While we think that heartworm might be from drinking bad water or eating bad food, the necropsy report from a dog that died of heartworm had no evidence that the heartworms were in the stomach, where water or food would go.
So students thought that maybe a dog would choke on the water or food, and it would enter the windpipe connected to the lungs, where the necropsy report did find evidence, but rather in the arteries. We first discussed how rare it would be for all the dogs in Pooch Province to choke on water or food, and secondly, the arteries didn't show any evidence of having damage to them, and the mouthparts of a heartworm don't look like they'd be able to "chew" through an artery. This basically gave us the info we needed to suggest it couldn't come from drinking or eating the heartworms. We also thought that the dog would have seen such large worms in his water or food, and this would deter the dog from eating it.
We also discussed how maybe the dogs are getting the heartworm from cuts, but our necropsy report didn't reveal any problems with the dog's skin (either actual wounds or scars). We also think that it's highly unlikely that the number of dogs with heartworm would also be dogs that are wounded with blood leaking out and then touching another dog. We discussed how most people with dogs are responsible, and out of our classes, most people said that when their dog has a cut, they took care of it immediately and wouldn't let their dog be active until it was healed.
This means we settled on the mosquito/tick/flea idea, which means that some sort of insect/bug is putting the heartworm into the blood of the dog it bites. We still have some questions about this, as mosquitoes are pretty small...which means that the heartworm has to be even smaller than the mosquito. We're not really sure if a worm starts as an egg, or as a really small worm, but we're thinking it certainly has to be smaller.
We came to consensus that the heartworm is entering the dog's blood through some other organism, most likely the bug that bites it.