Microfilaremia, the presence of heartworm offspring in the bloodstream of infected dogs, is relatively common. However, not all of heartworm infections result in these offspring circulating in the blood. These are referred to as occult heartworm infections and might be the effect of a number of factors such as single sex heartworm infections, host immune responses affecting the presence of circulating offspring (microfilariae) and most significantly, the administration of heartworm preventives.The onset and severity of illness in the pet is principally a reflection of the amount of mature heartworms present, the age of the infection and the amount of activity of their dog. Dogs with higher numbers of worms are usually found to have more severe heart and lung disease changes. Until the amount of adult heartworms exceeds 50 in a 25-kg dog (approximately 55 pounds), almost every one of the heartworms live in the reduced caudal pulmonary arteries (the arteries of the lower lung lobes). Greater quantities of heartworms result in their existence in the ideal chambers of the heart. In such infections, the most frequent early pathological changes brought on by heartworms are due to inflammatory processes that exist in and around the blood vessels of the lower part of the lungs in response to the presence of heartworms. Later, the heart may enlarge and eventually become weakened because of an increased workload and congestive heart failure may occur. An extremely active dog (e.g., working puppy ) is far more likely to develop serious disease with a comparatively small number of heartworms than an inactive one (e.g., a lap dog or couch potato). On occasion, a dog with a large number of heartworms may not just have worms in the heart, but also in the caudal vena cava (large main vein of the body) involving the liver and the center. In the event the heartworms aren’t removed surgically, this syndrome causes sudden collapse and death within two to three days.