Heartworms: How to Protect Your Pet

By: Trupanion Staff / Updated June 23, 2023

The heartworm (or Dirofilaria immitis) is a variation of roundworm, transferred by mosquitoes and one of the more dangerous parasites found in pets. Heartworms in pets is a serious health condition that can be difficult to treat once it progresses. So it’s important to be aware of common symptoms and prevention tips.

Quick facts about heartworms

• Heartworms are thread-like worm parasites that look like pale spaghetti at maturity.
• The heartworm parasite can affect both cats and dogs.
• Dog have been diagnosed with heartworm disease in all 50 U.S. states.
• Unlike other parasites, the heartworm can only be passed on by mosquitoes.

How heartworms infect pets

The heartworm is one of the only mammal-dwelling parasites to be transmitted exclusively by mosquitoes. While other common parasitic worms are transferred via feces, heartworms cannot be passed directly from one host to another.

Infection timeline

1. A mosquito bites an animal infected with heartworm and microscopic baby worms, known as microfilaria, are transferred to the mosquito.

2. Microfilaria grow and develop into ‘infective stage’ larvae over the next 10 to 14 days.

3. The infected mosquito then bites your pet, transferring developed larvae into the bloodstream.

4. The larvae grow into adult heartworms over the next six months and can then live in the host animal for up to 7 years.

Symptoms of heartworms in pets

There are three severity levels of heartworm disease in cats and dogs, and the symptoms associated with each can be quite different. The severity of your pet’s infection can be assessed by the level of symptoms they show signs of. According to the FDA, the classes of heartworm disease in pets are as follows:

• Class I heartworm disease - Animals will often show no symptoms at all, but there could be minimal indicators such as a mild cough. While Class I is the least severe form of the disease, the fact that the stage is asymptomatic (shows no symptoms) can make it difficult to spot. This makes it harder to treat in good time.
• Class II heartworm disease - is associated with a more severe cough, and often a reluctance to engage in physical activity. Dogs and cats with this level of infection may be lethargic, and even the most playful pets may become unwilling to run or play. 
• Class III heartworm disease - is the most severe form. Your pet will appear very unwell. Symptoms include anemia, intolerance to exercise and sometimes fainting. On closer inspection by a veterinarian, a Class III patient may also be found to have high blood pressure and an abnormal heart rate.

Are heartworms in pets dangerous?

As with many parasitic worms, blood loss is a serious risk for pets with heartworms. In the most severe cases of heartworms in pets, blood loss can lead to extreme lethargy, high blood pressure, and ultimately heart failure.

It is imperative that you immediately take your cat or dog to a veterinarian if you notice signs of uncharacteristic tiredness, difficulty breathing, weakness of body, bloody diarrhea, or collapse.

An electrocardiograph heart scan carried out by your veterinarian may reveal unusual rhythmic behavior, as well as any heart swelling or abnormalities. Spotting these signs early on could be the difference between health and suffering, or even life and death, for your pet.

Heartworm treatment

Heartworm treatment starts with parasite prevention. While young heartworms (microfilariae) can be killed off at home using a monthly prophylaxis, adult heartworms are more stubborn and require hospitalization. This is so that ‘adulticide’ medicines, a form of pesticide used to kill adult insects, can be professionally administered.