One of the useful advantages to having clinical or field veterinarians is the amount of data on disease that becomes available.

The prevalence (how frequently it shows) of disease, control methods, and successful treatment options are some of this data.

After discussions with local veterinarians, a disease I once thought was in scattered pockets is not apparently the case, rather it appears to be much more widespread.

Proponents of One Health will confirm that knowing disease prevalence is important – healthy environment, healthy animals and healthy humans are inextricably connected.

The disease of note today is heartworm disease – mostly a concern with dogs, but cats are not unscathed by it.

Cause – Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. The worms are spread through the bite of a mosquito. They mature into adults, mate, and produce offspring while living inside a dog. But, they can only do this after first living inside a mosquito for a short period.

Biology – In an infected dog, adult female heartworms release their offspring of tiny microscopic larva called microfilariae, into the dog’s bloodstream. When a mosquito bites the infected dog, the mosquito pulls these larvae into itself as it sucks blood. Over the next 10 to 14 days and under the right conditions, the microfilariae become infective When the infected mosquito bites another dog, the infective larvae are transferred to the dog through the bite wound. At this stage, the straw-like mouthpiece which the mosquito sucks blood through is too small for the infective larvae and they must instead must slither down the outside of it. In the newly infected dog, it takes about six to seven months for the infective larvae to mature into adult heartworms. The adult heartworms mate and the females release their offspring into the dog’s bloodstream, completing the lifecycle. Heartworm disease is not contagious, meaning that a dog cannot catch the disease from being near an infected dog. Directly, that is. Heartworm disease is only spread through the bite of a mosquito. Inside a dog, a heartworm’s lifespan is five to seven years. Adult heartworms look like strands of cooked spaghetti, with males reaching about four to  six inches in length and females reaching about 10 to 12 inches in length. The number of worms living inside an infected dog is called the worm burden. The average worm burden in dogs is 15 worms, but that number can range from one to 250 worms.

Tests – A blood antigen test detects specific proteins which are released by adult female heartworms into the dog’s bloodstream. In most cases, antigen tests can accurately detect infections with one or more adult female heartworms. The earliest that the heartworm proteins can be detected in a dog’s bloodstream is about five months after it is bitten by an infected mosquito. Another blood test detects microfilariae in a dog’s bloodstream. Dogs that are seven months of age and older can be tested for heartworms. A dog may appear healthy on the outside, but on the inside, heartworms may be living and thriving.

Signs – A serious disease that results in severe lung disease, heart failure, other organ damage, and death in pets, mainly in dogs. There are four classes, or stages, of heartworm disease. The higher the class, the worse the disease and the more obvious the symptoms. Class 1: No symptoms or mild symptoms such as an occasional cough. Class 2: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity. Class 3: More severe symptoms such as a sickly appearance, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest X-rays. Class 4: Also called caval syndrome. There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is risky, and even with surgery, most dogs with caval syndrome die. Not all dogs with heartworm disease develop caval syndrome. However, if left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death.

Treatment – Drugs which kill adult heartworms. Melarsomine dihydrochloride is currently most recommended. It is given by deep injection into the back muscles. Other drugs for killing adult heartworm are imidacloprid (not legal for use in Fiji) and moxidectin (we need to find a suitable cost-effective form of this). So-called slow-kill treatment method is often used when other treatments are cost-prohibitive or for medical reasons. These need to be discussed with your veterinarian before a choice is made. The treatment for heartworm disease is not easy on the dog regardless of protocol chosen. Treatment can be potentially toxic to the dog’s body and can cause serious complications, such as life-threatening blood clots to the dog’s lungs. However, the years the heartworms live in the heart and lungs is severely damaging – so treatment should be considered an acceptable risk.

Prevention – Many products are available to prevent heartworms maturing into adults in dogs. Most products are given monthly, either as a topical liquid applied on the skin or as an oral tablet. One product is injected under the skin every six or 12 months. Some heartworm preventives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas, ticks, and ear mites). All require a veterinarian’s prescription. Year-round prevention is best. Cats can also get heartworms after being bitten by an infected mosquito, although they are not as susceptible to infection as dogs. A cat is not a natural host of heartworms because the worms do not thrive as well inside a cat’s body. Heartworm disease in cats is a bit different than in dogs. Heartworms in cats do not live as long (average lifespan is only two to four years) or grow as long, and fewer of them mature into adults. Worm burdens are lower in cats than in dogs. Usually, a cat has only one or two worms. However, due to its relatively small body size, a cat with only a few worms is still considered to be heavily infected. In cats, it takes about one month longer than in dogs for heartworms to reach adulthood. The presence of microfilariae in a cat’s bloodstream is inconsistent, shortlived and uncommon – only 20 per cent of cats compared with 80 to 90 per cent of dogs with heartworm disease. It is harder to detect heartworm infections in cats than in dogs. Not all cats with heartworm disease show signs and rarely do they show signs of heart failure. Some cats are able to spontaneously rid themselves of heartworms while some infected cats die suddenly from heartworm disease without ever showing signs of being sick. Cats with heartworm disease may have very nonspecific signs that mimic many other cat diseases. There is no approved drug to treat heartworm disease in cats. Some prevention medications are available.

• JO OLVER is a Doctor of Veterinary Medicine with SPCA Fiji Islands. The views expressed are those of the author and do not reflect the views of this newspaper.