Heartworm and Interceptor for Dogs

Heartworm in dogs is potentially a deadly parasitic disease in which worms of the species Dirofilaria immitis infest the heart and associated blood vessels. Luckily, it is easily preventable with monthly tablets, chews or topical liquids.

The ingredients most commonly used to prevent heartworm are from a group of chemicals called macrocyclic lactones (MLs). One of the more common examples is milbemycin oxime, which is found in Sentinel and Interceptor. Milbemycin is derived from a sugar produced by a soil bacterium called Streptomyces hygroscopicus, subspecies aureolacrimosus.¹

Like all of the MLs, milbemycin works at different dose rates against different parasites. In the preparations used in dogs for heartworm prevention, the doses are also effective against heartworm larvae, roundworms, hookworms in cats and dogs, as well as whipworms in dogs. Sentinel combines lufenuron with milbemycin to provide protection against flea eggs, helping to break the flea life cycle. However it does not kill adult fleas.

Milbemycin works by interfering with the parasite's nerve transmission, causing a flaccid (floppy) paralysis subsequent death.^1,2 Milbemycin is effective in killing the young larval heartworm parasites which have been transmitted to the dog by a feeding mosquito. It is not effective in killing older adult worms living in the dog's heart. A once-per-month treatment clears any larvae which have been contracted in the past month.

Milbemycin is also known to be very effective in killing microfilaria, the offspring produced by adult worms in the animal's heart, but this is not currently a listed as a use on the label of any milbemycin-containing products. Due to this effect, dogs should be tested prior to commencing any heartworm preventative medications, as they can have mild reactions (thought to be due to an allergic response to high numbers of dying microfilaria). Symptoms can include labored breathing, vomiting and salivation.³

All of the ML products have very wide safety margins in mammals and adverse reactions are very uncommon. Allergic reactions to the drug bases are more likely than toxicity of the drugs themselves when used as recommended. The safety margin for milbemycin is similar to that of ivermectin, the best studied of the MLs.⁴

Milbemycin has a safety margin in most breeds of dog of well over 100 times the usual dose used for heartworm prevention. However in Collies the margin is about 16 times, so a large overdose is required to see any adverse signs even in this breed.¹ The reason for this increased sensitivity to the drug is that some breeding lines of Collies do not have a particular protein carrier in their blood-brain barrier which regulates the entry of MLs, so the drugs can move freely into the brain and cause nervous problems.

In very young animals, the blood-brain barrier is not completely closed so the drug can enter freely and cause nervous signs. This is one reason that milbemycin-based products are not recommended for newborn pups or kittens. They can be safely administered from four weeks of age and two pounds in weight for pups, and six weeks of age and 1.5 pounds in weight for kittens.3

At the doses used for heartworm prevention, it is unlikely that any of these symptoms will be noticable, but if you do see any of them, contact your veterinarian.

Prevention is definitely better than cure in the case of heartworm. Ensure that your dog is on preventative medication throughout the heartworm transmission season. This could mean year-round treatment in some parts of the country. Consult your veterinarian for further advice and to discuss an appropriate treatment for your dog.

Sentinel® and Interceptor® are registered trademarks of Novartis AG.