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Feline heartworm: from transmission to treatment

What are heartworms?

Heartworms (Dirofilaria immitis) are parasitic worms that normally live free floating in the right ventricle of the heart and nearby blood vessels that connect the heart to the lungs. This parasite is present in both Europe and America, and as the climate in the UK warms, owners should begin to bear this parasite in mind when choosing an antiparasitic product. Some UK vets have already started to recommend that a complete antiparasitic regime should include heartworm prevention.

How do cats become infected with heartworms?

Adult heartworms lay larvae (microfilariae) which live in a cat's bloodstream for several weeks before migrating to a mosquito (when it consumes blood from an infected cat). Microfilariae cannot mature into adult heartworms unless they pass through a mosquito. In 2-3 weeks, the microfilariae develop into larger larvae inside the mosquito and migrate to the mosquito's mouth where they then enter the cat when the mosquito bites it. The larvae take approximately three months to finish their migration to the heart, where they grow into adults.

The time from when the cat was bitten by an infected mosquito until adult heartworms develop, mate, and lay microfilariae is approximately 8 months. Infected cats usually have 1-4 heartworms at any one time which will live for 2 years or less.

The symptoms relating to heartworm in cats are vague, making diagnosis difficult. Heartworm usually appears as a respiratory problem and the following symptoms are commonly seen:

  • Coughing

  • Difficulty breathing

  • Vomiting

  • Lethargy

  • anorexia

  • Listlessness

  • Loss of appetite

  • Weight loss

  • Rapid heart rate

  • Fainting

  • Blindness

  • Collapse

  • Convulsions

  • Sudden death

What damage do heartworms cause?

In cats, heartworms often cause more damage to the respiratory system than to the heart. As a result, the illness caused by a cat heartworm infestation is now often referred to as Heartworm Associated Respiratory Disease (HARD). Immature worms get to the heart and then pass into the blood vessels going into the lungs where most of them die and cause local inflammation.

Your vet will probably perform one or several of the following diagnostics to confirm the presence of heartworm:

  • X-ray to see characteristic patterns of heartworm disease in the lungs and heart.

  • Echocardiography (heart ultrasound) to look for adult worms in the heart

  • Blood test to look for traces of the worm's presence

    • One test detects antibodies in the blood, formed by the cat's immune system in response to the worm infestation.

    • The antigen test detects proteins in the blood from breakdown of the worm.

The aim of treatment is to control the severity of the damage to the respiratory tract, and to prevent the continuation of the life cycle. Most vets will place the cat on anti-inflammatory doses of corticosteroids and a monthly heartworm preventive.

Medications used to prevent heartworm infections are called preventives, these do not kill the adult heartworm, for that an adulticide must be used.  The following drugs are used in heartworm preventative methods:

  • Ivermectin

  • Milbemycin oxime

  • Selamectin

  • Moxidectin