HATTIESBURG, Miss.—When Gleppy, a 2-year-old chihuahua mix, was found as a stray in Forrest County, she was full of energy and eager to greet anyone nearby.
But at Southern Pines Animal Shelter in Hattiesburg, staff discovered her biggest challenge wouldn’t be finding a home—it would be surviving a rising heartworm crisis.
“We diagnose about 1 in every 8 dogs with heartworm disease,” said Tamara Huff, shelter director of Hub City Humane Society. “Heartworms are transmitted by mosquitoes … it just takes one bite from one mosquito for your pet’s life to change.”
According to 2024 data compiled by the Companion Animal Parasite Council, 6% of dogs tested in Mississippi were positive for heartworms, a potentially fatal condition that can damage the heart, lungs and blood vessels if left untreated. The national average is 1%.
Of the five states with the highest heartworm infection rates, the American Heartworm Society said the Magnolia State ranks No.1—followed by Louisiana, Texas, Alabama and Arkansas.
In George County alone, 13% of dogs tested in 2024 had heartworms.
Twenty-nine percent of the nation’s veterinarians reported rising rates in their practices, according to a 2024 survey.
Heartworms Plague Shelters Across the South
Chris Duke, a board member for the National Heartworm Society, said the 1-in-8 figure is not unique to Hub City Humane Society. The veterinarian added that heartworms impact a majority of shelters in the South, where the environment gives the parasite an advantage.
“Heat, humidity, standing water like our bayous and bogs … encourage a lot of mosquito breeding,” said Duke.
Heartworms are spread when a mosquito bites an infected animal and then passes larvae to another. Over six months, the worms—which can grow up to a foot long—travel through the host’s bloodstream, eventually lodging in vital organs.
With warm temperatures and high mosquito activity stretching through most of the year, even a short lapse in prevention can be dangerous.
“It doesn’t take but a month or two of missed doses in south Mississippi to have a dog convert to heartworm-positive,” Duke said.
The biggest problem, he said, is inconsistent prevention—sometimes because of cost and sometimes because owners think someone else gave the dose.
“This happens more often than not. When there’s a couple, and they say you’re responsible, but you’re responsible, and then it doesn’t get done because somebody else thinks somebody else did it,” Duke said. “Occasionally you’ll have a dog that looks like it took the pill but spit it out behind the sofa … a partial dose can let you down, too.”
Some owners also believe indoor pets are safe.
“In Mississippi, we get mosquitoes inside; it’s a fact of our lives,” Huff said.
Without prevention, infection is inevitable, said Blake Crawford, a veterinarian at Animal Medical Center in Hattiesburg.
“Uncovered dogs, especially in the South, will get bit by a mosquito, and then it will inject (larvae) in them,” Crawford said.
Symptoms—which include coughing, low energy and exercise intolerance—can be easy to miss—and, when left untreated, the infection can be fatal.
“That’s why whenever (patients) come in for our annual visits, we recommend, ‘Hey, let’s do some blood work and let’s do a heartworm test,’” Crawford said.
Treatment Costs Strain Shelters, Pet Owners
Treatment is available for pets affected by heartworm disease.
Gleppy, who was adopted by Roy Howard Community Journalism Center reporter Justin Glowacki, is currently working through that process. For the next month, she’ll take two doses of an antibiotic every day to weaken the parasites. After that, she’ll start a series of injections to kill the worms.
The process requires restricted activity. A spike in heart rate can cause a dying worm to break loose, leading to a potentially deadly blockage in the lungs.
“Let’s say ‘Fluffy’ goes outside, if he sees a bird or a squirrel and decides to just run after it and get super excited—if the heart starts pumping really fast—it can break off a piece of that heartworm and cause a pulmonary embolism,” Huff said.
At the shelter, dogs in treatment are walked on a leash to help limit their activity.
“It’s a huge undertaking for our care techs,” Huff said.
Prevention is routine at Hub City Humane Society—but treatment strains the budget. Huff said care for a 50-pound dog can cost up to $4,000 when factoring in medication, veterinary services and staffing. In private clinics, Duke said treatment for a large dog can exceed $1,000.
The American Heartworm Society’s “fast-kill” method involves three injections over three months—along with antibiotics and anti-inflammatory medication. A “slow-kill” approach uses prevention and antibiotics over years, but Duke said it risks ongoing damage to the heart and lungs.
Prevention Is Easy—And Far Less Expensive
Veterinarians recommend regular use of government-approved preventives, such as monthly chewable tablets and topical treatments or yearly or twice-yearly injectables.
“It’s extremely easy to prevent—and a lot cheaper than the headache of treatment,” Crawford said.
Huff said Mississippi’s relentless mosquito population means prevention isn’t optional.
“There are some things in life you can gamble with,” she said. “Heartworms are not one of them. It just takes one bite from one mosquito for your pet’s life to change dramatically, and the monthly prevention is a lot more cost-effective than the treatment.”
