Just like the name says, Heartworm is a worm that lives near the heart. The heartworm is only transmitted through infected mosquitoes. Dogs are most commonly affected. Cats seem to be more resilient to the heartworm and cases of cats with heartworm are more common when the dog population is infected.
When a mosquito bites a heartworm infected dog, it ingests the tiny larval worms from the blood. These larvae become more mature in the mosquito. Then the mosquito will go to another dog and bite it, injecting the mature larvae into the dog. The mature larvae, which are now in the bloodstream, become adults and attach themselves onto the major lung artery. The adults reproduce and more larvae begin circulating into the bloodstream where another mosquito can ingest it. The lifecycle of the heartworm cannot be completed without the mosquito. It can take about 6 months for the mature larvae to develop into adults.
The most common symptoms of heartworm are excessive coughing, lethargy, exercise intolerance (out of breath with exercising), increased breathing effort and rate, weight loss, anaemia.
The larvae are very tiny and cause problems when the body produces an immune response to them in the small vessels. Organs likely to be affected are the lungs, liver and kidneys. The adults can be very large. If many are present they can block the main lung artery or even migrate to the right side of the heart leading to congestive heart failure. When the adult worms die, they detach from the artery and can block up other large blood vessels which could cause organ failure or even sudden death.
Many people think that Heartworm exists only in tropical climates, but there is an increasing number of isolated cases now, even in Metropolitan Melbourne. In another decade we could see more cases of heartworm. Southward spread may be from increased interstate travel with dogs, reduced mosquito control (due to concern of overuse of insecticides) and possibility of adaptation of heartworm to colder climates.
The heartworm products available from vet clinics, pet stores and supermarkets only kill certain stages of larvae so that they do not develop into the adults. Therefore heartworm prevention should start under 6 months of age as this is the time it will take for adults to appear. After 6 months of age, a dog may have adult worms. If prevention is to be started after 6 months of age then a heartworm test should be performed to check for adults worms. If the test is negative then heartworm prevention can be started (please be aware that this test is not 100% accurate). If the test is positive, then the adult heartworm will need to be killed first and prevention started afterwards. Prevention medication should always be given on time to avoid the chance of larvae developing into adults.
Treatment for adult heartworm can carry a significant risk as the dead adults could lead to blood vessel blockages and the medication can be toxic. Your pet may require a few days stay in hospital for treatment. Because of the risk involved with treating adult heartworm, prevention is always better than treatment!
At Heritage Veterinary Clinic, we offer the yearly Proheart injection for heartworm prevention. This is started at 6 months of age. The 2nd heartworm injection will be given at 15 months of age to coincide with the annual vaccination booster. The injections requires a larger needle which may cause some discomfort to your dog. Rare side effects that can occur would be an allergic reaction. We provide reminders in the mail so that owners do not have to remember when their pet is due.
The other heartworm preventatives are monthly treatments. This can either be in the form of a monthly tablet or flavoured chew (e.g. interceptor, heartguard), or a monthly spot-on product (e.g. revolution, advocate).