Summer is here, and the warm, humid weather is in our area full time – and so are the mosquitoes. Just like they do to humans, mosquitoes also feed on our pets, and in doing so they can transmit a dangerous parasite to them that can cause serious illness and even death: heartworms.

Adult female heartworms living in an infected animal, produce microscopic baby worms called microfilaria that circulate in the bloodstream. When a mosquito bites and takes a blood meal from an infected animal, it picks up these baby worms, which mature into “infective stage” larvae over a period of 10 to 14 days. Then, when the infected mosquito bites another animal, the infective larvae are deposited onto the surface of the animal's skin and enter the new host through the mosquito’s bite wound. Once inside a new host, it takes approximately 6 months for the larvae to mature into adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs and up to 2 or 3 years in cats – and during that time, they can cause significant damage and disease.

Heartworm prevention is paramount for our pets, and it should be given year-round to both cats and dogs. They work by killing the heartworms in the larval stage before they reach the heart. There are several options for prevention, and all are relatively inexpensive. 

Many of the preventatives have remained the same over time, but several have been added over the last years. At the same time, heartworms have also evolved. What’s important to know is that we have recently discovered a new strain of heartworms that has become resistant to some of the most common preventatives that we use. This is very alarming. These resistant heartworms have been primarily found in the Mississippi River Valley area, however with heartworm infected dogs being transported to other areas, these heartworms are starting to pop up in other regions of the country.

So which prevention is best? Some recent studies have shown that moxidectin specifically has increased efficacy against these known resistant heartworms. This drug is found in Simparica Trio and ProHeart and these products were shown to have the best killing action against these parasites. Ivermectin and milbemycin failed to demonstrate consistent efficacy against these resistant worms.

If your dog gets heartworms, there is treatment for it. Adulticide heartworm treatment takes two months to complete, is expensive, and can have some potentially serious side effects. Alternatively, a dog can have the “slow kill” treatment by using the monthly prevention and allowing the heartworms to live out their life cycles, but this is not in the best interest of the dog because the heartworms continue to cause damage during that time. The “slow kill” method is not recommended by the American Heartworm Society.

For cats, unfortunately, there is no adulticide treatment that is safe. If your cat gets heartworms and is lucky enough to survive long enough to be diagnosed, careful medical management of symptoms is needed. Some cats do ultimately survive, but irreparable damage will often occur.

Heartworms present a serious health threat for our pets, whether your pet is inside only or goes outside. Remember that it only takes one mosquito bite to cause an infection in an unprotected pet and either no prevention or missing just one dose of prevention can result in an infection in your pet. And if your pet happens to get the resistant strain of heartworm, the right prevention is key. Your veterinarian will guide you on the best products to use to protect your pet against this easily preventable disease, as you strive to provide your pet with a safe and healthy life quality.