Frequently Asked Questions: Heartworm

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• How Are Dogs Infected with Heartworm?
• How Common is Heartworm Disease?
• What Are the Signs of Heartworm Disease?
• How is Heartworm Disease Diagnosed?
• How is Heartworm Disease Treated?
• How is Heartworm Disease Prevented?

How Are Dogs Infected with Heartworm?

Adult heartworms are found in the heart and pulmonary arteries (the large blood vessels leaving the heart to the lungs) of infected dogs. On rare occasions they can migrate to other parts of the body such as the liver. They can survive for up to 5 years and during this time a female heartworm can produce millions of young heartworm. These young heartworms are known as microfilaria. They circulate in the bloodstream and tend to congregate in the small blood vessels of the body, particularly in the skin. In order to develop into adult heartworm the microfilariae must undergo several stages of development or "moults". These stages can only occur in a mosquito.

As many as 30 species of mosquitoes are known to be able to transmit heartworms. When a female mosquito sucks up a blood meal from an infected dog it also draws up many microfilaria. The microfilariae develop further inside the mosquito and after 10 to 30 days they migrate to the mosquito's mouthparts. The mosquito bites another dog and the microfilariae are injected into the dog.

Once inside their new host dog, the microfilaria live in the soft tissue (e.g. fat) for a period of time before migrating into the bloodstream and passing into the heart. It takes a further 2 to 3 months before the young heartworm have developed sufficiently to reproduce. The entire life cycle takes 6 to 7 months to be completed. One dog may have as little as 1 or as many as 300 adult worms.

How Common is Heartworm Disease?

The incidence of heartworm infection increases in warmer climates. 20% of unprotected dogs may be infected in certain areas of Queensland. In a recent study almost 9% of foxes on the outskirts of Sydney were infected with heartworm. So to extrapolate if you do not use a heartworm preventative on your dog then he/she will have roughly a 1 in 10 chance of being infected with heartworm.

All breeds of dogs can become infected. Longhaired breeds are just as susceptible to infection as shorthaired breeds.

Infection can occur at any age but because it takes a number of years for symptoms to develop the disease is most often diagnosed in dogs 3 to 8 years old. The disease is seldom diagnosed in a dog less than 1 year of age because the young worms take up to 7 months to mature following establishment of infection in a dog.

What Are the Signs of Heartworm Disease?

Adult worms cause disease by clogging the heart and major blood vessels leading from the heart. They interfere with the function of the valves in the heart. By clogging the vessels the blood supply to other organs of the body is reduced, particularly the lungs, liver and kidneys, leading to malfunction of these organs.

The signs of heartworm disease depend on the number of adult worms present, the location of the worms, the length of time the worms have been present, and the degree of damage to the heart, lungs, liver and kidneys from the adult worms.?Signs of heartworm disease include:

• Coughing. This is most often a soft, dry chronic cough.
• Exercise intolerance
• Lethargy
• Fainting
• Weight loss
• Laboured breathing
• Ascites. Swelling of the abdomen due to accumulation of fluid occurs when the right side of the heart is not functioning normally.
• Coughing up blood

How is Heartworm Disease Diagnosed?

A diagnosis of heartworm disease is made on the basis of:

• The dog's symptoms.
• The results of a physical examination.
• Blood Tests: Are the mainstay of diagnosing a heartworm infection.
• Xrays
• Ultrasound

How is Heartworm Disease Treated?

Heartworm disease can be treated if the infection has not already caused significant damage to the heart, lungs, liver and kidneys. In cases were organ damage is significant then it will be safer to just treat the organ damage rather than risk treatment to kill the adult heartworms.Treatment of heartworm disease is divided into 3 stages:

• Pre-treatment may be necessary in some dogs.
• Adulticide therapy kills the adult heartworms in the heart. Your vet will discuss the full course or treatment and home care if treatment is necessary.
• Blood Tests. A blood sample is tested for the presence of adult heartworm 3 months after treatment.

How is Heartworm Disease Prevented?

Heartworm disease can be easily prevented. There are essentially three broad classes of preventative medications available.

• Daily Medications. These medications have the advantage of being relatively inexpensive but they MUST be given EVERY day for life of the dog. If ONE day is missed the dog can become infected if bitten by a mosquito carrying heartworm on that day. We do not recommend their usage for this reason.
• Monthly Medications. There are several types of monthly preventative medications available. These include oral tablets, oral chewables and topical spot-ons. These products need to be given every month for your dogs life. If you miss a month your dog is at risk of becoming infected.
• Yearly Injections. An injection called Proheart SR12 is available that provides protection from heartworm infection for 12 months. The advantage of this form of treatment is that it does not require any medications to be given at home.