The surge in heartworm-positive animals, some of them on preventatives, has stirred debate about the cause and worries pet owners.
"The high incidence of positive cases in the Delta has made some of our clients very nervous," said Dr. Edwin Nordan, a veterinarian at Greenville Animal Clinic and Hospital. "Some have installed pesticide misting systems around the exterior of their homes to help reduce the number of mosquitoes their pets are exposed to. I understand their anxiety. We deal with positive cases every day, and it is a serious disease."
Heartworm disease is a potentially fatal infection transmitted by mosquitoes to dogs, cats and other mammals. Preventatives are relatively inexpensive medication given monthly that keep the parasite from maturing and eventually damaging the arteries of the animal's lungs and heart. Dogs are more often affected by heartworms.
Veterinarians say preventative treatment is still a pet's best defense.
"There are many factors associated with a successful heartworm prevention protocol," said Dr. Wally Mullen, an assistant clinical professor at Mississippi State University's College of Veterinary Medicine. "Right now veterinary medicine cannot definitively answer why there is such an increase in positive cases. All the different preventatives are still considered to be highly effective, especially when you compare the number of doses given to the number of cases associated with an unexplained lack of effectiveness."
Nordan and Dr. Bryant Anderson, also a veterinarian at Greenville Animal Clinic and Hospital, see the disease on a daily basis. Like many other veterinarians, they suspect the increase is most likely caused by more than one factor.
"The Delta has a large population of mosquitos and a species of mosquito that feeds all day long. So animals in the Delta presumably have a higher incidence of exposure," Anderson said. "The dog's immune system plays a role in fighting off infection. And different dogs metabolize medication differently. Medications will act longer in some dogs than in others and at various speeds.
"Preventatives are not active for the entire 30 days between doses. That is why it is so important for owners to strictly adhere to the correct dosing schedule," he said.
Other factors contributing to the increase in positive cases include the spread of existing mosquito species and the introduction of new ones, climate changes, urban sprawl, relocation of heartworm-positive companion animals and improper administration of heartworm preventatives and treatments.
The incidence of positive cases decreased between 2007 and 2010. However, numbers are still above those recorded before 2002. In hotbed areas, such as the Mississippi Delta, some reporting clinics were still seeing more than 100 heartworm positive animals per year in 2010, according to the American Heartworm Society.
Pet owners are urged to take pets to a veterinarian on a regular basis and follow current recommendations for heartworm prevention. Dog owners should start puppies on preventatives as early as possible. Even those with indoor-only pets should treat their animals every single month. Limit pets' outdoor activity during cooler times of the day, when mosquitos feed more. Eliminate standing water where mosquitoes can breed and have pets tested for heartworms at least once a year.
"Some people mistakenly believe that if they give the preventative at some point during every month, their pets are protected," Anderson said. "That is not the case. For example, if a pet gets medication on Sept. 1 and again on Oct. 7, there is a reasonable window of opportunity for a heartworm infection to occur.
"Ideally, clients should give preventatives every 30 days, especially here in the Delta and other areas that are experiencing extremely high numbers of positive cases," Anderson said. "Giving the medication on the same day of the month every month is also acceptable."
Always be sure the pet ingests the pill, and never split single doses of medication among multiple pets. The active ingredient in individual pills is not evenly distributed, so sharing pills would leave pets unprotected. If the pet vomits within the first hour of swallowing the medication, wait 24 hours and give another full dose of medication, Anderson said.