What Is Dirofilariasis and Why Is It Dangerous For Your Dog?

Dirofilariasis is potentially deadly disease which is caused by Dirofilaria immitis (heartworm) – parasitic worms from the group of nematodes (roundworms).

It infects dogs, cats and other mammals like wolves, foxes, ferrets and, in rare cases, people.

The dirofilariasis occurs in all areas of Bulgaria as the frequency of the disease is up to 16% (1, 2) of the population of dogs and the problem escalates more and more because of the low level of prophylaxis.


When in the dog are living adult worms they multiply and release into the blood microscopic larvae /microfilariae/.

If a mosquito bites a dog with heartworms, along with the blood it takes in the microfilariae, too.

Over the next 10-14 days the taken microfilariae are developing in the mosquito to larvae which are able to infect a new host.

Through biting the mosquito transfers the larvae to a new dog.

The larvae migrate into the circulatory system of the dog. For 6-7 months the larvae reach the heart and the pulmonary arteries where they develop into adult parasitic worms which can produce microfilariae.

Heartworms reach 30 centimeters in length and can live in the dog from 5 to 7 years.

In that way every season they are reservoirs for spreading the disease.



Dogs can develop symptoms of the disease years after being infected. Until then, however, the heartworms slowly damage the lungs and the heart.

Symptoms of the disease can include mild but persistent cough, denial of motion, weakness even after light activity, decreased appetite, weight loss.

Gradually, the blood flow through the affected arteries renders difficult and may lead to heart failure. The most common symptom of this is the collection of fluid in the abdomen and bloating.

If the number of the parasites is too large, it may lead to blockage of the blood flow through the heart and the lungs which is life-threatening and it is known as the Shepherd’s pipe syndrome.

Its symptoms may include sudden difficulty in breathing, pale gums, dark red or coffee-colored urine and unwillingness or inability to move.

Most dogs with Shepherd’s pipe syndrome don’t survive without surgical removal of the parasites.


The presence of the parasites is detected by a blood test. Despite this, however, in case of suspicions it may need additional ultrasound, X-ray and to repeat the blood test to catch the disease.


Testing for heartworms should be done once a year at the beginning of each prophylactic season (March-April) to ensure that the prophylactic was effective.

Sometimes you may need to do more than one examination per year, for example if you know that you have missed the deworming, you have changed the anti-parasite means or your dog has symptoms of dirofilariasis.

If the disease is detected at an earlier stage the treatment could be easier and with fewer complications.


Without treatment the dirofilariasis will deteriorate and it will lead to serious diseases. Unless there are medical contraindications for treatment, it should be conducted.

The treatment, however, also hides certain risks for the dog as the dead parasites can cause further damage to the lungs and the pulmonary arteries.

Before the treatment it should be performed a detailed examination, X-ray and blood tests to evaluate the level of the risk to the dog.

The treatment lasts several months and during that time the activity of the dog should be limited /caging except for short walks/. Otherwise there is a risk of complications or even death because the disintegrating parasites into the bloodstream are causing inflammation or blockage of blood vessels.

Even though the medication is very effective against adult heartworms some dogs may not be completely healed with one course of treatment.

The dog should be tested for dirofilariasis 6 months after following completion of the treatment to prove that all the parasites are killed. If the tests are still positive, it may be necessary a subsequent treatment.


The contraction of humans with Dirofilaria Immitis rarely leads to disease because the larvae cannot develop into adult parasites.

Much more serious is the infection with Dirofilaria Repens (subcutaneous dirofilariasis). According to recent data in Bulgaria (2) infection in dogs with Dirofilaria Repens reaches 7-8% which creates a real risk for people and it imposes strict prophylactic.


On the recommendation of the ESCCAP (European Scientific Council Companion Animal Parasites) in the endemic areas of the disease such as Bulgaria prophylactics is mandatory once every 30 days, at least from April to November.

The prophylactic is easy!

It is important, however, not to miss deworming!

There is already an easy solution for that!

Only one tablet monthly in the form of a tasty morsel will protect your dog simultaneously against dirofilariasis, ticks, fleas and gastrointestinal nematodes (worms, hookworms and flagellates worms).

Your vet will give you additional information and will test your dog for the presence of heartworms before the start of prophylactics.

(1) Dr. Kirova and team, Faculty of Veterinary Medicine, Thracian University, Stara Zagora, 2007
(2)Dr. Milen Kostadinov, Veterinary clinics Novet, a national survey from 2011

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4 thoughts on “What Is Dirofilariasis and Why Is It Dangerous For Your Dog?

    1. Вашият ветеринар най-правилно може да ви насочи към медикамента и как да се използва и кога за най-добър ефект 🙂

  1. Ако човек има пряк контакт с вече заразено куче от пети стадий, по точно контактът е с кръв от прорез на рана. Целите ми ръце бяха изцапан има ли опасност да се заразя,и какво трябва да предприема?

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