Mosquitoes and heartworm

Mosquitoes infect pets with heartworm

Heartworm: Health Risks and Prevention

Author, Stacey Kalinnikova, BVetTech for Professional Pet Sitting Etc.


What is heartworm?


Heartworms in cats and dogs, known by their scientific name Dirofilaria immitis, are worms that infiltrate the blood vessels associated with the heart and lungs.  Heartworms are spread using mosquitoes as vectors.  Adult heartworms lay larvae, called microfilariae, that are small enough to travel through and infect the bloodstream.  Mosquitos who feed on the blood of an infected animal ingest the microfilariae which are then transmitted on through subsequent mosquito bites to other animals.


After transmission, the microfilariae take approximately 6 months to mature into adult worms and travel through to the pulmonary vessels.  Adult heartworms can live for up to 5 to 7 years in dogs and 2 to 3 years in cats.  During this time, their presence within the system can cause white blood cell and platelet accumulation, damage to blood vessels, thickening of pulmonary walls, blood clots, interference with cardiac (heart) function, and reduced cardiac output.




Heartworm infection has been diagnosed throughout the 50 states in the U.S.  Risk factors include climate variation and strays or wildlife as carriers.  There are over 70 different species of mosquitoes that are capable of supporting the transmission of heartworm during feeding.  Rates of infection can vary each year, depending on the presence of mosquito activity in the area.


Symptoms of heartworm infection


Clinical signs generally do not appear until the later stages of heartworm infection.  Once the infection has progressed, common symptoms include:


  • Coughing or gagging
  • Respiratory (breathing) difficulty
  • Anorexia and weight loss
  • Lethargy
  • Difficulty keeping up with exercise
  • Vomiting
  • Pale mucous membranes
  • Pulmonary blockage due to a large number of worms leading to cardiovascular collapse


Diagnosis and routine testing


Diagnosis of heartworm involves blood tests as well as radiography and ultrasonography.  A routine annual blood test is recommended because there is still some risk of contracting heartworm despite prevention protocols in place.  Pets must also be tested for heartworm before initiating any preventative treatments as preventative medications can cause illness when given to infected animals.


Once a small blood sample is obtained, blood is measured for the presence of antigens and antibodies associated with heartworm larvae.  The initial blood test is usually performed in-clinic.  Additional samples may be sent away to a reference laboratory to confirm a positive diagnosis with complementary testing.


Radiography or ultrasound can also be used to look for the presence of heartworm.  Evidence of infection can be seen radiographically with changes to pulmonary arteries, enlargement of the heart, pleural effusion, and lesions within the lungs.




Treatment is initially aimed at stabilising the condition of the dog or cat and eliminating associated clinical signs through supportive therapy.


In dogs, if the infection is diagnosed before any extensive damage then there is generally a good prognosis.  Unfortunately, the elimination of heartworms can be a lengthy and expensive treatment.  The administration of drugs that kill the adult heartworms is usually required over several months, during which physical activity must be limited and the dog must be closely monitored.  Risks include dead heartworms causing a blockage of blood flow to the lungs.  Preventative medications are also given to kill the microfilariae and prevent reinfection.


Cats are treated with supportive therapy to control and eliminate damage caused by heartworm.  The drug administered to kill adult heartworms in dogs is not safe for use in cats.  On occasion, adult heartworms in cats can die off on their own due to natural resistance.  There can still be complications related to dead heartworms being present in the blood vessels.  Infected cats will require routine monitoring and preventative medications are given to avoid reinfection.




Prevention is the number one key to heartworm control in both dogs and cats.  Preventative treatment is recommended year-round for best control practices.  There are three types of preventative medications available – oral, topical (applied on the skin), and injectable.


Oral medications are available in the form of a pill or tablet and are given monthly.  They are often flavoured with malt or chicken to increase palatability.  Topical medication is available as a spot-on liquid treatment which is applied on the animal’s coat and permeates through the skin.  It is usually applied every 4-6 weeks, including in winter months.  Some clinics may offer an injectable prevention for dogs (not available for cats), which can be effective for 6 months.


Preventative medications are aimed at killing the heartworm microfilariae (larvae).  Strict adherence to treatment intervals is necessary as older larvae are likely to resist preventative medications and develop into adult heartworms.


A complimentary aspect to heartworm prevention is reducing mosquito exposure.  Limiting exposure can be achieved by keeping dogs and cats inside during hot afternoons and evenings when mosquitoes are most active, administering mosquito repellent, and avoiding areas of stagnant water.  Preventative medications are still required.




Heartworm infection has been diagnosed throughout the U.S. in all 50 states.  Heartworm infection can cause damage to pulmonary vessels, breathing difficulty, and reduced cardiac output.  A large accumulation of worms in the bloodstream can be fatal.


Heartworm infection is best avoided with consistent use of preventative medications.  Before any preventative medications are started, a blood test will be performed in-clinic to test for the presence of larval antigens.  Routine blood tests are then performed annually to confirm the effectiveness of preventative protocols.  Regular veterinary check-ups and a good prevention protocol are the key to heartworm control in dogs and cats.






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