This age old saying of which came first, the chicken or the egg is simplistic but puzzling for many. Even more puzzling to me, however, are many of the parasite life cycles. Even the roundworm of dogs is amazing. When someone brings in their new puppy for a physical examination I always recommend deworming medication. People often question this recommendation stating they hadn't seen any worms. Surprisingly, however most puppies have roundworms but may not be passing the microscopic eggs or adult worms so people usually do NOT see anything.
The dog roundworm developed a unique strategy of survival. An dog infected with roundworms passes microscopic eggs in their feeces that stay dormant until another dog ingests them - on grass or wherever they ended up. Once swallowed, the larvae travel from the intestine throughout the body. In many cases, especially if it is a young dog, they end up in the lungs, are coughed up and swallowed, and then become adult worms in the bowels.
That sounds complex enough, however, even more interesting is that in many adult dogs, these migrating worms sit dormant in cysts in the dog's muscles. When the dog becomes pregnant, these worms migrate out of their cyst and through the uterus into the unborn puppy. The puppy is born with worms and until these worms mature they may not be passing eggs, hence the reason for advising routine regular worming of puppies even if the caregiver is not seeing worms. As well, when eggs are passed, human exposure can result in migration of these parasites to form cysts in them as well.
Even more complex of a parasite, the Lancet Fluke, involves catle or sheep, flukes and ants. What 'fluke' of nature would allow the development of a parasite with nine separate life stages and two intermediate hosts
Once inside the sheep or cow, these flukes travel to the liver from the intestine and lay eggs that travel down the bile duct into the intestine and out. Now, with the parasite eggs on the ground along comes a snail that ingests the feces and also ingests the fluke eggs. These eggs hatch into a larval stage that is irritating to the snail so it covers them with slime and passes them out as slime balls. Along comes the ant who finds these slime balls delicious and ingests them.
Once they infects an ant, some remain infective in the ant while another one of the parasites travels to the brain of the ant affecting it's nesting behaviour changing it so that rather than have it return to its colony it climbs to the top of a grass blade and clamps on, making it more vulnerable to being eaten by sheep and cattle. Once eaten by cattle or sheep the amazingly complex life cycle continues.
Parasite life cycles fascinate me, but knowing that many can also infect people causes great concern to me as a veterinarian so remember to regularly deworm your pet.
