Page 230 - parasitology for medical and clinical laboratoryprofessionals

210 CHAPTER 9

before preparing and eating meals can prevent the eggs or contaminate the vegetable garden. The eggs
from entering the human digestive tract. Employing are highly resistant to weather conditions and can
these simple practices in some areas has practically remain viable for months. The eggs have to be
rid the populations of hydatids such as Echinococcus swallowed by an animal (intermediate host) to
infections in both humans and domestic animals, even form hydatid cysts.
when the disease was prevalent at one time.

GENERALIZED LIFE CYCLE INFECTION IN HUMANS
OF THE TAPEWORM Human infection does not occur from eating infected
meat that includes organs and entrails. People usually
The tapeworm needs two hosts to complete its life cycle:
become infected by accidentally swallowing the tape-
1. Intermediate host—these hosts vary depending worm eggs passed in dog feces. This occurs from pet-
upon the region of the world and the intermediate ting the animal without adequately washing hands and
hosts available there, but which typically include then eating or smoking and then making contact with the
the grazing animals. In some areas, grazing animals mouth using dirty hands. The human takes the role of
such as pigs, cattle, goats, and horses most com- the intermediate host by substituting for sheep, horses,
monly are hosts. In Australia, dingoes (wild dogs), or cattle, and perhaps other herbivores. The organisms
wallabies, and kangaroos are the animals most travel through the blood vessels before lodging in organs
commonly infected with tapeworms. The normal where they form watery hydatid cysts full of tapeworm
fauna of the various geographic regions of the world heads (scoleces). This is known as hydatid disease or
that graze in areas where canids also exist provide echinococcosis. Hydatid disease is not contagious and is
the conditions that may lead to infections with spe- not passed by person-to-person contact, which is why a
cies of the Echinococcus genus. Infection begins human is considered a dead-end host.
when the grazing animal eats grass infected with
the feces from a dog or other canid species that is Symptoms of Hydatid Disease
infected with tapeworm eggs. The eggs hatch in the
animal’s gut into embryos (called oncospheres). Typical symptoms of hydatid disease will depend on the
These embryos penetrate the wall of the intestine severity of the disease and which organs are affected. For
and are carried in the bloodstream to vital organs E. multilocularis, the most commonly affected organ is
such as the liver, lungs, or brain where they can the liver. The kidneys, brain, and lungs are sometimes
develop into hydatid cysts. These cysts contain affected. In rare cases, hydatid cysts may form in the
around 30 to 40 tapeworm heads (the first segment thyroid gland or heart or within bone. E. granulosus is
of the tapeworm). A mature fertile cyst may contain found in more diverse tissues and organs than E. mul-
several million such heads. tilocularis. Signs and symptoms may not occur for con-
2. Definitive host—such as dogs and other canine siderable lengths of time following the contraction of an
species. Infection begins when the animal eats infection, possibly even up to months or several years
wastes that contain hydatid cysts. The swallowed later. In some cases, no symptoms are ever experienced
cysts burst and the tapeworm heads travel to the and presence of the disease may be purely accidental.
gut and attach themselves to the intestine wall. The When symptoms do occur, they may include any or all of
tapeworms are mature after about 6 weeks. An adult the items from the following list:
E. granulosis tapeworm is only 6 mm, or about ■ Stomach upset frequently accompanied with
1/4 in., in length. Thousands can inhabit the gut diarrhea
of an infected animal. Each mature worm grows
■ Unexplained weight loss that in many cases in-
and sheds the last segment of its body about every
cludes anemia
2 weeks. This last segment contains immature
eggs. The eggs are passed from the animal’s body ■ Distended abdomen
in fecal material and may stick to the animal’s hair ■ Extreme fatigue and feelings of weakness