Pre-existing conditions are not covered by pet insurance companies. Pre-existing conditions are problems or diseases that your pet may have shown symptoms of or been diagnosed with before the effective date of the policy. Most insurance companies will use the date you first noticed the problem, not necessarily the day it was diagnosed by a veterinarian. Some companies may cover a problem that occurred previously if it was "cured" and not considered a chronic condition (no symptoms or treatment within the last 6 months).
From reading reviews that policyholders write on the various companies, this is a reason that is frequently given when a claim is rejected. See previous post. How can you avoid this? Buy a policy when you first acquire a puppy or kitten and/or before your pet manifests any signs of illness. There will be a waiting period for illnesses - usually 2 weeks or 1 month (varies depending on the company) and 24 to 72 hours for accidents. If your pet gets sick or injured before you purchase a policy or during this waiting period, it is unlikely the condition will be covered. Clients suddenly become interested in pet health insurance when they are in my office and their pet has a serious problem. Unfortunately, that's not the best time to inquire about it.
During the application process, you will usually have to answer several questions about any previous problems your pet may have had. You should be completely honest when answering these questions. Knowingly misleading the insurance company about your pet's previous problems is called fraud and is grounds for the policy to be cancelled. Depending on your answers to these questions, the insurance company may issue a policy on your pet with no exclusions or they may request further information from you and/or request your pet's past 12 - 24 months medical records. Even if you aren't required to send in medical records during the application process, you will likely be required to send in medical records when you file the first claim. If you have forgotten to mention something during the application process, it may become evident when the company reviews the medical record and a condition could be considered pre-existing and excluded from coverage.
Therefore, you should tell the insurance company during the application process that you want to know in writing if there are any conditions that will be excluded from coverage and for how long because they are considered pre-existing. Most of the insurance companies will do this if you make this request, and it is worth asking about so that there aren't any surprises down the road. The last thing you want to do is pay several months/years of premiums only to find out that a claim is denied because the insurance company considers a condition pre-existing before you bought the policy.
The goal is transparency on your part to reveal any known-about prior medical problems to the insurance company and transparency from the insurance company to reveal (when the policy is initially written) what, if any, pre-existing medical problems are excluded from coverage. If one or more conditions are excluded from coverage and you elect not to insure your pet, you can usually cancel the policy for a refund of premium as long as you haven't filed a claim.
If your pet is older when you apply for a policy, the insurance company may request your pet's medical records to review and/or require a physical exam and/or lab testing to make sure your pet doesn't have a chronic condition that would preclude coverage for illnesses.