Dog Internal parasites: Heartworms

Heartworms are mosquito borne internal parasites that can grow into foot long worms that lodge themselves in the right ventricle of the heart. These can cause a lot of damage to the heart as well as the lungs. Untreated heartworms are deadly. Veterinarians can prescribe a heartworm preventative that at the same may also avoid roundworms, hookworms and whipworms. Heartworms go through several life stages before they become adults infesting the heart of the host animal. The worms require the mosquito as an intermediate stage in order to complete their life cycle and so at least two animal hosts other than the mosquito are required for the heartworm to reproduce. A mosquito ingests heartworm larvae, called microfilariae, from an infected host. The mosquito then transfers the larvae to another uninfected host when next it feeds. The microfilariae then go through several changes to reach adult form, eventually traveling to the right side of the heart to reproduce. Reproduction results in the dispersal of microfilariae into the bloodstream where ingestion by another feeding mosquito spreads the microfilariae to another host.
At this stage, the host dog will likely be asymptomatic. Once the infestation reaches a certain concentration in the lungs, the now adult worms migrate from the pulmonary artery to the right side of the heart and begin to reproduce in earnest, filling the blood with microfilariae. At this point the host will begin to show symptoms of infestation. These symptoms can manifest earlier or increase in severity depending on the activity level of the animals as infestation reduces cardio-pulmonary capacity. Very active animals may experience symptoms at lower heartworm concentrations and have more severe symptoms than less active animals.
Heartworms can reach up to 12 inches in length and can remain in the host's heart for several years.
Many dogs will show little or no sign of infection even after the heartworms have matured. To some degree these dogs may be described as seeming to age slightly faster than normal as the worms slowly damage the lungs, kidneys and liver. These animals usually have a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections will quickly show the classic symptoms of heartworm disease. Early symptoms include a cough, especially on exercise, and early exhaustion upon exercise. More advanced cases progress to severe weight loss, fainting, coughing up blood, and, finally, congestive heart failure.