Texas Leads Nation in Canine Heartworm Cases as Parasite Spreads to New Regions
April 27, 2026 – Texas has emerged as the nation’s leader in canine heartworm cases for the first time, according to new data released by the American Heartworm Society. The group’s 2025 Heartworm Incidence Map, published earlier this month, shows the highest concentration of infections in eastern Texas, highlighting a growing concern for pet owners and veterinarians.
The survey, conducted in early 2026 using data from more than 1 million heartworm tests performed in 2025, found that veterinary clinics in Texas reported an average of nearly 50 heartworm-positive dogs each. That represents a 3.78% infection rate, up from 2.97% in 2022.
Heartworm, a potentially fatal disease spread by mosquitoes, has traditionally been most prevalent in warm, humid regions such as the Gulf Coast. However, the latest findings show the parasite expanding into cooler and drier areas previously considered low risk. Newly affected regions include parts of southern California, southwest Colorado, and even southern Maine.
“This survey underscores how resilient and adaptive heartworm transmission can be,” said Dr. Marisa Ames. “We’re now seeing cases in places where they were once rare or nonexistent.”
Beyond Texas, other states with high case counts include Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, and Tennessee. Notably, the survey found no state in the U.S. to be completely heartworm-free.
Experts point to several factors behind the spread, including increased movement of infected dogs through adoption programs, rising populations of wildlife carriers like coyotes, and extreme weather events that boost mosquito populations. Additionally, lapses in preventive care and pet relocation to higher-risk areas have contributed to the trend.
Despite the concerning rise, veterinarians emphasize that heartworm disease remains largely preventable with routine medication. The AHS continues to urge pet owners nationwide to maintain consistent prevention, as the parasite’s expanding footprint makes vigilance more important than ever.
