Did you ask your vet for help on that question? I have several friends that take in strays. The vet provides the ivomec.

The best thing to do is ask your vet for assistance.

Ivermectin is not generally considered dangerous to give dogs that are heartworm-positive, although it is possible that they'll have a bad reaction if they have a lot of circulating microfilaria, according to what my vet says. So the best idea is still to test them before starting it.

It will not cure them of heartworm infection, but it can keep them from getting even more heartworms (ivermectin kills the microfilaria). If the dog doesn't have a bad infestation, with the parasite's lifespan being a few years, it's possible after that time to end up with a heartworm-free dog. It can be better than doing nothing if conventional treatment isn't an option. Heartworm infestation does DAMAGE to the dog, and ivermectin (Ivomec) won't undo the damage. Please CONSULT A VET about this.

Standard preventative dosage that I have been told by several vets over the years is one-tenth of a cc of Ivomec (which is a brand name) per ten pounds, given orally once a month. I do not have Collies or herding types, some individuals of those breeds can be sensitive to ivermectin, and I have never had problems with this dosage.

Use a needle and syringe to withdraw the correct amount, remove the needle and squirt into the dog's mouth. My dogs won't take a treat with the stuff on it.

Air and light are enemies of Ivomec. Keep the rubber stopper tightly in the bottle (which is why you need a needle and syringe) and store the bottle in a cool, dark place. It does not need refrigeration.

The old medicine that was dangerous to give to heartworm-positive dogs was DEC, or diethylcarbamazine, also known as Filaribits. It was given daily. I don't know how or why the danger of giving that old medication to positive dogs got carried over to ivermectin, but it did. DEC hasn't been in common use, to my knowledge, for several decades.

Here is a link that might be useful: Heartworm treatment

I would NOT give my dogs the recommended dose on the Ivomec vial (.1cc/10 pounds) because that's for cattle. Dogs are not cattle. If you read Merial's information you will see that the dosage for swine is lower because swine are not cattle either. The concentration in Heartgard was designed for dogs, so I keep close to that dose, but am not overly concerned about precise accuracy because there is a lot of experience that shows mild overdoses are normally not harmful except for the breeds that have been mentioned. The precise dosage of Ivomec 1% for dogs is 0.0027 cc or ml per 10 pounds. This based on the concentration in Heartgard for dogs. I verified the concentration and dilution with a vet, a pharmacist and a chemist.