Heartworm Statistics by State

Heartworm disease is transmitted by mosquito bite, is preventable with regular medication, and can be fatal if untreated — which is why the Companion Animal Parasite Council (CAPC) tracks real test results from veterinary clinics nationwide and publishes them as a continuously updated map, broken down by state.

The figures below reflect CAPC's live data as of August 2026: more than 12.5 million canine heartworm antigen tests run so far this year, with results aggregated state by state. Because 2026 isn't over yet, these are year-to-date totals, not final annual figures.

Canine Heartworm Positivity Rate by State

The Companion Animal Parasite Council (CAPC) aggregates real diagnostic test results from veterinary clinics and labs across the US — over 12.5 million canine heartworm antigen tests so far in 2026 alone — and publishes the results as a live, continuously updated map. Unlike survey-based estimates, this is actual positive/negative test data, not modeled projections.

The Heartworm Belt

Nine of the ten highest-rate states — Mississippi (6.10%), Louisiana (3.84%), Arkansas (3.41%), Alabama (3.19%), Texas (2.86%), Oklahoma (2.29%), Georgia (1.64%), South Carolina (1.56%), Tennessee (1.46%), and North Carolina (1.22%) — form a contiguous block across the Deep South and lower Mississippi River Valley. Mississippi's rate of 6.10% is more than 33 times North Dakota's rate of 0.18%, the lowest in the country. This matches CAPC's own long-documented pattern: warm, humid climates support mosquito populations (heartworm's transmission vector) year-round, while colder states see transmission limited to a short seasonal window.

What This Data Does and Doesn't Show

These are positivity rates among dogs that were tested, not the infection rate across the entire dog population in each state. Dogs get tested for heartworm for many reasons — annual wellness screening, before starting/renewing preventive medication, or because a vet suspects infection — and testing rates themselves vary by state and by veterinary practice patterns. A state's true population-wide infection rate could run higher or lower than its tested-positivity rate. CAPC's data is still the most current, largest-scale, real-world signal available, updated continuously as new lab results come in throughout the year.

For general national pet health statistics, see our pet health statistics page.

Lyme disease follows a nearly inverse geographic pattern — see our Lyme disease statistics by state page for the comparison.

Giardia breaks from both patterns entirely — see our giardia statistics by state page.

Cats can also test positive for heartworm, though at a much lower rate — see our feline heartworm statistics by state page.

The Big Picture

Heartworm risk in the US is heavily regional, not evenly distributed — a dog in Mississippi tests positive at more than 30 times the rate of a dog in North Dakota. That gap tracks climate and mosquito activity closely, which is why national veterinary guidance generally recommends year-round heartworm prevention regardless of location: even low-rate states still have non-zero positive cases, and CAPC's own multi-year maps show the highest-risk zone gradually expanding beyond its traditional Southeastern core.