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Epidemiology of Nematodes, Cestodes, and Trematodes 133

generalized anemia and the appearance of not being eating contaminated raw or undercooked meat, fish,
well. Hookworm infections are often merely referred to or grains. Taenia solium cysticercosis or Hymenolepsis
as “worms” or a symptom such as urticaria or itching of nana are directly transmitted through ingestion of
the skin. eggs from human feces and Echinococcus eggs come
Beginning just before and into the early part of the primarily from the definitive hosts, dog or fox, with
twentieth century, the U.S. Public Health Service con- humans as the intermediate host. Reinfection with
cluded that the “germ of laziness” associated with hook- adult tapeworms is common but second infections
worm disease had been found as a common malady in with larvae are rare, apparently due to developed
the American South. Progressive pernicious anemia seen immunity. There are literally thousands of trematodes,
in the southern United States was found to be caused by divided into two subclasses. The smaller subclass,
A. duodenale, and testing of schoolchildren in the 1900s Aspidogastrea, includes about 100 species as obligate
showed heavy infections in the group. An investigator parasites of mollusks that may also infect turtles and
also noticed that symptoms of the hookworm disease fish, including cartilaginous fish. The other subclass,
in animals were similar to those of many poor Whites in Digenea, which shows the most trematode diversity, is
the rural South. Philanthropist efforts by the industrialist obligate parasites of both mollusks and vertebrates and
John D. Rockefeller and President Theodore Roosevelt occasionally of cartilaginous fish.
worked to diminish the rate of parasitic infections in the Trematodes are known commonly as flukes and
rural population through the Country Life Commission. in the definitive host the eggs are shed along with
Efforts to improve farm life through general sanitation feces from the host. Eggs shed in water release free-
and awareness of means for preventing the diseases be- swimming larval forms infective to the intermediate
gan to bear fruit by a lessening of the cases of hookworm host, where asexual reproduction occurs. Both tissue
infections and resulting diseases. flukes that infect organs and systems of the body, and
But in the 1930s, during the Great Depression, blood flukes are two major divisions of the trematodes.
public officials again found that many children again Blood flukes inhabit the blood in some stages of their
showed signs of hookworm. In the 1920s, a hookworm life cycle and include the three main species of the
eradication program in the United States spread to the genus Schistosoma: Schistosoma mansoni, S. japonicum
Caribbean and Latin American countries where well- and S. hematobium.
established epidemics, particularly in Brazil, prevailed. The life cycles of some of these flukes are complex,
Both control of new cases and treatment of existing cases and may also be classified according to the environment
were necessary. in which they are found. The life cycle of schistosomes
Not only children but adult males who work in require fresh water in order to infect a host, so interior
contaminated soil of farms and with a variety of farm lakes in the endemic areas of the world are often con-
animals were often heavily affected by hookworm infec- taminated. Humans become infected by cercaria from
tions. This life cycle for hookworms requires warm and miracidia that infected the snails found in fresh wa-
sandy or loamy soil with substantial rainfall, conditions ter, where these cercaria penetrate the skin and travel
met in many areas of the United States. Infective larvae through the blood vessels of their human hosts to the
of Necator americanus survive at higher temperatures heart, lungs, and portal routes of circulation.
than Ancylostoma duodenale so the cooler climates Severe symptoms and signs are routinely found
of Europe are more favorable for propagation of that range from a mere nuisance splenomegaly (spleen
A. duodenale. enlargement), diarrhea, and lymphadenopathy. In the
The most important cestodes are the tapeworms bladder, granulomatous lesions, hematuria, and some-
Taenia solium (pork tapeworm), T. saginata (beef times urethral occlusion occur, perhaps leading to blad-
tapeworm), Diphyllobothrium latum (fish or broad der cancer in chronic cases. In the intestine, polyps may
tapeworm), Hymenolepis nana (dwarf tapeworm), form that result in life threatening dysentery. In the liver,
and Echinococcus granulosus and E. multilocularis the eggs cause periportal fibrosis and portal hyperten-
(hydatid). Diet and contact with animal excrement sion, resulting in hepatomegaly, splenomegaly, and as-
pose the greatest risks, where larvae infect a host by cites (swelling and distention of the belly).