The severity of the infection depends on many factors including the number of mature worms, the length of the infection, and a dog's activity level. (pethealthnetwork.com)
Cats may only have a few mature worms or might only be infected with immature worms. (waterlooanimalhospitalok.com)
To avoid accidentally causing mature worms to die off, a heartworm test (4Dx) is administered annually, prior to refilling your heartworm prescription. (veterinariannephiladelphia.com)