She received annual checkups at her veterinarian's office and took a chewable heartworm tablet once a month. Still, last spring, she tested positive for the deadly parasite.
"I was just devastated," said Delilah's owner, Emily Pieralisi, of Leland. "This dog went with me everywhere. I kept thinking, 'Oh, my gosh. Am I going to wake up and she's going to be dying?' "
Delilah's story is one of a growing number coming from Mississippi Delta-area pet owners who say their dogs contracted heartworms despite taking regular, preventative medication.
The increasing reports of heartworm-positive dogs in communities along the Mississippi River have spurred research into the preventative drugs' failures and whether there could be heartworms that have become genetically resistant to the medications.
"We always thought it was a compliance issue - that not every medicine works in every dog or that an owner thought they gave their dog the pill but actually forgot or the dog threw up the medicine," said Mark Russak, director of Student Affairs at Mississippi State University's College of Veterinary Medicine.
"But we're seeing more cases than ever before," he said. "It's my gut feeling that it's resistance."
Scientists had believed that because of the short life cycle of a heartworm, it's not genetically possible for them to become resistant to the poison used to kill them, American Heartworm Society President Sheldon Rubin said.
But newer research, led by Byron Blagburn, a veterinary medicine professor at Auburn University, suggests there are reports of heartworm positive dogs that can't be explained by owners failing to give preventative drugs properly.
Blagburn presented his findings last week at an American Heartworm Society's symposium in Memphis. He could not be reached for comment.
If resistance is proven, it could be bad news for pet owners as well as manufacturers of popular heartworm preventatives.
"This category of drug that has been used for many, many years to prevent heartworms would have to be reworked and researchers would have to determine what genetically occurred in that particular worm that has made it resistant," Rubin said.
"We're not taking this lightly, nor are the companies who make these products."
Merial, maker of one of the most popular brands of heartworm prevention known as Heartgard, does not believe the resistance hypothesis is credible.
"We've seen no evidence of our product being any less effective," company spokeswoman Natasha Mahanes said.
The company is named as a defendant in a class-action lawsuit filed in August 2009 in federal court in the Northern District of Mississippi. In the suit, several pet owners in the Delta area say their dogs contracted heartworms while taking Heartgard.
Greenville police officer Susan Graziosi has had two dogs test positive for heartworms in the last four years even though she said they took the most popular preventatives, Heartgard and Interceptor.
While she isn't part of the Heartgard law suit, she said drug manufacturers should be more honest with buyers.
"At the very least, they should tell dog owners of the risk so I can make a decision on what I want to use," Graziosi said.
The American Heartworm Society estimates one in 50 dogs in the United States has heartworms, according to a national survey the society conducted in 2004.
More than half of those are not on preventative drugs.
While preventative medications such as Heartgard and Interceptor are available at vet offices and cost about the same as a gourmet coffee treat per dose, heartworm treatment can be expensive - costing upward of $1,000 - and take several months, Rubin said.
"It's very serious, and in some cases it can be fatal," he said.
Elmer Landess, a veterinarian with Gates & Gates Veterinary Hospital in Clarksdale, said he doesn't believe in drug-resistant heartworms.
"It's probably not possible," he said. "That's just my speculation."
Rather, Landess points to exposure.
The Delta region is the "heart of heartworms" because of the prevalence of mosquitoes that carry the parasites and transmit them from dog to dog.
"Dogs that spend most of their time outdoors get exposed to mosquitoes at such overwhelming numbers, (the preventative drugs) just can't get all the immature worms," he said.
But veterinarian Edwin Nordan of Greenville Animal Clinic and Hospital said he has seen dozens of cases like Delilah's each year and is inclined to believe in the possibility of resistance.
"We have had people who swear they mark their calendars on the first of the month and watch the dog to be sure it swallows the medicine and doesn't spit it up," Nordan said. "But we are one of the poster children for this."
Nordan said his practice performed about 1,450 heartworm tests each year in 2007 and 2008. Of those dogs, about 50 each year tested positive despite being on a regular preventative regimen.
Several theories as to why are floating among vets and academics.
One is that hurricanes Katrina in 2005 and Gustav in 2008 blew mosquito populations north where they found a fertile breeding ground in the Delta, with its abundance of water and warm temperatures.
Another is blood tests are more accurate at detecting heartworms in their early stages.
And another is that a new, more aggressive breed of mosquito that can carry more of the parasite has been introduced to the Delta through the Coast's ports.
"It could be a combination of those things," Nordan said. "But we still feel that there is something else there besides those that's causing this."
Fortunately for Pieralisi, Delilah overcame her diagnosis and survived months of treatment. But Pieralisi says she's still upset by what she and her pet endured. While Delilah still takes a preventative medicine, she wonders whether it is doing any good.
"It's frustrating to pay every month for seven, eight, nine years and think everything's fine," Pieralisi said.
"You put your trust in that pill only to find out it didn't work."