Internal parasites Infestation of internal parasites such as roundworms and heartworms can cause coughing. Roundworms live in the canine intestine. The dog can become infected by licking or eating soil contaminated with eggs. As they migrate up the windpipe, the larva can cause bouts of coughing. Puppies can be born with roundworms if the mother is infested.
Pregnant bitches can carry encysted roundworm larva in their tissues.
In the last stages of pregnancy, the larva are emitted, enter the bloodstream, and travel to the puppies through the placenta. Worming the mother before birth is ineffective against encysted larvae. If a dog is in good condition, a light infestation of roundworms is seldom a problem. Heartworms are transmitted to dogs by mosquitoes. The mosquito bites the infected dog, ingests the tiny heartworm microfilaria, provides a host for the next stage of development of the parasite, then transfers the juvenile larvae to the bloodstream of the next dog she feeds on.