The short answer
Lyme nephritis is an immune-mediated kidney disease that develops in a small subset of Lyme-exposed dogs. It’s frequently fatal once symptomatic, and it begins damaging kidney function long before any outward signs appear. That’s the main reason we recommend a urinalysis (looking for proteinuria) on dogs that test positive on the 4DX.
What’s happening in the kidney
In a typical Lyme exposure, Borrelia burgdorferi spirochetes circulate at low levels and the dog’s immune system responds with antibodies. In most exposed dogs (~95%), nothing further happens — they’re seropositive but asymptomatic for life. About 5–10% develop clinical Lyme: lameness, fever, lethargy, swollen joints. That responds well to doxycycline.
In a smaller subset, the immune response itself becomes the problem. Antibody-antigen complexes deposit in the glomeruli of the kidney — the tiny filtering structures responsible for keeping protein out of urine. The deposits trigger inflammation, the filtration barrier breaks down, and protein starts leaking into the urine (proteinuria). Over time, the glomeruli are destroyed, and the kidneys fail.
This is immune-mediated, not infectious — antibiotics that kill the spirochete don’t reverse the kidney damage that the immune complexes have already caused. By the time outward signs appear (vomiting, weight loss, increased thirst and urination, severe lethargy), substantial kidney function has often already been lost.
How common is it?
Honest answer: rare in the absolute, but devastating when it happens.
- Lyme nephritis affects a small percentage of Lyme-exposed dogs — well under 5%.
- The mortality rate among dogs with established Lyme nephritis is high — many do not survive despite aggressive treatment.
So the math: the chance any given exposed dog develops Lyme nephritis is small. But the chance is non-zero, the outcome is often fatal, and the only practical way to catch it early is to screen for proteinuria on positive 4DX dogs.
Why screening matters
This is the part that connects the 4DX to the rest of the picture. The 4DX SNAP is a simple in-clinic test that detects Lyme antibodies (among other things). A positive Lyme on a healthy dog is not a treatment trigger by itself — most exposed dogs never get sick.
But it is a screening trigger. The next step on a positive 4DX is a urinalysis to look for proteinuria. The urine protein-to-creatinine ratio (UPC) tells us whether the kidneys are leaking protein. If the UPC is normal, we monitor and re-check. If the UPC is elevated, we have early Lyme nephritis on our hands — and now we have something to do about it before the kidneys are destroyed.
For more on the 4DX itself, see our 4DX test explainer.
Treatment approach
If we catch proteinuria early, treatment combines doxycycline (to clear any residual Borrelia), supportive medications to reduce glomerular pressure and protein loss, a kidney-friendly diet, and serial monitoring of kidney values, urine protein-to-creatinine ratio, and blood pressure. The exact medication mix is tailored case-by-case in line with current internal-medicine guidance.
Outcomes depend heavily on how early we catch it. Dogs caught at proteinuria-only stage with normal kidney values often do reasonably well long-term. Dogs caught at the symptomatic stage (vomiting, weight loss) have worse outcomes.
What this means for prevention
The takeaway from Lyme nephritis isn’t to panic about Lyme exposure. It’s to not be casual about it either. Specifically:
- Year-round tick preventive on dogs — particularly in Halton, where ~38% of blacklegged ticks are now Borrelia-positive.
- Annual 4DX SNAP test — even if your dog is on preventive. The test catches breakthrough exposure.
- Urinalysis on every Lyme-positive dog — even if asymptomatic. This is the only practical early-warning system for nephritis.
- Consider the Lyme vaccine for dogs with frequent off-leash forest exposure or prior tick history — particularly if proteinuria is already a finding.
- Aggressive bite response on known exposures — daily checks, fast removal, save the tick for eTick.ca, schedule a 4DX 6–8 weeks later.
What about cats?
Cats don’t develop clinical Lyme disease and don’t develop Lyme nephritis. The kidney problems we worry about in cats are different (chronic kidney disease related to age, diet, dental health, infection, or hyperthyroidism). For more on cats and Lyme, see our post on feline Lyme exposure.
For the bigger picture, see our 2026 field guide on ticks in Halton.
Key takeaways
- Lyme nephritis is an immune-mediated kidney disease in a small subset of Lyme-exposed dogs.
- It begins damaging kidneys before any outward signs appear.
- The only practical early-warning system is a urinalysis (UPC ratio) on every Lyme-positive 4DX dog.
- Caught early, prognosis is meaningfully better than caught late.
- Year-round preventive, annual 4DX, and prompt urinalysis on positives are the best protection.
- Cats do not get Lyme nephritis — Lyme isn’t a clinical disease in cats.
References
- ACVIM Consensus Statement on Lyme Disease in Dogs (2018).
- Companion Animal Parasite Council — Lyme disease guidelines.