The canine heartworm or the worm heart disease is a parasitic disease caused by nematode worms transmitted through mosquito bites. It can be fatal if it is not detected in time, so early diagnosis is essential. We explain how to recognize the first symptoms in your dog and the most effective prevention methods.

What is filariasis?

Filariae are a group of parasitic nematodes that affect domestic and wild carnivores, especially canids. There are species of filariae that infect our dogs, such as Dirofilaria repens or Dipetalonema recondite, settling in the subcutaneous tissue. 

Still, Dirofilaria immitis or ‘ heartworm’ is undoubtedly the most damaging filaria since it is located in the pulmonary arteries and the right ventricle of the heart, causing absolute havoc at the cardiorespiratory level and even the death of the animal.

How the canine heartworm parasite is transmitted?

Canine heartworm disease is transmitted through the bite of a vector, mainly mosquitoes of the genus Culex. This means that direct contagion between dogs is not possible. The presence of the vector is necessary. 

Mosquitoes bite the sick individual, ingesting larvae called microscopic microfilariae. These microfilariae evolve within the vector, becoming infective larvae, which are inoculated with the mosquito’s saliva on the skin of a healthy dog.

The larvae migrate through the subcutaneous tissue and muscle until they reach the bloodstream. They feed on the blood components and develop until they reach the adult worm stage, which is located in the pulmonary arteries and, in the final stages of the disease, in the right ventricle of the heart.

This phase from when they are inoculated into the dog until they are adults is the incubation period, and it can last up to 6-8 months. Adult worms can measure up to 18 cm (males) or 30 cm (females). 

When they copulate, they generate microfilariae that are distributed throughout the circulation, being able to be ingested again by a mosquito when biting the infected dog, thus closing the heartworm’s life cycle. Microfilariae have a life span of between two and 18 months, and adult worms can live up to seven years.

Factors that predispose your pet to heartworm

Heartworm disease is common in southern regions of Europe, including Spain, Italy, or Greece. 

Dirofilaria immitis is more present in some specific geographical areas, considered hyperendemic due to the high population density of the transmitting mosquito, such as Tenerife, Huelva, Zaragoza, Salamanca, or Menorca.

Usually, the incidence is higher in the spring and autumn seasons, although as a result of climatic changes, the transmission period is being extended, and the prevalence zones are expanding.

Furthermore, the disease is being detected for the first time in regions of northern Europe due to the movements of animals from endemic countries, whether for tourism or adoptions of homeless dogs. Therefore, it is imperative to rule out the infection by testing before acquiring a pet or sending it abroad.

Heartworm Disease

Symptoms and Diagnosis of Heartworm Disease

The canine heartworm or worm heart is a disease of dogs that can become serious and fatal if not detected and treated early. Although its name suggests cardiac symptoms, heart failure only occurs in advanced cases and is essentially a pulmonary disease since the location of adult worms in the pulmonary arteries.

Symptoms of canine heartworm disease

The initial symptoms presented by the pet with heartworm disease are:

  • Cough chronic.
  • Moderate to severe dyspnea (shortness of breath).
  • Syncope after exercise or excitement.

Advanced heartworm symptoms in dogs are:

  • Symptoms of congestive heart failure: ascites, vascular oedema, anorexia, weight loss, dehydration
  • Heart murmurs and arrhythmias (atrioventricular fibrillation).                 
  • Thromboembolism due to worm death: causes dyspnea and hemoptysis.
  • Pulmonary hypertension (vena cava syndrome) during the migration of worms from the pulmonary artery to the right ventricle: generates dyspnea, murmurs, hemolysis, hemoglobinuria (dark urine), and death.

How to detect heartworms in your dog

Dogs with heartworms can remain asymptomatic for a long time, but during this period circulating microfilariae in the blood or adult heartworm antigens can be detected so that effective treatment can be started as soon as possible. 

  • Direct observation of microfilariae: A blood sample is taken, and a blood smear is made to observe the microfilariae under the microscope directly. Previously, microfilariae could be filtered or concentrated to be more easily detect. A negative result does not rule out infection since 30% of dogs infected with adult worms do not have microfilariae. 
  • Detection of heartworm antigens: using ELISA or immunochromatographic tests (rapid tests), the presence of adult heartworms can be detected in the blood. The result may be harmful if the incubation period is not completed (6-8 months after infection).
  • Imaging: In the advanced stages of the disease, changes in the lungs and heart can be seen on x-ray. Cardiac arrhythmias are detected by electrocardiography.