Heartworm
Pathogen
Dirofilaria immitis -nematode (roundworm)
Vector
Mosquitoes (e.g. Aedes, Culex, Anopheles)
Geographical distribution
Especially Southern Europe, reaching as far north as the Czech Republic, Slovakia, and Romania. Heartworm has also been observed in imported dogs in Finland. While mosquito species capable of acting as vectors for heartworm are present in Finland, the parasite cannot currently complete its lifecycle within mosquitoes under Finnish conditions (due to the lack of a sufficiently long, continuous warm period).
Transmission
Dirofilaria immitis is transmitted through mosquito bites. Its lifecycle is similar to that of D. repens. Mosquitoes ingest microfilariae (the juvenile stages of the worm) from the bloodstream of an infected dog (or less commonly, a cat) during a blood meal. These microfilariae migrate to the mosquito’s body cavity, undergoing developmental changes. Eventually, they move to the mosquito’s mouthparts and develop into an infectious larval stage. The mosquito transmits these infectious larvae into a dog (or less commonly, a cat or human) during its next blood meal. The larvae enter the host’s skin through the bite site and migrate and develop in the subcutaneous and muscle tissues over several months. From there, the larvae mature into young adults, penetrate the blood vessels, and travel through the bloodstream to the heart. Once in the heart, the young adults continue to mature for a couple of months before the female worms begin producing microfilariae, the juvenile stages, which enter the host’s bloodstream.
Symptoms
The symptoms of a heartworm infection often begin with coughing and reduced exercise tolerance. Smaller dogs require fewer adult heartworms to cause symptoms than larger dogs, as their relatively smaller heart "fills up" more easily. The condition typically progresses gradually to right-sided heart failure, with symptoms including difficulty breathing and fluid accumulation in the abdominal cavity. Worms in the vena cava can cause circulatory disturbances affecting the liver. Dead worms, whether due to medication or natural causes, can create blockages in the pulmonary vessels when they dislodge from the heart. Once they reach the lungs, they may cause localized inflammatory reactions.
Prevention
Preventive treatment with microfilaricidal macrocyclic lactones is recommended monthly during mosquito season (in Southern Europe, at least from May to November, and year-round in high-risk areas). When traveling from Finland (a non-endemic area) to Eastern or Southern Europe (endemic areas) for trips shorter than a month, the animal should be treated at least once within 30 days of exposure.
If staying in the area longer (over 1 month), the medication should be repeated every 30 days and once again within 30 days of leaving the risk area upon returning home. In some European countries, an injectable formulation offering year-long protection (moxidectin) is available and can be a useful alternative to monthly oral medications when staying long-term in high-risk areas.
Mosquito-repellent products can also be beneficial, such as the Scalibor collar (deltamethrin) or Vectra 3D spot-on solution (permethrin + dinotefuran). However, these products must NOT be used on cats!
Other considerations
Dirofilaria immitis can also infect humans, but it very rarely develops into an adult heartworm. The infection most commonly causes only localized skin reactions.
Lähteet:
Culverwell C, Uusitalo R, Korhonen E, Vapalahti O, Huhtamo E, Harbach R. The mosquitoes of Finland: update distributions and bionomics. Medical and Veterinary Entomology 2020, 35:1-29
ESCCAP 2019 (European Scientific Counsel Companion Animal Parasites). Guideline 05 Control of Vector-Borne Diseases in Dogs and Cats, Third Edition.
Saari S, Näreaho A, Nikander S. Elinympäristönä koira, koiran loiset ja loissairaudet. 1. p. Fennovet Oy, Helsinki, Suomi 2016
Saari S, Näreaho A, Nikander S. Canine parasites and parasitic diseases. 1.p. eBook, London, United Kingdom 2019