A recent study, using data from the Golden Retriever Lifetime Study and published in Frontiers in Veterinary Science, shows less than 40 percent of dogs were on a heartworm preventative. This is alarming because heartworm infection is on the rise for numerous reasons.

Heartworm disease isn’t as prevalent in Wisconsin as it is further south. The 2023 incidence rate for Dane County was 1 in 300 dogs, but for dogs in Alabama, the risk increased tenfold. Is this relevant if you don’t travel with your dog? Yes, because many dogs being adopted at shelters and rescues in Wisconsin are brought up from Alabama and Texas. Not all of these dogs have been treated for heartworms, and many are incubating the microfilaria (baby heartworms) and don’t test positive for four to six months after adoption.

Mosquitoes spread heartworms. A mosquito will take a blood meal from one dog then fly up to three miles to another dog and get another meal. If the first meal contains microfilaria, then microfilaria will be injected into the next dog, which can develop into adult heartworms that infect the dog’s pulmonary artery. If your “never leaves my Wisconsin backyard” dog lives next to a newly adopted rescue dog from the south, there is a high probability that they are sharing mosquito friends. If your dog isn’t on a heartworm preventative, he could be the 1 in 300 that tests positive on his yearly heartworm test.

Climate change is also impacting mosquitoes. Typically, mosquitoes are the most active from May through September, but they can be out looking for a meal anytime the temperatures are above 50 degrees Fahrenheit. In Madison, there were seven days above 50 degrees in February, and for all of 2023, the average temperature in Dane County was 50.4 degrees.

Finally, because heartworm preventatives aren’t given year-round by many owners (not even in the south), heartworms are developing resistance to some of the more commonly used preventatives. This is scary!

Mosquitoes are the wimpiest of the parasites that like to feed on our pets. Fleas don’t die until temps go below 45; ticks don’t die, but become dormant at temps below 45. Deer ticks are the most active October through May, as long as daytime temperatures are above 32 degrees. Looking at last year’s average temperature in Dane County, only January 2024 saw average temperatures below 32 degrees. There’s a reason your veterinarian recommends year-round flea and tick prevention.

Unlike heartworm disease, there is a much higher prevalence of tick-borne diseases in Wisconsin. Lyme disease is diagnosed in almost 7 percent of dogs and anaplasmosis in over 12 percent.

Though dogs are more likely to get heartworms and tick diseases, cats are not immune. None of us want our pets to get any of these infections, so why aren’t they getting year-round preventatives? Let’s discuss the many options for heartworm, flea, and tick prevention so you can decide which gives your pet the best protection that fits your lifestyle and budget.

When I graduated from veterinary school many years ago, my first practice carried Filaribits. These were given daily as a heartworm preventative and could potentially kill a dog if given to one with heartworms. Soon after joining that practice, a new monthly preventative, Heartgard, was introduced. Heartgard contained ivermectin, which does a great job killing microfilaria, even in a heartworm-positive dog. Heartgard Plus, which contains both ivermectin and pyrantel pamoate, controls both heartworms and several intestinal worms (roundworms and hookworms), and is still broadly prescribed today. Many generic versions of Heartgard are available and may be less expensive, but still very effective. A similar drug, selamectin, is found in topical heartworm preventatives, like Revolution. Efficacy is the same, but some people prefer a topical medication instead of one their pet has to eat.

Other heartworm preventatives were developed with a different active ingredient. Interceptor contains milbemycin, which can also be found in some generic heartworm preventatives. It’s equally as effective as ivermectin. Moxidectin is another preventative in the same family as milbemycin. It’s found in Advantage Multi, a monthly topical preventative, and Proheart 12, a once-a-year injection. All heartworm preventatives require a prescription from a veterinarian.

Some flea and tick preventatives don’t require a prescription, but for the most part, these aren’t very effective. Inexpensive flea collars are a waste of money. They only repel fleas hanging out near the collar, and the active ingredient is often an organophosphate, which can be deadly to cats and harmful to humans. If you like the idea of a collar, I recommend the Seresto brand, which contains flumethrin and imidacloprid—both have been used extensively in other products for flea and tick control. The ingredients slowly diffuse out of the collar and into the oil layer of the pet, distributing over their body and providing protection for up to eight months. They’re very safe for both pets and humans.

One reason cited for poor compliance with heartworm prevention is that some pet owners thought what they were using prevented heartworms, fleas, and ticks when it was really only a flea/tick preventative or vice versa. This is not surprising with the vast number of different brands of heartworm, flea, and tick preventatives available.

In 2010, a new group of oral flea and tick prevention was discovered. These isoxazolines were found to be highly effective against fleas, ticks, and sarcoptic and demodex mites in dogs. Soon, every pharmaceutical company had developed their own compound, and dogs were being prescribed Nexgard, Bravecto, Credelio, or Simparica, and cats were receiving oral Credelio or topical Revolution Plus or Nexgard COMBO. They aren’t recommended for dogs with seizures, but have otherwise been well-tolerated by dogs and cats. In addition to being efficacious against fleas and ticks, it’s now rare to see a dog with skin issue due to mites.

In the last several years, combination products have become available. Nexgard Plus and Simparica Trio are both all-in-one monthly chewables. Now, you only need to remember one pill each month. In addition to heartworms, fleas, and ticks, the preventative also covers infection with roundworms and hookworms. Elanco is coming out with their version, Quattro, which will also eradicate tapeworms.

On the near horizon is a once-a-year flea/tick injection from the maker of Bravecto, Merck Animal Health. When given with a once-a-year Proheart 12 injection, you won’t need to remember to give your dog any preventatives each month.

We have come a long way from daily Filaribits, and, hopefully, we can greatly increase the percentage of dogs getting heartworm, flea, and tick preventative. Our pets deserve to be parasite free!

Lori Scarlett, DVM is the owner and veterinarian at Four Lakes Veterinary Clinic. For more information, visit fourlakesvet.com.