Heartworm is a life-threatening disease of dogs and sometimes cats, yet it is very preventable. Sometimes even the most educated and caring of clients are naïve to the facts regarding heartworm disease.

Last week I encountered a veterinary client new to South Florida who knew little of this awful nematode that is transmitted by mosquitoes and lives in the heart of dogs. As a veterinarian who has practiced in a non-heartworm-endemic area (the Pacific Northwest) and now a heartworm-endemic area (South Florida), I’d like to offer some thoughts on heartworm preventatives that you may not have considered.

Transmission of heartworm disease is dependent on the climate of a region. Here in sunny South Florida it can be transmitted year round. In northern climates, it is typically transmitted during the warmer months of the year. Experiments have shown that the infective larval stage must mature within the mosquito and this requires an ambient temperature of 57 degrees. Speak with your veterinarian as to whether you wish to treat only during the warmer months or if you wish to choose the safer option of treating year round. Most veterinarians require heartworm testing annually before prescribing or authorizing one of the many monthly medications available. Testing for heartworm is a simple blood test that can be done right in the veterinary clinic. For clients who may have difficulty administering or remembering to give a monthly medication, there is an injectable heartworm preventative for dogs that lasts for 6 months.

Many of the heartworm preventatives are very effective intestinal dewormers as well. The spectrum of parasites that a medication treats varies from medication to medication. Most vets encourage routine fecal tests at least annually. If your pet comes up positive for an intestinal parasite, that may guide your doctor to choose a particular heartworm preventative. For example, if your pet has been diagnosed with whipworms, vets will typically recommend monthly milbemycin (the dewormer in Sentinel, Interceptor and Trifexis). If your pet has been diagnosed with hookworms, your vet will likely send you home with one of the ivermectin/pyrentel combinations (such as Heartgard Plus, Iverhart or Triheart) as pyrentel is very broad spectrum against hookworms. If your pet has shed worm eggs into your yard, you want to give a monthly dewormer to treat a possible reinfestation. Some of the heartworm preventatives are even paired with medications to treat for fleas. Your doctor can guide you toward the medication best suited for your pet.

Even if you live in a non-endemic area, you cannot say that your pet will never be exposed to heartworm disease. Our society is very mobile. Your pet may not venture far from home, but do you know where your neighbor dogs have lived? Heartworm disease is transmitted via mosquito bites. Perhaps your neighbor dogs are from Texas or New Orleans. By keeping the majority of pets on heartworm preventatives, we can prevent a non-endemic region from becoming an endemic region.

If you and your veterinarian deem it appropriate to have your pet on a heartworm preventative, keep your receipts. A pet who tests positive while on a heartworm preventative is very uncommon, but I have seen it happen twice with topical heartworm preventatives. Both of those pets were absolute fuzz buckets with very thick coats. Both of those owners were able to provide the manufacturer proof of timely purchasing of the medication and the company helped pay for the expensive heartworm treatment. The most common cause of lack of efficacy of a heartworm preventative is lack of client compliance. Sometimes folks purchase the medication and then leave it in the drawer where it sits forgotten. Many people opt to give the monthly preventative on the first of every month. I do this for my own pets, and I also put a monthly repeating reminder on my online email calendar. Even if I forget, my calendar sends me a monthly reminder.

It is much cheaper to prevent heartworm disease (particularly if you use one of the generic heartworm preventatives) than to treat heartworm disease. It can be prevented for as little as 5 dollars per month. The medication used to treat heartworm in dogs (melarsomine) was on back-order for much of 2012 – not only is it very expensive, your vet may not be able to get it. Cats cannot be treated with melarsomine, and there is no medical cure available at this time for felines. The treatment protocol and further information can be found on the American Heartworm Society’s website.

If ignored, heartworm infection can result in right sided heart failure, pulmonary thromboembolism and even death. Heartworm disease is easily prevented.


NOTE: Consult your Veterinarian first to make sure my recommendations fit your special health needs.

Dr . Joi Sutton

Dr . Joi Sutton

Dr. Joi Sutton is a 1993 graduate from Oregon State University. She has practiced both in emergency medicine and general practice. Dr. Sutton has done extensive international volunteer work and is the President and Founder of Veterinary Ventures, a nonprofit organization that takes teams of veterinarians to undeveloped countries for humane medical care. She also runs a small animal practice in South Florida.
Dr . Joi Sutton