Heartworms

Incidence

Heartworm disease caused by Dirofilaria immitis occurs mainly in dogs, but has also been observed in cats, horses, reindeer, humans, ferrets and black bears. The highest incidence remains in the southeastern states of the United States, but around a hundred cases a year are diagnosed in Quebec. Mosquitoes carry the infection from dog to dog, and the worm needs the mosquito to complete its reproductive cycle.

Reproduction cycle

An infected dog has adult worms in its heart and in the blood vessels that connect the heart to the lungs. The female worm produces larvae or microfilariae. Microfilariae are very small and can only be seen under a microscope. Microfilariae are released into the bloodstream, where they can remain for up to two years. If microfilariae are not ingested by a mosquito when it bites the animal, they die. If ingested by a mosquito, microfilaria will go through two different stages to become infectious. If the mosquito bites a dog, the infective larva migrates into the stinger and penetrates the skin.
The larva remains under the skin for around two months. During this time, the larva undergoes two further mutations before entering the circulation and travelling to the heart. Once in the heart, the larva grows into an adult and produces microfilariae, starting the cycle all over again. The complete cycle, from the moment the larva penetrates the skin until microfilariae are produced by adults, takes about six months. Dogs kept outdoors in areas with a lot of vegetation are the most exposed. The most dangerous time is early evening, when mosquitoes are most numerous.

Clinical signs

At the onset of infection, most dogs appear normal, and clinical signs develop slowly and may not be evident until three years after exposure to the parasite. Not all heartworm-carrying dogs are therefore ill. Some dogs may have had heartworms for months or even years and their owners are unaware of it. Eventually, in most cases, signs will begin to appear. Most clinical signs result from the increased workload of the heart associated with increased pressure in the arteries that carry blood to the lungs. The first clinical signs are lack of energy and reduced exercise tolerance. Other signs include coughing, lack of appetite, abdominal distension and weight loss. The most severe signs are difficulty in breathing, loss of consciousness, coughing up blood and even death.

Diagnosis

At present, no single diagnostic method provides a complete picture of the disease, and several tests are often required. The simplest and most effective screening test is to look for microfilariae in the blood. If microfilariae are present, the chances are high that adult worms are present. Most microfilaria tests involve drawing blood from your dog, processing the sample using filters and dyes, and looking for microfilaria under a microscope. Disadvantages with this method include the fact that not all heartworm-carrying animals necessarily have circulating microfilariae, and that other worms are very similar to microscopic evaluation.
Other, more sophisticated tests are designed to detect small parts (antigens) of the adult worm or microfilariae. Blood screening is recommended every year. It should be remembered that some heartworm-carrying dogs may have no microfilariae in their blood, so a negative test does not prove beyond doubt that the dog does not suffer from the disease. Other examinations, such as chest X-rays and immunological tests, are important for diagnosis.

Treatment

Although there are treatments for heartworm disease, prevention is far preferable. Not only can this disease be fatal, but the treatment used is dangerous. Since heartworm is so easy to prevent, it's a shame that dogs are left unprotected, and many contract this terrible disease or even die as a result of treatment.
There are several ways to treat heartworm disease. Many treatments involve killing the adult worms first. This is the adulticide treatment. This is done by injecting thiacetarsamide (an arsenic derivative) intravenously several times in succession. This part of the treatment requires hospitalization of the patient, as the arsenic derivative is not only toxic for the heartworm, but also for the dog.
The second phase of treatment consists in killing the microfilariae. The first part of the treatment eliminates only the adults living in the heart. It is now necessary to kill the juvenile form, the larvae. For this, the patient must be reassessed between the third and fourth week following the initial treatment. This “microfilaricide” treatment requires no hospitalization for the patient. However, he must return two weeks later for blood tests to ensure that there are no more microfilariae in his blood.
The third phase of therapy is directly linked to the prevention of reinfection. This treatment is given at home by the owner. It involves oral medication to help the dog never contract heartworm disease again.

Prevention

The three most popular preventive treatments on the market today are: Hearthgard (Merk), Interceptor (Ciba) and Filaribits (Norden). All are taken orally. The active ingredient in Heathgard is ivermectin. It comes in tablet or chewable cube form and is given once a month. Ivermectin should be used with caution in colleys, although it appears that the very low doses used in Hearthgard should not cause toxicity. Interceptor is the latest drug on the market for the prevention of heartworm. The active ingredient is milbemycin. It is also given once a month and has the advantage of also controlling hookworms. Hearthgard Plus also contains broad-spectrum dewormers. Research carried out by the Interceptor company indicates no toxicity at doses fifteen to twenty times higher than the recommended dose, even in colleys. Filaribits is rarely used, as it has to be given once a day.
All these products are highly effective if used correctly. They are not a treatment for heartworm disease. It is essential to test the dog for heartworm, as these drugs can make the animal very ill if it tests positive.
The duration of preventive treatment depends on the length of the mosquito season. It is generally agreed that the animal should be on preventive treatment from one month before the mosquito season to one month after. In the Quebec region, we give six tablets (one for each month) from May 1 to November 1. Once again, it's essential that the dog be tested before starting treatment.

Vivianne Venisse MDV