I dont understand why your incidence rate tied to the percent who participated in the survey tells you anything about what percent purchased something. Choice to participate in a survey, or how many you decide to sample does not tell you anything about what percent chose another action (to purchase). You need to look for actual data on what percent purchased and owned pets.
The answer to your question depends on your research question. is the population you are interested in total canine households or those that purchased meds? Statistics won't tell you that, or research design. It depends what you want to know