Clinical judgment required. This calculator is a decision-support tool, not a substitute for veterinary consultation.

Heartworm Susceptibility Window CalculatorCanine

Calculate the heartworm (Dirofilaria immitis) infection susceptibility window for dogs. Given a last negative antigen test date and a first positive antigen test date, estimates the earliest and latest possible dates the dog was bitten by an infected mosquito. Uses the prepatent period of heartworm: antigen may appear as early as 150 days (5 months) post-infection, and is reliably detectable by 210 days (7 months). Helps determine the timeframe of mosquito-transmitted infection for treatment planning with melarsomine (Immiticide) adulticide protocol, doxycycline pre-treatment, and macrocyclic lactone (ivermectin, milbemycin, moxidectin, selamectin) preventive restart. Also estimates when immature adult worms arrived at the pulmonary arteries (70–120 days post-infection). Relevant for: heartworm positive dog, new heartworm diagnosis, heartworm treatment timeline, prevention gap analysis, missed heartworm prevention dose, slow-kill vs fast-kill protocol timing, American Heartworm Society guidelines, three-dose melarsomine protocol scheduling. Common queries: 'when was my dog infected with heartworm' or 'heartworm infection window calculator' or 'how long has my dog had heartworm'.

Last Negative Antigen Test Date

First Positive Antigen Test Date

Earliest Possible Infection Date

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Latest Possible Infection Date

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Susceptibility Window

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Earliest Adult Worm Arrival

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Latest Adult Worm Arrival

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Preventive Efficacy Cutoff

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120 date

240 date

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