A Positive Heartworm Test Is Just the Beginning
If your dog tests positive for heartworms, the next question matters just as much as the diagnosis itself: how serious is it? A new study found that dog heartworm bloodwork — the kind of routine blood test your vet may already run — can reflect how far the disease has progressed. In a review of 35 heartworm-positive dogs, researchers found that the worse the infection, the more clearly it showed up in standard blood chemistry results. Liver and kidney markers were especially good indicators.
This means your vet doesn’t have to guess how sick your dog is. A simple blood draw can give them real, measurable clues — and help them decide the safest path forward for treatment.
Why Just Testing for Heartworms Isn’t Enough
Most dog owners know the basics: heartworm disease is caused by parasitic worms that live in the heart and lungs. Mosquitoes spread it, and it can be serious — even life-threatening if left untreated. Standard heartworm tests (called antigen tests) confirm whether the parasite is present. But they don’t tell you how much damage has already been done.
That gap matters. A dog with a mild early-stage infection needs a different treatment approach than a dog whose heart and lungs are already under heavy strain. Before this kind of research, vets often had to rely mostly on physical exams and imaging to estimate disease severity. This study looked at whether something simpler and more routine — a blood chemistry panel — could offer the same kind of insight.
How the Study Was Done
This was a retrospective cohort study — meaning researchers looked back through existing medical records rather than running a new experiment. They reviewed the files of 35 dogs that had already been diagnosed with heartworm disease and examined the bloodwork results on file.
Here’s what they looked at:
- Hematology: A complete blood count, which measures red blood cells, white blood cells, and platelets — the tiny cell fragments that help blood clot
- Biochemistry panel: A blood chemistry test that checks how well the organs are working, including the liver and kidneys
- Disease severity: Each dog’s heartworm infection was categorized from mild to more advanced based on clinical signs and testing
The team then compared the blood results against the severity ratings to see if any patterns emerged.
What the Researchers Discovered
Bloodwork Mirrors How Sick the Dog Is
The standout finding: as heartworm disease got worse, the abnormalities in routine bloodwork followed right along. Dogs with more severe infections showed more significant changes in their lab values. This wasn’t random noise — it was a consistent pattern across the group.
Think of it like a car’s dashboard warning lights. Standard bloodwork is already measuring dozens of things every time it’s run. This study found that in heartworm-positive dogs, those measurements shift in predictable ways as the disease progresses — like warning lights that get brighter the more serious the problem becomes.
Liver and Kidney Markers Were the Clearest Signals
The markers that tracked disease severity most closely were liver and kidney values. The liver processes toxins and supports the immune system. The kidneys filter waste from the blood. When heartworms damage the heart and lungs, it creates a chain reaction — poor circulation puts extra stress on both organs, and that stress shows up in lab results.
Elevated liver enzymes (such as ALT and ALP) or kidney markers (such as BUN and creatinine) in a heartworm-positive dog can be a sign that the infection has already moved beyond the early stage. A dog that looks relatively fine on the outside may have measurable organ strain showing up in the blood.
What This Means for Dog Owners
Ask for a Full Blood Panel — Not Just a Heartworm Test
If your dog is diagnosed with heartworms, ask your vet whether a complete blood chemistry panel has been run. The antigen test only confirms the parasite is there. A blood chemistry panel tells you — and your vet — what the infection is actually doing to your dog’s body.
This matters because treatment for heartworm disease isn’t the same for every dog. The standard treatment involves a drug called melarsomine, which kills the worms. But the process is tough on the body. Dogs with significant organ strain may need to be stabilized first, or monitored more closely during treatment. Knowing the starting point helps your vet build a safer plan.
Practical Steps After a Positive Diagnosis
- Request full bloodwork, including a complete blood count and chemistry panel, as part of the initial workup
- Ask your vet which specific values they’re watching — especially kidney and liver markers
- Keep a copy of the baseline results so changes can be tracked as treatment progresses
- Follow your vet’s exercise restrictions — physical activity can worsen complications in heartworm-positive dogs
- Schedule follow-up blood tests as recommended, so your vet can see whether organ function is improving after treatment
When to Call Your Veterinarian
Contact your vet promptly if your heartworm-positive dog shows any of these signs:
- Unusual tiredness or weakness
- Coughing, especially after light activity
- Reduced appetite or weight loss
- Faster or labored breathing at rest
- Pale gums or fainting
These could indicate that the infection is progressing or that the body is under significant stress — both situations where bloodwork changes would be most important to catch.
Limitations to Keep in Mind
This study reviewed records from 35 dogs, which is a small group. The results are a meaningful starting point, but larger studies with more dogs — across different breeds, ages, and geographic areas — would give us stronger evidence. Because it was retrospective (reviewing past records rather than following dogs prospectively), the researchers couldn’t fully control for other health conditions that might also affect blood values. And the study didn’t track how bloodwork changed in individual dogs over the course of treatment, which would be a valuable next step.
The Bottom Line
Routine heartworm bloodwork can do more than just check a box. In a study of 35 heartworm-positive dogs, standard blood chemistry results reflected how serious the disease was — with liver and kidney markers proving especially useful. For dog owners, the takeaway is straightforward: if your dog tests positive for heartworms, make sure your vet runs a full blood panel as part of the workup. That extra information could directly shape how — and how safely — your dog is treated.
Heartworm disease is serious, but it’s also manageable when caught and staged properly. The more your vet knows about what’s happening inside, the better equipped they are to help your dog through it.
This article summarizes peer-reviewed research for educational purposes. Always consult with your veterinarian for personalized advice about your pet’s health and behavior.