The onset and severity of disease in the pet is principally a manifestation of the amount of adult heartworms present, the era of the disease and the level of activity of the dog. Dogs with higher numbers of worms are generally found to have more severe heart and lung disease changes. Until the number of mature heartworms exceeds 50 in a 25-kg dog (approximately 55 pounds), nearly all the heartworms live in the lower caudal pulmonary arteries (the arteries of the lower lung lobes). Higher quantities of heartworms result in their existence in the ideal chambers of the heart. In such infections, the most common early pathological changes brought on by heartworms are due to inflammatory processes that exist in and around the arteries of the lower portion of the lungs in response to the presence of heartworms. Later, the heart may enlarge and become weakened due to an increased workload and congestive heart failure may occur. A very active dog (e.g., working puppy ) is more likely to develop serious disease with a relatively 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 at the center, but also in the caudal vena cava (large main vein of the lower body) involving the liver and the heart. If the heartworms aren't removed surgically, this syndrome causes sudden collapse and death within two to three days.Canine heartworm disease is broadly distributed throughout the United States. Heartworm infection was found in dogs native to all 50 states. All dogs, irrespective of their age, gender, or habitat, are susceptible to heartworm disease. The maximum infection rates (up to 45 percent ) in puppies (not maintained on heartworm preventive) are observed in 150 kilometers of the Atlantic and Gulf coasts in the Gulf of Mexico into New Jersey and along the Mississippi River and its major tributaries. Additional regions of the United States might have lower incidence rates (5 percent or less) of canine heartworm disease, though some areas have environmental, mosquito population and pet population variables that allow a greater neighborhood incidence of heartworm infection. Regions where heartworm disease is common possess diagnosed infections in puppies as young as one year of age, with most areas diagnosing illnesses chiefly between the ages of eight and three decades. Although there are differences in frequency of disease for a variety of groups of dogs, all dogs in all regions should be considered in danger, placed on prevention programs and also often examined by a vet.
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