Heartworm is a parasite which is passed from animal to animal by the bite of a mosquito.
Darwin's warmth and heavy rain creates the perfect breeding ground for mosquito-borne diseases like heartworm.
We have recently diagnosed cases of Heartworm infection in dogs at All Pets Veterinary Hospital, and there have also been numerous cases of Heartworm infection reported in other tropical areas of Australia recently.
Heartworms are the most life-threatening worms for dogs and cats. They reside in the heart and pulmonary arteries, causing heart failure and eventually death if untreated. Adult worms are 10-30cm in length and about 1mm in diameter. Worms multiply and release offspring into the dog's bloodstream, causing damage to organs in the body.
Signs of heartworm can include:
- Tiring easily with exercise
- Loss of appetite
- Enlarged or swollen abdomen.
In dogs, clinical symptoms usually appear only when the disease has reached a severe stage (usually 3-5 years from the time of infection). In cats symptoms can appear much earlier.
If you think your pet may be infected you should talk to your vet immediately about specific diagnostic testing.
Heartworm can be difficult to treat once a dog has been infected – treatment is lengthy, and can be risky. Strict confinement is required during the treatment process, usually for several months.
Prevention is as easy as a yearly injection (or 6 monthly for puppies) or a monthly tablet, chew, or spot on.
My pet has been on Heartworm prevention, is there any risk of them having Heartworm?
If your pet has been on a Heartworm preventative medication since they were 8-12 weeks of age, and have always received medication on the same date each month (for the monthly preventions), or have received a Proheart Heartworm injection on time each year, then the risk of your pet having Heartworm infection is extremely low, these medications are very effective at preventing Heartworm infection when given on time.
If you try to give your pet monthly preventions, but are sometimes late, or miss the occasional dose, we recommend having a Heartworm test done to ensure that your pet has not picked up Heartworm. For dogs it may then be worth switching to the Proheart Heartworm injection, as this will keep your pet protected for the entire year. Talk to us about the best options for your pet, if you have been using a combined intestinal worming, heartworm, +/- flea prevention and wish to change to the Proheart injection, we can discuss alternative flea control and worming options. While intestinal worms can be easily treated, this is not the case for Heartworm, and effective prevention is very important.
Read on if you would like more information about Heartworm...
Heartworm in Dogs
How Heartworms are Transmitted?
Heartworms are transmitted by mosquitoes. When an infected mosquito bites a dog it transfers larvae. The larvae migrate inside the dog until they reach their final site (heart and pulmonary arteries) in about 3-4 months. Here they grow to maturity (macrofilariae) in a further 3 months. They then produce larvae (microfilariae) which can survive for about 2 years in the bloodstream.
When a mosquito bites an infected dog, it picks up these larvae and can transmit the infection to other dogs.
Over time, the presence of adult heartworms in the heart and pulmonary arteries causes an inflammation and thickening of blood vessel walls. This increases blood pressure and the cardiac effort to push blood in these vessels. As a result, the dog may develop heart failure, which can eventually lead to death.
Symptoms include sporadic coughing and tiredness. As time passes, the cough becomes chronic and is accompanied by difficult respiration, particularly during and after exercise, mild anaemia and lethargy. In advanced cases, the dog may collapse after even light physical exertion. Most dogs eventually develop congestive heart failure.
Treating Heartworm Infection
Treating heartworm infection is a long and risky process. Dying heartworms and their larvae (microfilaria) may cause shock and embolism. During therapy, dogs have to be strictly monitored for side effects, and their activity restricted for a few several months. In advanced cases, health will not be restored even after effective treatment.
For all these reasons it is now clear why prevention is so important.
In contrast to therapy, heartworm prevention is safe, easy and effective. For dogs, options include monthly tablets or chews, a yearly injection (if given as a puppy it is repeated after 6 months), or a monthly spot-on. All of these options are considered very effective; however the monthly treatments must be given on time every month. If doses are missed, or given late, heartworm infection could occur.
Heartworm prevention should be started by 8 weeks of age (or 12 weeks of age if giving the Proheart injection). If a dog is 6 months of age or older and has not been on Heartworm prevention, they should be tested for heartworm infection before starting treatment. Those with infections must be treated against adults and microfilariae before a prevention programme is started. If the heartworm test is negative, Heartworm Prevention should be started. The Heartworm test is repeated again after 7 months (the length of the Heartworm life cycle) as the initial test only picks up adult heartworms and early infections could be missed.
Heartworm in cats
Although it is not as well known as in dogs, cats can definitely also get Heartworm. An incidence of 2% to 14% of all cats has been reported for endemic areas, making heartworm a concern for any cat living where there are mosquitoes.
The cat is not a natural host for the heartworm, which means the migrating larval heartworm is not likely to complete its life cycle.
Whereas a moderate heartworm infection in a dog would involve 25 to 50 adult heartworms, infected cats typically have less than six adult worms. Because the feline heart and blood vessels are so small, these few worms can wreak havoc. In a dog, six worms or fewer might not be considered worth treating. In a cat, a single worm could easily represent a lethal infection.
Whereas worms found in the canine heart can reach lengths up to 14 inches, the average length of worms found in feline hearts is only 5 to 8 inches long.
While an adult heartworm can expect to live 5 years in a dog, it will only live 2 to 3 years in a cat, probably due to the cat's strong immune reaction.
Heartworm disease in cats is caused by the inflammatory reaction generated by the worm's presence.
Symptoms of infection tend to be more immune-related than heart-failure related. Cats develop more of a lung disease, complete with respiratory distress, and chronic coughing or vomiting. Feline heartworm disease is often misdiagnosed as feline asthma. Sudden death may occur just as it may occur in infected dogs.
In cats there are two phases where the disease can exert symptoms. The first is when immature worms reach the lung and pulmonary arteries, as early as 75 to 90 days after infection The second phase where problems can occur is when the worm dies. Since cats are not the natural host for this parasite, most immature worms that make it to the lung are killed. The presence of the dead worm is extremely inflammatory.
The effects of this kind of widespread inflammation can reach far beyond the lung and circulatory system. The kidney can be affected as well as the gastrointestinal tract and even the nervous system.
Luckily, Heartworm prevention in cats can also be easy and effective. A monthly spot-on treatment, or a monthly tablet can be given to prevent Heartworm. There is currently no yearly injection option for cats.
Testing for Heartworm in cats is not as easy as in dogs, however if your cat has not been on Heartworm prevention, or is showing any signs that they may have Heartworm infection, talk to your vet about testing options.