What percentage of dogs infected with heartworms are over 2.5 years?

• A. 60%
• B. 80%
• C. 50%
• D. 90%

Answer: (B)

A key idea here is that infection risk for heartworms accumulates as dogs live longer in an area where mosquitoes transmit the parasite. Heartworms live for several years in a dog, so once a dog becomes infected, the infection can persist and be detected several years later. Because of this, the pool of infected dogs is skewed toward older ages; most infected dogs have had enough time to be exposed and develop a detectable infection. That tends to place a large majority of infected dogs above 2.5 years, and about 80 percent is the typical estimate used. The other values would under- or overstate how age relates to infection: fewer than 80 percent would underrepresent the accumulated risk with age, while a much higher figure would ignore the presence of some younger-infected dogs and ongoing new infections.