease was performed. Although a commercial quantitative assay for C6 antibody levels is available, C6 levels are not predictive of disease.¹

• If proteinuria is identified, treatment with doxycycline is warranted; however, other causes of proteinuria should be considered.
• Antimicrobial treatment of clinically normal, seropositive dogs without proteinuria is likely not warranted.

Identification of seropositive but clinically normal dogs is common in areas where B burgdorferi is endemic. Although doxycycline is clearly indicated in animals with clinical evidence of Lyme disease, there is controversy about the need for treatment of clinically normal dogs (Table 2). The recent ACVIM consensus statement on Lyme disease did not have a consensus approach to management of seropositive, clinically normal, nonproteinuric dogs.¹ Most panelists did not recommend treatment because of concerns about overuse of antibiotics, lack of evidence that treatment reduces illness, lack of evidence that treatment can clear the bacterium from all tissues, and the fact that reinfection is common in endemic areas. There was consensus that subclinical Lyme-seropositive dogs should be regularly (eg, 2-3 times per year) re-evaluated for proteinuria, regardless of whether antibiotics are used.

Merlin’s prognosis was excellent. His seropositive result indicated exposure to a tick weeks or months before the wellness visit. Because signs of disease were not present, it was unlikely that disease related to that exposure would develop. Seropositivity is common in endemic areas, yet disease is anecdotally fairly rare, meaning most exposed dogs mount an immune response and therefore become seropositive but do not develop disease.

Lyme nephritis is of concern because of the severity of disease; however, it is rarely reported, and the pathogenesis of disease and factors that predispose to disease are unknown.² B burgdorferi is rarely found in the kidneys of dogs with Lyme nephritis, and disease manifests as an immune complex glomerulonephritis.^3,4 As such, immune-mediated disease—rather than active infection—is the apparent etiology, raising questions about whether an infection to treat is still present by the time Lyme nephritis is identified or whether disease occurs during a postinfectious state. Although there is no universal approach to seropositive but clinically normal animals, treatment of seropositivity in the absence of proteinuria is likely not warranted. The owners submitted urine samples from Merlin twice over the next year, and proteinuria was not identified. Merlin was clinically normal at his next annual wellness visit. His owners had been diligent with tick prophylaxis and had not observed any ticks on him since the previous visit. At his latest wellness examination, antibodies against B burgdorferi were identified but were likely residual levels from his prior exposure. Twice-yearly rechecks were recommended to assess for proteinuria.