Heartworm – The Silent Killer

mossieThe Silent Killer

Heartworm is a silent killer of dogs and cats and by the time you notice the tell tale signs of the disease, the damage that has been caused is serious.


How do dogs get infected?

Heartworms are spread by mosquitoes (Dirofilaria immitis) where the infective larvae enter the bloodstream and move to the heart and adjacent vessels. It can take a number of years before dogs show outward signs of infection. Consequently, the disease is diagnosed mostly in 4 to 8 year old dogs. The disease is seldom diagnosed in a dog less than 1 year of age because the young worms (larvae) take up to 7 months to mature following establishment of infection in a dog.

What do heartworms do to the dog?

Adult worms cause disease by clogging the heart and major blood vessels leading from the heart, by doing this the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.    The disease initially causes a cough which progressively becomes worse. The dog becomes inactive and lethargic due to the weakening of its heart. It will not be able to tolerate exercise without coughing. In severe cases, fluid leaks out of the blood vessels and accumulates in the lungs and the lower part of the abdomen. This fluid gives the dog’s abdomen a ‘pear-shaped’ appearance, resembling the shape of a balloon filled with water.

How is heartworm diagnosed?

In most cases, diagnosis of heartworm disease can be made by a blood test that can be run at your veterinary practice. Further diagnostic procedures are essential, in advanced cases particularly, to determine if the dog can tolerate heartworm treatment.

How can I prevent my dog getting heart worm?

  • Monthly tablet or Spot-On. Many of the monthly preparations also help to control intestinal worms.
  • Once a year Heartworm Prevention Injection ideally administered at the same time as your dogs annual vaccinations.
  • If your dog is currently on a monthly heartworm preventative they can be easily switched on to the annual injection. (The once a year injection is not suitable for cats)
  • Puppies can be given the once a year injection as early as 3 months but due to their rapid growth it may need to be repeated at 6 months of age as the dose rate is determined by the weight of the dog.


For more advice on heartworm prevention talk to one of the team at Vet Cross