Heartworms: all you need to know

The heartworm (or Dirofilaria immitis) is a variation of roundworm, transferred by mosquitoes and one of the more dangerous parasites found in pets.

What are the symptoms of heartworm?

There are three severity levels of heartworm disease in cats and dogs. The severity of your pet’s infection can be assessed by the level of symptoms shown.

• Animals with Class I heartworm disease 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 of the infection. 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 dangerous?

As with many parasitic worms, blood loss is a serious risk for pets infected with heartworm. In the most severe cases, blood loss can lead to extreme lethargy, very high blood pressure, and ultimately heart failure. It is therefore imperative that you take your cat or dog immediately 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 swelling or abnormalities in the heart. Spotting these signs early on could be the difference between health and suffering, or even life and death, for your pet.

How to treat heartworm

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. Some pets will only be in hospital for a short time while the infection clears up, but others may need to stay longer, depending on the progression of the disease and whether there are any other complications such as blood clotting. In cases where a large number of adult worms are present in the right ventricle of the heart, a surgical procedure may be needed to remove the parasites.

Following the administration of an adulticide, your pet’s activity should be restricted as much as possible. Just like a human, your cat or dog’s body needs time to recover, and this recovery will only be slowed or harmed by activity. Your veterinarian will recommend any changes to food intake following treatment, but a restricted sodium diet is often recommended.

How to prevent heartworm

There are a number of easy steps owners can take to minimize an animal’s chances of picking up heartworms. Here’s our simple 4-step plan to keep your pet heartworm-free:

1. GUARD AGAINST: For dogs and cats at risk due to their health or environment, regular prophylaxis should be given as a prevention against heartworm infection. 
2. BLOOD TESTS: While more difficult in cats, regular blood testing in dogs can often detect any early signs of heartworm disease before it develops too far. 
3. DON’T GET NIBBLED: Heartworms are transferred exclusively by mosquitoes, so one easy way to minimize the chances of the disease is to avoid contact with the little biters! Consider using pet-safe mosquito repellent sprays when out and about. 
4. TRY THE PET STORE: Many effective heartworm preventatives are available to buy in store. Speak to your veterinarian about which option is best for your cat or dog.

What do heartworms look like?

Heartworms are a thread-like parasite. Adult heartworms look like pale spaghetti.

How do heartworms infect?

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