Heartworm in Cats: 6 Months, 1 Dose a Month, 2 Tests

A heartworm infection starts with one mosquito bite and ends with 1-3 adult worms in the pulmonary artery about 6 months later. There is no approved adulticide for cats, so the whole strategy is prevention: a prescription product every 30 days, for 6 months in low-risk areas and 12 months where mosquitoes never stop. Before the first dose, two tests rule out an infection you would otherwise be feeding.

The 6-month clock

Larvae picked up by a mosquito need 10-14 days of warm weather inside that mosquito before they can infect a cat. After the bite, L3 larvae molt to L4 in about 8-12 days, then travel to the heart and pulmonary arteries and reach adulthood at roughly 170-200 days, which is why 6 months is the number every guideline uses. Females start releasing microfilariae around month 6-7, and that timing sets the earliest reliable test date. A cat infected in July is not detectable by antigen test until January or February.

Once a month, not once a season

The preventive works by killing larvae during a narrow window in the first 30-45 days after the bite, so the dosing interval is 30 days, not 60 or 90. Skipping 2 weeks leaves a gap that larvae simply walk through, and a missed month restarts the clock. In a zone with 7 or more mosquito-active months, the schedule is 12 doses a year with no break, because a 20 °C afternoon in December is enough for a mosquito to feed. Your vet chooses the product and the strength under prescription; do not split, halve or borrow a dog product, since several dog formulations are toxic to cats.

3 risk zones, 3 schedules

The zone depends on how many months your local mosquitoes stay active, which tracks nights above roughly 15 °C. Count the months you actually see mosquitoes at dusk, then match the row below, and add 1 extra month of cover at each end if the cat goes outside.

Zone	Mosquito-active months	Doses per year	Tests per year	Earliest test after season (weeks)
A: high, warm nights	7-12	12	1	6-8
B: moderate	4-6	8-10	1	6-8
C: short, cool summer	1-3	6	1	8
Cat travelling to zone A	7-12	12	2	8
Zone A plus outdoor access	7-12	12	2	6-8

The 2 tests, and what each one misses

Test one is the antigen test, an ELISA on 0.5-1 mL of serum or plasma that detects a protein from adult females; it turns positive 5-7 months after infection and can read false negative with only 1-3 worms, which is exactly the burden most cats carry. Test two is the modified Knott or a direct smear for microfilariae, which needs 1 mL of EDTA blood, takes 10-15 minutes to run, and confirms that adult females are present and reproducing. Roughly 20-30% of infected cats are microfilaria-negative, so the pair is stronger than either alone and neither replaces a chest radiograph when signs are present.

Why test before the first dose

Giving a preventive to a cat that already carries adult worms does not treat the infection, and in a microfilaria-positive cat it can trigger a reaction as dying larvae load the circulation. Standard practice is antigen plus microfilaria testing first, then starting the monthly product 30 days later if both are negative. A positive result changes the plan to monitoring, cage rest and managing the airway, with recheck imaging at 6-12 months because some cats clear the infection on their own.

Mosquito math around the house

A female mosquito takes a 3-5 microliter blood meal and may bite 3-5 times before she is done, so one insect at the window is not one exposure. Screens of 1.5 mm mesh or finer block most species, standing water in a 2 cm saucer or a 5 L bucket breeds larvae in 7-10 days, and dusk between 19:00 and 21:00 is peak feeding time in July. A cat that never goes outside still meets mosquitoes on a balcony 20 minutes a day; that is why indoor cats appear in clinic reports at all.

What infection looks like, and why prevention is the plan

Most infected cats carry only 2-3 worms, and the damage comes from the worms dying rather than living, which is why sudden death can be the first sign in a cat that looked normal 24 hours earlier. Common findings are a soft cough, vomiting that is not hairballs, and exercise intolerance over 4-8 weeks. With no approved adulticide, care is supportive: corticosteroids for the airway, oxygen when needed, and restricted activity for 4-6 weeks to lower the risk of thromboembolism. Repeat antigen testing at 6 and 12 months tells you whether the cat cleared the worms.

Adjacent summer risks that travel with this one

Three other hazards ride along with mosquito season. If the cat shares a paddling tray with the family, the water play guide covers the rinse-and-dry routine that keeps a 2 cm puddle from becoming a nursery. Ticks arrive on the same dusk walks, and the tick removal walkthrough gives the 2 mm mouthpart rule and the 24-hour window for pulling one off. Before you spray anything for mosquitoes, check the toxin lookup: permethrin, common in dog spot-ons and yard sprays, is lethal to cats at a fraction of the dog dose.

5 steps for mosquito season

• Count mosquito-active months at dusk and pick the zone row that matches.
• Set a phone alarm for the same date every 30 days, not every 60.
• Test antigen plus microfilaria before the first dose, then yearly.
• Screen windows with 1.5 mm mesh and empty standing water every 7 days.
• Keep the cat indoors from 19:00 to 21:00 in July and August.

Terms mentioned in this article

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