Canine heartworm disease can be fatal for your dog, but is easily controlled by using the correct medication.
What Is It?
Canine Heartworm Disease is caused by worms which live in the right side of the heart and develop by feeding on the surrounding blood. They are long, thin worms, up to 30cm long. The number of worms harboured by infested dogs is variable, however, up to 200 have been found in the one dog. The disease is essentially produced by the activity of the worms and the dog's own immunological response to the presence of the worms.
How Is It Transmitted?
Heartworm is spread to dogs by mosquitoes, which harbour the microscopic larval stage of the worm (L3) within their own bodies. When infected mosquitoes feed, they inject the larvae into the dog and these migrate into the bloodstream, eventually reaching the heart and lungs where the final stages of development into adult worms is completed. It takes about 6 - 7 months from the time of infection to the time when the adult worms begin to produce the tiny offspring (microfilariae), which are infective to the mosquitoes at feeding, and so the cycle continues.
Is My Dog At Risk?
Because infection is through mosquitoes and is not possible by direct dog to dog contact, ALL dogs are at risk. Further, since Perth has a Mediterranean climate, our mild conditions mean that we get mosquito activity all year round, hence there is year-round risk of infection for all dogs.
Does It Affect Other Species?
Rare cases of heartworm have been reported in humans and even cats, however infections are not usually patent and as such, do not produce the severe clinical signs seen in dogs.
How Does It Affect My Dog?
Heartworm disease is a multi-systemic disease that ultimately affects the heart, lungs, liver and the blood system itself. The course of the disease is protracted, and it may take 1 - 3 years after infection before any clinical signs become evident. For this reason, many cases of heartworm infestation are detected by routine heartworm blood tests in clinically healthy dogs. In the early stages of the disease, clinical signs are referable to the effects on the heart and lungs, including exercise intolerance (tiring easily), chronic intermittent cough, weight loss and sometimes a dry coat.
In more severe cases, congestive heart failure may develop, producing marked coughing, retching, abdominal distension, weakness and lethargy. By this time, the internal organs (liver and kidneys as well as heart and lungs) have become severely compromised. An even worse form can occur, where the animal becomes very depressed and may collapse, have difficulty breathing and become pale and/or jaundiced. These signs are associated with severe heartworm disease and the associated red blood cell destruction. The prognosis for dogs at this stage, even with treatment, is grave.
Diagnosis of Heartworm Disease
Until the advent of monthly heartworm preventatives, (originally Heartgard, Endovet and now also Proheart) diagnosis of Canine Heartworm Disease was routinely made by microscopic examination of blood to detect the presence of heartworm offspring (microfilariae).
Occasionally, an occult infection occurs, where there are adult worms in the heart, but no circulating microfilariae. In this case, even though the dog has heartworm disease, the microfilarial test will not show it. Similarly, the monthly preventatives will kill (either straight away , or cumulatively) the circulating microfilariae, so again a microfilarial test proves negative even if the dog has adult worms in the heart. For these reasons, in a dog showing clinical signs or a dog that has been on a monthly preventative and lapsed for less than 6 months we use an antigen test - an immunological test for the presence of adult worms in the heart.
Unfortunately, the antigen test tends to give a large number of false results, presenting us with the dilemma of what to do with a dog that is clinically normal, yet tests antigen positive. The choices are either perform chest radiographs, a full blood count and biochemistry panel, and possibly electrocardiograms to support (or otherwise) the antigen result, or to repeat the antigen test. This could be done, say at the next vaccination time, or sooner if clinical signs of the disease develop. Since there are likely to be a significant number of false positive antigen results, the latter option of repeating the antigen test at a later date would seem the most sensible.
Prevention of Canine Heartworm disease
There are many heartworm preventative medications available today. The most common ones are monthly oral or topical (spot-on) medications, as well as the option of a yearly injection (similar to a vaccination). All preventatives are almost 100% effective in preventing heartworm infestation, provided they are given as directed. Remember, preventative treatment must be given throughout the dogs life in order to provide protection.
Any dog older than 5 months of age should be blood tested by your veterinarian BEFORE preventative treatment can be started, or re tested if for any reason the preventative treatment has lapsed.
Please ask your veterinarian to advise you on testing and re testing procedures, and to discuss the various preventatives available. You CAN change from monthly to yearly prevention quite safely and vice versa. Please ask us how.
Treatment Of Existing Heartworm
Whilst treatment of Heartworm disease is possible, it involves several phases and thus is quite protracted. Pre-treatment assessment using blood tests, radiography, urinalysis and electrocardiography are usually necessary before treatment can be considered. Then in hospital treatment and strict rest at home, to follow, before the final microfilariacidal step completes the treatment protocol. Repeat blood tests to assess the efficacy are required later on. The prognosis is dependent upon the severity of disease and organ damage, although the success rate for treated dogs is high.