The American Heartworm Society (AHS) recommends annual heartworm tests for all dogs, even those who receive regular prevention. This is because there are cases where the preventive may not be fully effective, and the test will help detect heartworms in the early stages when they are easier to treat.
If your dog has been off prevention for a while, there is a chance that they could have contracted heartworms. Giving prevention medication to a dog with undetected heartworms can be dangerous and potentially lead to serious health complications.
By skipping the test and restarting prevention without knowing the status of your pet's heartworm status, you may be setting yourself up for a potential problem down the road. If your dog were to develop heartworms later on and you hadn't tested, you may be liable for the cost of treatment, which can be expensive.
It is always better to be safe than sorry. For the best protection for your pet and peace of mind, it's recommended that you have your veterinarian perform a heartworm test before resuming prevention.