Mosquitoes are nasty pests—they buzz in our ears, produce itchy bites, and can ruin a lovely evening outdoors. What’s worse, mosquitoes can transmit dangerous and life-threatening heartworm infections to our pets. The good news, however, is that heartworm infection and disease is easily and affordably preventable. April is National Heartworm Awareness Month, so your Palm City Animal Medical Center team is here to share their wealth of knowledge that will keep your pet safe from heartworm disease. 

Heartworm life cycle and transmission in pets

Heartworms are parasitic worms that grow up to a foot long, and live in the heart, lungs, and surrounding blood vessels of dogs, wolves, foxes, and coyotes. They can live in other mammals as well, including cats, but because these are not ideal hosts, they don’t allow full life cycle completion. Adult worms will mate inside the host and produce microfilariae (i.e., tiny immature worms) that circulate in the bloodstream, where mosquitoes can pick them up while feeding. The immature worms stay in the mosquito for 10 to 14 days, move to the mosquito’s mouthparts, and are then deposited on their next host’s skin when the mosquito feeds. The microfilariae migrate through tissues, mature in the lung blood vessels in about six months, mate, and then start the life cycle over again. Adult worms can live five to seven years in dogs and two to three years in cats. 

How heartworms affect pets

Heartworms affect dogs and cats differently, since dogs are natural hosts and cats are not. Dogs may host anywhere from 30 to several hundred worms, because the worms are long-lived and able to reproduce inside the host. Large worm burdens typically lead to more severe signs, and more complicated treatment, but any number of heartworms can cause lasting heart and lung damage. In the early stages, dogs with heartworm infection may not show signs, but in the later stages, signs include:

  • Coughing
  • Exercise intolerance
  • Decreased appetite and weight loss
  • Fluid accumulation in the abdomen (i.e., ascites)
  • Heart failure
  • Caval syndrome (i.e., sudden collapse and death when heartworms fully block heart arteries)

Cats tend to harbor only one to six worms, because immature worms cannot reproduce, and often die before reaching adulthood. Worms that do reach maturity in cats can cause significant respiratory system damage, which is termed heartworm-associated respiratory disease (HARD). Heartworm signs in cats mimic other diseases, and may include:

  • Coughing or wheezing
  • Vomiting, loss of appetite, and weight loss
  • Fainting or seizures
  • Difficulty breathing or panting
  • Sudden death

How heartworm disease is treated in pets

Once heartworms are detected, treatment should commence as soon as possible, to minimize ongoing damage from the adult worms. Treatment for dogs usually involves several injections, over the course of multiple months, of an arsenic-derived medication injected deep into the muscles around the spine. The injections are painful, and your pet needs hospitalization that day. When heartworms die, they can cause life-threatening clots in the lungs, or allergic reactions, so medications such as steroids often are given for the treatment duration, and your pet’s activity level must be severely restricted to prevent these complications. 

The drug that kills adult heartworms in dogs isn’t safe for cats, so no treatment is possible for cats. Cats may be treated for respiratory inflammation and associated symptoms during the worms’ lifespan, and closely monitored with periodic X-rays and bloodwork, but only some cats are lucky enough to survive heartworm infection. Heartworm-positive cats should be started on a monthly preventive to ensure they cannot be infected by new heartworms.

Heartworm disease prevention and testing in pets

Heartworm treatment can be expensive and painful for your dog, and isn’t an option for cats. Instead of waiting for heartworms to infect your pet and cause irreparable damage to their heart and lungs, prevent infection altogether with monthly prevention products. These products, which are available for dogs and cats in oral, topical, and injectable formulas, work by killing the microfilariae before they mature into adults, are highly effective, and are a much better alternative than heartworm treatment.

Heartworm testing is a key part of a preventive strategy, as well. Adult dogs previously not on a preventive should first be tested, as heartworm-positive dogs can, rarely, have a serious reaction to a preventive. Puppies younger than 7 months can start preventives without a test, since they aren’t old enough to have adult heartworms. All dogs should be tested yearly for heartworms, including those on monthly preventives. Because of the time heartworms take to reach adulthood and become detectable, dogs who weren’t previously on preventives should be tested every six months for one year, and then yearly. 

Cats suspected of having heartworm disease may be screened with a blood test checking for adult and immature heartworms, which can be harder to detect in cats. Imaging with X-rays and ultrasound may also be necessary to confirm diagnosis. 

Heartworm disease can be devastating for your pet, but you can easily prevent this infection with monthly medication from your veterinarian. If your pet is due for their annual wellness visit and heartworm test, or you’d like to learn more about the disease and get your pet started on a heartworm prevention plan, contact us to make an appointment with your Palm City Animal Medical Center team.