Offspring can be discovered in the blood (pre-patent period) approximately six to seven months after the infective larvae from the mosquito go into the dog. The male heartworms (four to six inches in length) and the females (10-12 inches) become fully grown about one year after disease, and their life span in dogs appears to average around five to seven years.In experimentally induced infections of heartworms in dogs, the proportion of infective larvae developing to adults is high (40% to 90 percent ). On the other hand, the percentage of infected dogs out of which mature worms have been recovered is virtually 100 percent. The amount of worms infecting a dog can be very large, with the amount of worms in puppies ranging from one to approximately 250.
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