How many dogs per year get infected with heartworm?

I was just wondering an actual number of dogs that become infected with heartworm per year? I read somewhere that about 250,000 dogs are diagnosed with heartworm per year, which is actually pretty small, given the dog population is about 60,000,000 dogs in the US. So how many dogs really get heartworm each year?

Canine heartworm infection is widely distributed throughout the United States. Heartworm infection has been found in dogs native to all 50 states. All dogs, regardless of their age, sex, or habitat, are susceptible to heartworm infection. The highest infection rates (up to 45%) in dogs (not maintained on heartworm preventive) are observed within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries. Other areas of the United States may have lower incidence rates (5% or less) of canine heartworm disease, while some regions have environmental, mosquito population and dog population factors that allow a higher local incidence of heartworm infection. Regions where heartworm disease is common have diagnosed infections in dogs as young as one year of age, with most areas diagnosing infections primarily between the ages of three and eight years. Although there are differences in frequency of infection for various groups of dogs, all dogs in all regions should be considered at risk, placed on prevention programs and frequently examined by a veterinarian.

I work in a vets office and we see quite a few dogs that come in with heartworms. dogs can get it over and over again if its not being treated monthly by using a heartworm medication. Alot of people also dont have 90 bucks laying around for the heartworm medicine and that number can vary greatly. there truely isnt a way to know how many dogs get heartworms in a year because vets dont have to write it down or keep track of it. they write it in there chart and thats it.