Heartworm disease

Symptoms an owner can spot at home

• A persistent soft, dry cough, often first noticed after exercise
• Tiring quickly, or a previously fit dog reluctant to keep up
• Weight loss and a dull coat as the infection becomes established
• A swollen abdomen from fluid accumulation in advanced disease
• In cats, intermittent coughing and wheezing easily mistaken for asthma, or vomiting unrelated to eating
• Frequently nothing at all in the early months, which is why testing exists

When to see a vet

• Sudden collapse, pale gums, laboured breathing, or coffee-coloured urine, which can indicate caval syndrome and is a surgical emergency
• Any persistent cough or exercise intolerance in a dog, particularly one whose prevention has lapsed
• Sudden difficulty breathing or collapse in a cat, which can be the first and only sign of heartworm in that species
• A missed dose or a lapse in year-round prevention, so testing can be timed correctly before restarting
• Before starting preventive medication in a dog over about seven months old, which requires a test first

What it is

A mosquito takes up immature heartworms when feeding on an infected animal, and deposits them on the skin of the next animal it bites. They migrate and mature over roughly six months, ending up as adult worms in the pulmonary arteries and the right side of the heart, where they obstruct blood flow and inflame vessel walls. Adult worms live for years and reproduce, so an untreated infection worsens.

Dogs are a natural host and can carry large worm burdens. Cats are not: worms often fail to reach adulthood, and a cat's disease is driven by a severe inflammatory response in the lungs rather than by worm numbers. That distinction explains why a cat with only one or two worms can suddenly become critically unwell, and why feline signs so often resemble asthma.

Prevention, which is the entire point

Preventive medication works by killing the immature stages acquired in the preceding weeks, so it is retrospective rather than protective going forward. That is why dosing intervals matter and why a late dose leaves a gap. Current guidance is year-round prevention rather than seasonal, since mosquito activity is unpredictable and compliance is far better with a routine that never stops.

• Year-round preventive medication for dogs and cats, prescribed by your vet for the species and weight.
• Annual testing for dogs, even on continuous prevention, because doses get missed, spat out or vomited.
• Testing before starting or restarting prevention in a dog older than about seven months.
• Mosquito reduction as a supporting measure: remove standing water, and keep animals in at peak mosquito activity where practical.
• For cats, prevention matters more rather than less, precisely because there is no approved treatment once infection is established.

Treatment in dogs

Treating an infected dog is possible but demanding. It involves a course of injections to kill adult worms, and strict exercise restriction for months, because dying worms break up and fragments can lodge in the lungs. That restriction is the part owners find hardest and the part that most affects the outcome. Treatment is costly, and the disease frequently leaves permanent damage to the heart and lungs even when it succeeds.

In cats there is no approved treatment to kill adult worms. Management is supportive, aimed at controlling the inflammatory lung disease while the infection runs its course. This asymmetry is the strongest argument for feline prevention, and it is routinely skipped by owners of indoor cats, who are not as protected as they assume.