Heartworms infect your pet through a mosquito’s small bite wound. Once inside an animal, the heartworm takes at least 6 to 7 months to become sexually mature before the infection can be detected by a heartworm test.
The heartworms actually continue to grow in size after reaching sexual maturity & the females start to pass microfilaria (babies) into the blood. This is why some pets are infected with numerous worms. The mass of twisted & intertwined heartworms in dogs can serve as a significant mechanical blockage to the normal flow of blood.
Think of a garden hose. If pieces of debris block the hose, pressure builds up due to the obstruction of the flow of water. This is what happens to the heart and blood vessels when more & more heartworms congregate within the right ventricle. The smaller your pet (the host) is, the fewer worms it takes to cause a problem.
Once the heartworm becomes an adult it will continue to produce young for many years in the dog. Adults have been documented to live in the heart around 5-7 years in dogs (2-3 years in cats).