The dog days of summer are here. These days can be glorious, but beware of the downsides for your pet. Obvious risks to Fido are easy to avoid because they are on the forefront of our mind. These include water dangers, accidents, heatstroke and overheating cars, tick diseases, and indigestion from barbecue morsels.

Hidden risks lurk in Fido’s presence, too. Heartworm disease is one of those, and it can make your pet seriously ill.

You may not give heartworm disease much thought. After all, unlike our neighborhood deer tick, you never actually see heartworms. Maybe they fleetingly cross your mind when you remember Fido’s monthly heartworm prevention. But you may forget a month here or there. Oh well, that’s not a big deal, right?

That depends, but you are playing Russian roulette with Rover if you skip heartworm prevention.

Heartworms are unique parasites. They are spread from one animal to another by a mosquito bite. That’s why these glorious summer days are especially risky for dogs that aren’t on heartworm prevention. The more mosquitoes, the greater the risk.

A single mosquito bite can deposit enough baby heartworms into your dog to make him sick later. He may not show signs for a year or more, but during that time, the parasite matures and invades big blood vessels in your dog’s chest and around his heart. This can lead to lifelong complications.

Baby heartworms — that the mosquito deposits into your dog — are so tiny that they can only be seen with the aid of a microscope. Within a few months, though, they mature into big worms. These adult heartworms resemble angel hair pasta, in length, overall size and color.

During their maturation, they invade the great vessels around the dog’s heart. As you can imagine, a handful of these pasta-like worms can clog the vessel. The heart is unable to pump properly and has to work extra-hard to try to overcome this obstacle. Eventually, this leads to heart failure and death of the dog.

As the heartworm disease process is progressing in the pooch, you may notice signs of heart failure. These would include coughing, exercise intolerance, heavy breathing, abdominal distension, collapse and, eventually, death.

Meanwhile, the adult pasta-like heartworms are living in Fido’s chest, mating and producing lots of microscopic baby heartworms. These immature parasites will circulate in the blood, waiting for another mosquito to bite Fido. They will be sucked up, along with Fido’s blood. If that mosquito bites another dog, the new dog will also become infected with heartworms. And so the cycle goes.

The good news is that heartworm disease is preventable. Typically, a chewable pill is given once every month. This medicine works backward and kills any baby heartworms that were deposited by a mosquito into your dog in the past month. If you miss a month, you also miss that window of opportunity to halt heartworm disease in your dog.

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If Fido develops heartworm disease, the only safe and effective option is treatment with an arsenic-based medicine. This treatment takes several months to complete. It causes discomfort for the pet, and it’s not inexpensive.

A big risk for heartworm disease to our pet dogs in the Newburyport area is from wildlife. Heartworms can infect animals other than dogs. Coyotes, foxes, raccoons and weasels are also susceptible to heartworm disease. This wildlife can serve as a reservoir to supply mosquitoes with an ample source of baby heartworms for infection of pets.

Other pets are susceptible to heartworm disease, including cats and ferrets. The disease in cats is quite different from that in dogs. Instead of heart failure, it typically causes asthma-like symptoms such as wheezing and coughing.

Unfortunately, there is no safe heartworm treatment for infected cats. However, monthly heartworm prevention is available. Even indoor cats should receive monthly prevention, because mosquitoes get into our homes, too.

Although the greatest risk for heartworm disease is during the summer, pets should receive prevention year-round. Here are some reasons why:

First, New England seasons are unpredictable. The warm season can extend well into the fall, or start early in the spring.

Second, mosquitoes can survive all winter, even in Essex County. Many will find sheltered “micro-climates” such as sheds, mulch piles, under decks or near furnace vents. If we have a mild day in January, they will come out from dormancy and look for a blood meal. That’s why we sometimes find a mosquito buzzing around our house in the middle of winter.

Third, most heartworm prevention actually contains medicine to prevent other common intestinal parasites, some of which can spread to people.

Heartworms cause an invisible but dangerous disease in pets. Remember your pet’s monthly heartworm prevention. An ounce of prevention is worth a pound of cure.


Dr. Heidi Bassler practices at Bassler Veterinary Hospital at Crossroads Plaza in Salisbury. Do you have questions for her? Send them to