Heartworm Horrors

Prevent heartworm with monthly prevention to control symptoms.

Monthly prevention is the easiest and least expensive approach to controlling heartworm disease in pets, yet lack of compliance to the recommended 12-month prevention regimen leads experts’ opinion as to why infections are on the rise.

Mosquito-friendly environmental changes, movement of infected wild and domestic animals and persistence of vectors are other causes for a rise in diagnosed cases, according to the American Heartworm Society.

“The revised AHS canine and feline heartworm guidelines reiterate the need for year-round prevention, even in northern states,” says Tom Nelson, DVM, the AHS heartworm guidelines symposium chair and a member of the Companion Animal Parasite Council (CAPC). He also owns  the Animal Medical Center in Anniston, Ala. “The recommendations we make today for dogs and cats are the result of ongoing heartworm research discoveries.

Heartworm damage is similar to damage caused by smoking, says Dr. Cristiano von Simson. “The amount of cigarettes a person smokes can affect the damage done to their body, just like the larger the infestation, the greater the damage.”

“We once thought 12 months might not be necessary in cold winter months, and we also thought cats weren’t affected by the disease,” Nelson continues. “We now know that it takes only a couple of warm days to activate mosquitoes, which will be looking for a blood meal. We also know that if dogs have been diagnosed in a certain area with heartworm, that it’s likely local cats not using prevention can be infected, too.”

The AHS notes that the Food and Drug Administration considers a positive heartworm test on a dog taking preventives a lack of product efficacy. But other factors may be at play, including appropriateness of dosage or administration consistency.

Nelson says LOE is probably caused by owners missing doses or giving doses off schedule, applying the product improperly, host immune response to drugs and different levels of metabolizing the drug within each species.

“Veterinary medicine still has a long way to go to get pets on year-round heartworm prevention,” says Michael Murray, DVM, technical marking director for Merial in Duluth, Ga. “Research shows that only about 15 percent of dogs that are regular veterinary patients receive 12 doses of heartworm preventive per year, and fully 64 percent of dogs receive no heartworm preventive at all.”

Client education is the best way to create awareness of the importance of disease prevention, says Cristiano von Simson, DVM, MBA, director of veterinary services at Bayer Animal Health in Kansas City, Mo.

“Giving clients tips like ‘Give prevention on the same day you pay your house mortgage’ or telling them that 10 years of prevention would cost about the same as treating an existing infection can increase compliance,” Dr. von Simson says. “It can take hearing it a few times for clients to realize the importance of prevention.”

New Study

Authorities say testing capabilities are not where they’d like to see them. While some tests have been highly effective, others might not detect an existing infection.

The AHS is in the early stages of procuring funding and developing a plan for a comprehensive, side-by-side study of all available heartworm tests, Nelson says.

“There hasn’t been a test of this nature for 10 years, and there are many more tests on the market today,” he said. “We need an independent, controlled study using about 200 to 300 infected shelter dogs to examine which tests are most effective.”

Just as fecal tests should no longer be considered negative simply because parasites or eggs are not detected, Nelson says heartworm tests shouldn’t be considered clear-cut negative, either.

“A non-positive heartworm test result simply means no reproductive female worms were producing glycoprotein,” Nelson says. “Additionally, positive antigen tests should be tested for microfilariae. This will identify if the patient is a reservoir of infection and lets veterinarians know there’s potential for severe reaction if administering a microfilaricide to a dog with a high microfilarial count.”

Spreading awareness of feline heartworm disease is a top priority, Nelson says, because there isn’t an effective test nor an available treatment, and even fewer cats than dogs are getting monthly prevention despite being the more common pet.

“The revised heartworm guidelines include a new educational initiative to help veterinarians and owners implement the recommendations and reduce heartworm infection incidence,” Nelson says.

“However, veterinarians need to believe feline infections are an issue and educate cat owners with conviction that feline heartworm disease is serious and prevention is necessary,” Nelson adds.

Because heartworms don’t reach the adult stage in cats at the same frequency as canine infections, tests today often miss positive feline infections because the antigen produced by mature female heartworm isn’t present.

Similarly, only three to 10 adult worms develop in about 75 percent of infected cats. These worms could easily be all male or too few female worms to show positive results. This is compared to the approximately 60 adult worms that develop in almost 100 percent of infected dogs.

With dogs, “A beagle that weighs about 10-15 kG can test positive with just three adult female worms, but a larger dog would need a larger infestation before testing positive,” Nelson says. “The concern with this is that disease in the larger dog may go undetected for much longer, which causes more physical damage to the animal’s body.”

Von Simson says heartworm damage is similar to damage caused by smoking.
“The amount of cigarettes a person smokes can affect the damage done to their body, just like the larger the infestation, the greater the damage,” von Simson says. “The longer a person smokes, the more harmful it is to them, just as the longer an animal has heartworm disease, the more damaging it can be.”


The FDA-approved drug Immiticide (melarsomine dihydrochloride), produced by Merial, has been in short supply even before Merial’s Aug. 4, 2011, letter to veterinarians indicating difficulty obtaining the product’s active ingredient. This shortage has increased use of alternate—and less efficacious—methods of treating an active infection.

“The age of the worm plays a role in how certain treatments will combat the disease,” Nelson says. “A heartworm-positive dog can have 1-month-old to 7-year-old heartworms. Periods of susceptibility to macrocyclic lactones and melarsomine exist, creating a treatment gap. There are times when D. immitis is not considered to be susceptible to either treatment.”

AHS guidelines say the gap can be eliminated by administering a macrocyclic lactone preventive for two to three months before administering melarsomine. This treatment will eliminate migrating larvae less than 2 months old and allow worms 2 to 4 months to reach an age at which they have been shown to be susceptible to melarsomine.

“The use of doxycyline in combination with Ivermectin or other heartworm preventive drugs has been shown to reduce pathology, the number of adult worms and the infective potential of microfilaria in dogs,” Nelson says.

“In addition to the obvious discomfort and physical harm these parasites cause, owners must keep their pets on restricted activity for long stretches of time,” Nelson continues. “This critical part of treatment is almost never fully followed.”


Owners believe their indoor cat or dog is safe from heartworm disease, experts say.

“It might be a truer belief for cat owners than dog, but owners think indoor pets are safe from mosquito bites and therefore heartworm disease,” Nelson says. “This just isn’t true. Mosquitoes come indoors often and pets can become infected sitting on the family couch.”

Von Simson says multi-purpose heartworm prevention often sounds appealing to owners because one medication controls several types of parasites.

“Multi-products are available for dogs and cats which can help improve heartworm prevention compliance while keeping other parasites at bay, too,” von Simson says.

AHS adopted Merial’s year-long educational “Think 12 in 2012” campaign to keep 12 months of heartworm prevention on the minds of veterinarians and owners.

Client education handouts, case studies and scientific articles will be available to all veterinarians, who will be encouraged to support the mission for increased compliance. This program will also remind veterinarians that they need to remind owners of this need regularly.

In April, CAPC released a first-of-its-kind heartworm prevalence and forecasting campaign, derived from a weather predictions and expert advice. The forecast predicts heightened heartworm prevalence this spring because of above-mentioned temperatures.

“We want everyone to be especially vigilant in protecting their pets from risk that parasites pose in every state,” says Byron Blackburn, M.S., Ph.D., a ACAPC board member and professor at Auburn University. “It’s important to remember that almost all parasites are completely preventable.

AHS Suggested Adulticide Therapy

Pretreat dogs with a monthly preventive in combination with doxycycline before administering melarsomine.

Use a three-dose regimen of melarsomine (2.5 mg/kg body weight) for both symptomatic and asymptomatic dogs. The regimen should include an initial dose, followed at least one month later by two injections 24 hours apart.

Methods using only macrocyclics as an adulticide are not recommended.

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