Golden puppy

If your dog's home is permanently in the UK, don't worry about heartworms.

© Pixabay

What are the first signs of heartworms in dogs?

By Dr Holly Graham BVMedSci BVMBVS MRCVS Veterinarian

Updated on the

What’s one of the main things that all dog lovers fear? Heartworms, which cause dangerous and potentially fatal damage to a dog’s body. Here's how to avoid these worms and their horrific effect.

Heartworms can be a huge worry for dog owners in the Americas and certain parts of Europe, but thankfully these parasites aren't found in the UK. If you're planning on holidaying or moving abroad with your pet, heartworm is something to be aware of. The parasite can have devastating effects on your pet’s health, and can be fatal if left untreated.

Heartworm, or dirofilariasis, is caused by an internal parasite known as dirofilaria immitis. These large worms can reach 36cm in length during adulthood, and can survive for up to five years in your dog's body. The blood-borne parasite infests the heart and pulmonary artery, and infections can be life-threatening. Without appropriate treatment heartworms will multiply in great numbers, and can cause long-term damage to the lungs, heart and arteries of affected dogs. Unless treated quickly and effectively, your dog may suffer the effects of a heartworm infection for the rest of their life.

First signs of heartworms in dogs

It can take a long time before a dog shows any signs of heartworm infection – it may even be years before signs are noticed. The microfilariae or immature worms take 5-7 months to mature into adults, so the disease is rarely seen in puppies or very young dogs. Unfortunately by the time clinical signs are seen, the disease is well advanced.

The signs of heartworm depend on the number of adult worms present in the body, the location of the worms and the organs involved. Early signs of heartworm infection include:

  • A soft or dry persistent cough
  • Shortness of breath or exercise intolerance, a loss of stamina or lethargy
  • Weakness.

Symptoms are usually worse following exercise, and may become more severe as the infection progresses.

How do dogs get heartworms?

The life cycle of the heartworm is complicated, and requires mosquitoes as intermediate hosts. Female mosquitoes ingest the microfilariae when they bite a dog infected with heartworm. The microfilariae develop inside the mosquito into infective larvae, which enter the dog's body when the mosquito bites to feed. The infective larvae migrate through the bloodstream and move into the heart and surrounding blood vessels, where they mature into adults. The bite of a single infected mosquito is enough to infect your dog with heartworm.

What happens if a dog has heartworms?

Once a dog is bitten by an infected mosquito, it takes around 6-7 months for the larvae to develop into an adult heartworm. These adult worms, measuring up to 36cm in length, clog up the large blood vessels and smaller vasculature of the internal organs causing (sometimes) irreversible damage. It's possible for a dog to have up to 300 adult worms living in their body at once. As the worms begin to obstruct the vital blood vessels to the lungs, heart and kidneys, the blood supply to these organs is greatly reduced. If the blood flow is reduced enough, these organs can malfunction completely.

What are the symptoms of heartworms in dogs?

Heartworms are sneaky and can live for a long time in a dog's body undetected. Symptoms often take years to show, and by that time the damage may be too great to reverse. Common signs of heartworm infection include:

  • A persistent dry cough
  • Difficulty breathing and shortness of breath
  • Weakness and lethargy
  • Loss of stamina
  • Weight loss
  • Disorientation or fainting during physical activity
  • Dry coat
  • Pot-bellied appearance or bulging ribs
  • Seizures
  • Nosebleeds.

A vet may be able to pick up a heart murmur (an abnormal noise heard with a stethoscope on examination), which may help with the diagnosis once clinical signs are apparent. If you have any suspicion that your dog may be infected with this parasite, you should discuss your concerns with a vet immediately. Early diagnosis by a vet is essential.

How do vets diagnose heartworms in dogs?

Most of the time, a vet will run a simple blood test to screen for heartworm. In areas where heartworm is endemic, regular testing is recommended for dogs who aren't on preventative treatment. These tests aren't necessary in the UK, but speak to a vet about potential blood testing if you're planning on travelling abroad.

Sometimes further testing is required to diagnose heartworm. These tests may include further bloodwork and chest x-rays to confirm the diagnosis and determine how advanced the disease is.

What is the treatment for heartworms in dogs?

Treatment varies from case to case, but if your dog is diagnosed with heartworms, you can expect a combination of medication, surgery and exercise restriction. Treatment of heartworm in dogs depends on the severity of your dog’s symptoms. Dogs are categorised into four groups:

  • Category I: dogs without symptoms, surprise cases.
  • Category II: dogs with mild symptoms e.g. cough, signs of heart disease on chest x-rays and some abnormalities on bloodwork.
  • Category III: severely affected dogs with changes on x-rays and bloodwork, unwell animals.
  • Category IV: dogs who are dying from heartworm.

Exercise restriction is key in treating this disease. Dogs who are unaffected or who are not showing signs require strict exercise limitation following medical therapy. In comparison, dogs showing signs require exercise restriction prior to beginning any treatment. The larval stages of heartworm are killed by monthly treatments of avermectin or milbemycin. If larval stages are killed off effectively, adults can't develop and infection is prevented.

Worst cases of heartworm

Adult worms are treated with a drug called melarsomine. This is given via intramuscular injections and usually requires 2-3 injections to clear the infection. Exercise restriction is particularly important following this treatment. It's not often vets encourage dogs to be lazy, but lazing around is perfect in this case.

Dogs who are acutely affected and are at the point of death are treated as surgical emergencies. They may require physical removal of the worms via their jugular vein. These dogs are very, very sick.

How do you prevent heartworms in dogs?

If your dog's home is permanently in the UK, you needn't worry about heartworm and no preventative measures are required. If you're planning on travelling to another country where heartworm is present, it's essential to consider how you can prevent infection with heartworm while you're travelling.

Medication via tablets or spot-on treatments are easy to use. They are highly effective in preventing your pet from becoming ill. Discuss your requirements with a vet prior to travel, as heartworm treatment usually requires a prescription.

Mosquito bite prevention is key. This can be achieved by using either a spot-on treatment or a drug collar. Along with this, you should use a medication to stop worms developing in your dog should they get bitten. This can be done through another form of spot-on treatment or tablet, which should be discussed with a vet prior to travel. Most treatments need to begin before you head off on holiday, and should continue for a short while once you're back on home ground.

How would you know if your dog has heartworms?

Most dogs with heartworm don't show any signs of illness until the infection has progressed significantly. If you live in an area where heartworm is common, or have travelled to an area where it may be lurking, consider any signs such as coughing, exercise intolerance or lethargy as a potential sign of heartworm.

How long can a dog live with heartworms?

If caught early enough and treated successfully, your dog may make a full recovery and live a normal life. Unfortunately as many dogs don't show any signs of illness until things are getting serious, there can be damage to internal organs that isn't reversible – getting a diagnosis fast is important in saving your dog's life.

How likely is it for a dog to get heartworms?

Dogs in the UK don't carry heartworm, so it's nearly impossible for them to catch it here. Our climate doesn't suit the lifestyle of the worm, and infected dogs are usually ones who have travelled outside of the UK, or have been imported from a country where this is rife. If you regularly travel to a country where heartworm is prevalent, they are at an increased risk of catching this parasite.

Can heartworm in dogs be cured?

Heartworm is something that can be cured, provided the signs of disease are noticed early and the heart, lungs and kidneys haven't sustained permanent damage. It's possible to kill all stages of heartworm with medication, but this has to be done in a controlled manner.

When should I talk to a vet?

There are many different causes of coughing, lethargy and general illness in dogs that could mimic the signs of heartworm. If you live in the UK, it's very unlikely that these signs are caused by the parasite, but these signs should always be checked out by a vet.

More advice on...

What did you think of this advice article?

Thanks for your feedback !

Thanks for your feedback !

Leave a comment
Connect to comment
Want to share this article?