Microfilaremia, the presence of heartworm offspring in the blood of infected dogs, is comparatively common. However, not all of heartworm infections cause these offspring circulating in the blood. These are referred to as occult heartworm infections and might be the effect of a number of factors like single sex heartworm infections, host immune reactions affecting the existence of circulating offspring (microfilariae) and most importantly, the management of heartworm preventives.The onset and severity of disease in the pet is principally a reflection of the amount of adult heartworms present, the age of the disease and the amount of activity of the dog. Dogs with higher numbers of worms are usually found to have more severe heart and lung disease affects. Until the number of mature heartworms exceeds 50 at a 25-kg puppy (approximately 55 pounds), almost every one of the heartworms reside in the reduced caudal pulmonary arteries (the blood vessels of the lower lung lobes). Higher numbers of heartworms lead to their own existence in the right chambers of the heart. In such ailments, the most common early pathological changes caused by heartworms are due to inflammatory processes that occur 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 diminished 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 relatively small number of heartworms compared to an inactive one (e.g., a lap dog or couch potato). Occasionally, a dog having a large number of heartworms may not only have worms at the center, but also from the caudal vena cava (large main vein of the lower body) between the liver and the heart. In the event the heartworms aren't removed surgically, this syndrome causes abrupt collapse and death within two to three days.
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