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Frequently Asked Questions | Contact PetDrugs.com

Heartworm Questions Answered

Q. Does my dog need to be tested for heartworm before taking Heartgard?

Yes. Before an adult dog commences heartworm prevention medication, it is recommended that a blood test should be performed to ensure the dog does not already have adult heartworms living in it's heart. The prevention medications are not effective in killing adult heartworms. A veterinarian will need to treat the dog with a different medication if it has adult heartworm. On rare occasions, dogs with heartworm infection can have a shock-like reaction if given preventative medication, which is why it is important to rule this out first! Dogs which have been on a different heartworm medication can simply swap over to Heartgard when their next dose is due, so long as there have not been any doses missed. If any doses have been missed, consult your veterinarian for advice. Dogs under 4 months of age do not have to have a heartworm test, as they cannot have any adult worms in their heart. It takes around 6 months from the time a dog is infected by a mosquito to the time adult heartworms will be present in the heart - obviously this is impossible in a four month old puppy! Most veterinarians will recommend prevention starts at the time of their final puppy vaccinations. However, Heartgard can be started as early as 6 weeks of age.

Q. Do I need to give a heartworm preventative year-round?

Mosquitoes transmit heartworm. Heartworm preventatives work by killing any baby worms transmitted by a mosquito in the past month, before they can make their way to the heart and cause problems. The length of the transmission season for heartworm varies throughout the US, depending on the temperature. The use of heartworm preventatives may be considered seasonal in some of the northernmost parts of the United States, with low temperatures during winter, but is required year round in the southern parts. The heartworm preventative season that veterinarians recommend depends upon the transmission pattern in the state and the individual dog's activity profile. Your veterinarian is the best person to advise you what is appropriate for your dog, based on all of these factors. Remember that if you travel out of state with your dog, the preventative season may change.

Q. What are the signs of overdosing heartworm medications?

Adverse reactions are very uncommon with heartworm medications - allergic reactions to the bases of the drugs are more common than toxicity of the drugs themselves. The safety margin for ivermectin, one of the most common ingredients used, in most breeds of dog is well over 100 times the usual dose used for heartworm prevention, but in collies is about 16 times - so still a large overdose is required to see any adverse signs even in this breed. The reason for this is that some breeding lines of Collies do not have a particular protein in their blood-brain barrier which regulates the entry of ivermectin (and other drugs of the same class, called macrocyclic lactones and including drugs such as milbemycin, selamectin etc), so the drug can move freely into the brain and cause nervous problems. Its entry is not blocked by the protein carrier system, where as in other breeds, this protein slows the entry of the drugs into the brain. In very young animals, the blood-brain barrier is not completely closed, so again, the drug can enter freely and cause nervous signs, and this is one reason that the macrocyclic lactones is not recommended to be given to newborn pups. The signs seen with ivermectin toxicity include dilated pupils, vomiting, drooling, depression, unsteady gait, and tremors. The signs can progress to coma or even death in some cases. But at the doses for heartworm prevention, it is extremely unlikely that any of these signs would occur, so owners can feel confident in giving the tablets to their dog. However, should any of these signs be seen, it is recommended that a veterinarian is contacted immediately for advice and appropriate treatment.

Q. What is the minimum age that Heartworm products can be used in puppies?

Heartgard and Generics can be used from 6 weeks in pups, as can Revolution. Sentinel and Interceptor can be used from 4 weeks of age and 2lb weight in pups. Most veterinarians will encourage you to commence heartworm prevention in your puppy before 8 weeks of age, depending on the when they were born in relation to the heartworm transmission season. Your veterinarian will be able to advise when the heartworm transmission season in your area is, and when the best time to start your pup on preventative medication is.

Q. What is the minimum age that Heartworm products can be used in kittens?

Revolution can be used in kittens from 8 weeks of age. Interceptor can be started at 6 weeks or 1.5lbs in weight. Discuss with your veterinarian when the heartworm transmission season in your area is (this varies from state to state) and they will be able to advise the appropriate time to start your kitten on preventative medication.