First of all, where do you live? If you live in an area where there are mosquitos for a large part of the year, then nothing works like heartworm prevention. If you live in an area where you don’t have mosquitos for most of the year, you can give it for only part of the year, but you really have to pay attention to the temperatures, because that affects when heartworms are transmissible.
Monthly heartworm prevention is actually good for about 45 days, due to the particular stages in the lifecycle of the immature heartworm that it kills. They just recommend giving it monthly because it’s easier for humans to remember to give it on a particular day. So if you want to give it every 40 days instead, you can, as long as you have a foolproof way of making sure you are reminded to give it.
Your dog has possibly been unprotected half the time, so could have been infected. Some dogs that are heartworm positive go into anaphylactic shock when given heartworm preventative, not a heart attack, but they can still die from it.
The natural heartworm preventatives have to be given multiple time a day every day, so are not really a good idea for most people.
If you want the least toxic regular heartworm prevention, then go with regular Heartgard, or an equivalent.
