When we think of our beloved dogs, who hold a special place among our pets, we often remember the joyful and healthy moments we’ve shared with them. However, as dog owners, we are also aware that they can face various health problems. But are we conscious of how crucial early diagnosis is for these conditions?

The focus of this blog, heartworm infection, is one of those diseases where early detection is extremely important, and it holds particular significance for dogs.

So, what is heartworm disease in dogs?

Canine heartworm disease is a parasitic disease commonly seen worldwide. Although the adult form is known as heartworm, it primarily settles in the pulmonary arteries. The disease is caused by Dirofilaria immitis, a nematode transmitted by various mosquito species acting as carriers.

What is Dirofilaria immitis?

Dirofilaria immitis is a nematode parasite transmitted through the bite of mosquitoes, particularly from the genera Culex, Aedes, and Anopheles, which serve as intermediate hosts (vectors). While dogs are the primary hosts, it can occasionally infect cats, and more rarely, horses, humans, and sea lions. The parasite goes through several larval stages (L1, L2, L3, L4, L5) during its life cycle.

What is the Pathophysiology of the Disease?

The disease actually begins when infected mosquitoes deposit third-stage larvae into the dog’s skin while feeding on its blood. These L3 larvae migrate through the subcutaneous, muscles and connective tissue, developing into the L4 stage. By around the 70th day, the migrating larvae reach the pulmonary artery, where they mature into adult forms and grow to an average length of 10 cm. Once the adult parasites reach sexual maturity, the females release microfilariae (L1) into the bloodstream, which can be ingested by uninfected mosquitoes. Through this blood-feeding process, the life cycle of the worms is completed via the infected mosquitoes.

Heartworms also harbor an endosymbiotic bacterium called Wolbachia spp.. Eliminating these bacteria has adverse effects on heartworms, and it is thought that Wolbachia triggers an immune response in the host as well.

Life Cycle of Dirofilaria immitis

The worms residing in the dog’s circulatory system cause pneumonitis and thickening of the pulmonary artery walls, leading to inflammation in the blood vessels. As these worms die within the circulatory system, they cause thromboembolism, blocking the pulmonary artery and its branches. As the disease progresses, lung lesions occur, and in more severe cases, congestive heart failure can develop.

What Are the Clinical Symptoms in Dogs?

Most dogs infected with Dirofilaria immitis do not exhibit clinical symptoms. However, the following signs may appear:

• Typical respiratory symptoms such as coughing, tachypnea, and dyspnea

• Intolerance to exercise and physical activities

• Loss of appetite accompanied by weight loss

• Syncope (fainting)

• Ascites

• Hemoglobinuria

• In more severe cases, Caval syndrome (a condition caused by the blockage of the superior vena cava)

Diagnosis of the Disease

Heartworm infection is often detected during routine annual check-ups conducted with awareness. There are highly sensitive diagnostic tests available that detect female antigens in the circulatory system. However, because these tests target female antigens, if only male parasites are present or if the parasite load is very low, the test may yield a negative result.

Microfilariae (L1) can be detected in blood smears or directly in peripheral blood using the Knott technique.

While urine analysis, blood tests, and thoracic X-rays are primarily used to assess the severity of the disease, adult worms can sometimes be seen directly on thoracic X-ray images. Additionally, differential diagnosis can be made by identifying larvae in stool samples, based on the patient’s medical history and observed symptoms.

What Should Be Done in Positive Cases?

It is important to remember that dogs diagnosed with heartworm can recover with early diagnosis and proper treatment. Pet owners must take their animals for annual health check-ups and closely monitor them. The goal of treatment in positive cases should be to eliminate the adult parasites and prevent further damage to the lungs, pulmonary vessels, and heart.

For more detailed information about treatment protocols and preventive measures, consulting a veterinarian is the most appropriate course of action.

This blog post has been verified by Doctor of Veterinary Medicine Elif KENAR.

PHOTOGRAPH

TOPARLAK, M. TÜZER, E. (2012) VETERİNER HELMİNTOLOJİ. İstanbul, Türkiye: Müfit TOPARLAK, Son erişim tarihi: 22.03,2024

SOURCES

MERAL, Y., BAKIREL, U., & ÇAKIROĞLU, D. (2007). Echocardiographic Diagnosis of Heartworm Disease (Dirofilaria immitis) in a Dog. Journal of Istanbul Veterinary Sciences(3), 1-10.

SCHAER, M. GASCHEN, F. (2019) Köpek ve Kedilerin Klinik Hekimliği. Konya, Türkiye: Nuri ALTUĞ, Son erişim tarihi: 22.03.2024

TOPARLAK, M. TÜZER, E. (2012) VETERİNER HELMİNTOLOJİ. İstanbul, Türkiye: Müfit TOPARLAK, Son erişim tarihi: 22.03,2024