When Parasites Attack the “Wrong” Host

Hydatid cysts from the lung. These fluid-filled spheres are also known asbladder worms.

When Parasites Attack  the “Wrong” Host Not only do we humans host a number of our own parasites, but occa sionally, worms indigenous to other animals attempt to take up residence in us. When this happens, the parasite cannot complete the life cycle in its usual  fashion,  but  in making  the  attempt,  it  can  cause  discomfort  and even death in some cases One  such  parasite  is Echinococcus  granulosus, a  tiny  intestinal tapeworm that causes hydatid cyst disease. The parasite’s usual de?ni tive host is the dog, though it can be transmitted to other carnivores and herbivores  in egg and  larval stages. Humans become  involved  through frequent,  intimate contact with  fur, paws, and  tongue, especially while and migrate through the body, eventually coming to rest in the liver and “kissing” their dog. If a person accidently ingests eggs, the larvae hatch lungs. There they form compact, water-filled packets called hydatid cysts (L. hydatos, water drop). The bursting of these capsules gives rise to an allergic reaction and also proliferates the worm by seeding other sites Another  disease  that  arises  from  invasion  by  animal worms  is by both dog and cat hookworm (Ancylostoma). It is acquired when hu. cutaneous larval migrans, also described as a creeping eruption caused mans walk barefoot through areas where dogs and cats defecate, enabling the larvae to enter the skin. The larvae cannot invade completely and so re main just beneath the skin, producing lesions that become so itchy and in?amed  that  the person cannot eat or  sleep. Another migratory  infection called visceral larval migrans is caused by the dog ascarid Toxocara that is ingested in egg form with contaminated soil. The larvae hatch in the in testine and migrate extensively through the liver, lungs, and circulation Likewise, “swimmer’s  itch”  results when  larval  schistosomes of birds and small mammals penetrate a short distance into the skin and then die. The disintegrating worm sensitizes  the humans and causes  intense itching and discomfort.