Heartworm Treatment Calculator for Dogs

How to use this heartworm treatment calculator

1. Enter the dog's weight in kg or lb and the first treatment day.
2. Record how the antigen result was confirmed, whether microfilariae were seen, and the clinical class.
3. Choose the protocol. The recommended three-injection protocol suits every dog except one in caval syndrome.
4. Set the preventive type and the tablet sizes you stock, then press Build treatment plan.
5. Print the clinic schedule for the record and give the owner the calendar with the exercise dates.

How does the heartworm treatment protocol work?

Treatment kills the heartworms in stages so the lungs are never hit by the whole worm burden at once. The preventive kills larvae under 2 months old, and doxycycline removes Wolbachia, the bacteria the worms carry. Dogs given doxycycline and ivermectin before melarsomine had milder pulmonary artery lesions and almost no thrombi [2]. In 94 treated dogs, the group given doxycycline had 6.5% respiratory complications against 19.1% without it [1].

Melarsomine kills the adult worms. One injection followed a month later by two injections 24 hours apart kills about 99% of adult worms, against about 90% for the two-injection label protocol. A one-month wait between the end of doxycycline and the first injection did not consistently worsen lung lesions in experimentally infected dogs [8]. In 35 dogs with class 1 to 3 disease treated this way, all survived and all 32 followed long term became antigen negative [7].

How are the doses calculated?

Tablets round to the nearest quarter, or half for 20 mg prednisone. The result shows the mg/kg actually delivered, flags any dose more than 20% off target, and turns red when the smallest tablet piece overdoses by more than 50%.

How strict does cage rest need to be?

Exercise is the largest risk factor for complications. Dead worms lodge in the lung arteries, and higher blood flow during activity tears the inflamed vessel walls. From the first injection until 4 weeks after the last one, the dog lives in a crate or small pen and goes out on a lead only to toilet. Lead walks then lengthen gradually, with no running or play until 6 to 8 weeks after the last injection. Food puzzles and lick mats keep the dog busy without raising the heart rate.

Which signs after an injection are an emergency?

• Coughing blood, or a sudden worse cough. Usually pulmonary thromboembolism, most often 7 to 10 days after an injection.
• Fast or hard breathing at rest.
• Collapse, fainting or pale gums.
• Red-brown urine with sudden weakness. Caval syndrome: the worms have moved into the right heart. Most dogs die within 2 days without surgical extraction.
• Facial swelling, vomiting or collapse within an hour of an injection. Anaphylaxis.

Swelling and soreness over the injection site for a few days is expected and is managed with the pain relief sent home.

When are the other protocols used?

Shortened protocol. Melarsomine starts on day 30, 3 days after the doxycycline course ends, then day 60 and 61. In 76 dogs treated this way, all were antigen negative and free of microfilariae 6 months later [5].

Label two-injection protocol. Two injections 24 hours apart with no pre-treatment kill about 90% of adult worms, and 76 to 81% of dogs become antigen negative. Any dog still positive needs treating again, so I recommend the three-injection protocol for every dog except one in caval syndrome.

Non-arsenical treatment. Monthly topical moxidectin with doxycycline killed 95.9% of adult worms in experimental beagles [3], and 21 of 22 client-owned dogs became antigen negative at a median of 234 days [4]. The worms die over months, so the dog needs strict rest the whole time. It is indicated only after a reaction to melarsomine or when another disease limits the dog's prognosis. It is contraindicated in working dogs, dogs that cannot be rested, and suspected preventive-resistant infections.

What if treatment is delayed?

If the doxycycline course was at least 5 mg/kg BID for 4 weeks and the preventive has continued without a gap, the first injection can wait up to 6 months without repeating doxycycline. If injections 2 and 3 are delayed after the first, they can wait up to 6 months, and must still be given within 24 hours of each other. Both deadlines appear under key dates.

Common mistakes

• Starting a preventive or doxycycline on a single unconfirmed antigen test.
• Giving the first preventive to a dog with microfilariae without pre-treatment and 8 hours of observation.
• Letting rest slip in the weeks after injection 3, when the most worms are dying.
• Giving injection 3 into the same side as injection 2.
• Using aspirin to prevent clots. It has no proven benefit and may do harm.
• Testing for antigen too early. Antigen from dead worms lingers, so wait 9 months after the last injection.

Heartworm treatment FAQs

Veterinary review and sources

Written and reviewed by Dr Duncan Houston, veterinarian and founder of ASK A VET™. Last clinically reviewed 21 October 2026.

1. Nelson CT, Myrick ES, Nelson TA. Clinical benefits of incorporating doxycycline into a canine heartworm treatment protocol. Parasit Vectors. 2017;10(Suppl 2):515. PMID 29143657.
2. Kramer L, Grandi G, Passeri B, et al. Evaluation of lung pathology in Dirofilaria immitis-experimentally infected dogs treated with doxycycline or a combination of doxycycline and ivermectin before administration of melarsomine dihydrochloride. Vet Parasitol. 2011;176(4):357-60. PMID 21292403.
3. Savadelis MD, Ohmes CM, Hostetler JA, et al. Assessment of parasitological findings in heartworm-infected beagles treated with Advantage Multi for dogs (10% imidacloprid + 2.5% moxidectin) and doxycycline. Parasit Vectors. 2017;10(1):245. PMID 28526088.
4. Ames MK, VanVranken P, Evans C, Atkins CE. Non-arsenical heartworm adulticidal therapy using topical moxidectin-imidacloprid and doxycycline: a prospective case series. Vet Parasitol. 2020;282:109099. PMID 32450463.
5. Carretón E, Falcón-Cordón Y, Falcón-Cordón S, et al. Variation of the adulticide protocol for the treatment of canine heartworm infection: can it be shorter? Vet Parasitol. 2019;271:54-56. PMID 31303204.
6. Savadelis MD, Day KM, Bradner JL, et al. Efficacy and side effects of doxycycline versus minocycline in the three-dose melarsomine canine adulticidal heartworm treatment protocol. Parasit Vectors. 2018;11(1):671. PMID 30587225.
7. Romito G, Pane E, Guglielmini C, et al. Efficacy and tolerability of the American Heartworm Society therapeutic protocol in dogs affected by heartworm disease without caval syndrome. J Small Anim Pract. 2024;65(1):39-46. PMID 37888802.
8. Moorhead AR, Evans CC, Sakamoto K, et al. Effects of doxycycline dose rate and pre-adulticide wait period on heartworm-associated pathology and adult worm mass. Parasit Vectors. 2023;16(1):251. PMID 37491306.