Biology Reference
In-Depth Information
INTRODUCTION
Among the helminth parasites that infect humans, the roundworm
Ascaris lumbricoides is the most widely distributed across the globe and
one of the most difficult to control. This is largely due to its very high
fecundity and, concomitantly, high transmission potential. However, the
inability of the human host to mount an effective immune response to
block parasite entry despite repeated exposure, and the robust nature of
the egg infective stage excreted in the feces of infected people which have
good tolerance to a wide range of environmental conditions, are also
central to its wide distribution and often high abundance.
Its abundance in many regions of the world with poor sanitation
facilities has resulted in much research on its biology and epidemiology
(see Chapter 7), and on how best to control infection and spread (see
Chapter 15). Some of the most detailed studies were carried out under the
auspices of the Rockefeller program on the control of soil-transmitted
helminths in the southern regions of the United States, Panama, Puerto
Rico, China and West Africa in the early 1900s. Researchers such as
Norman Stoll, who worked at the Rockefeller Institute for Medical
Research at Princeton and the Rockefeller University in New York after
graduating from Johns Hopkins University, and who coined the phrase
“This Wormy World” in 1947, created the template for the epidemio-
logical study of helminth infections and the methods employed to
measure the prevalence and intensity of infection based on fecal egg
output from infected individuals. These methods are still widely used
today.
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Many of the papers published by Stoll and his many collaborators
such as William Cort from Johns Hopkins in the first three decades of the
1900s still stand today as some of the best andmost detailed studies of the
biology and epidemiology of intestinal nematodes.
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Following on from
this pioneering work, others contributed greatly from countries such as
Japan and South Korea after the Second World War, where control
programs based on improved sanitation and chemotherapy, concomitant
with economic growth and development, resulted in parasite eradication
in most urban areas during the period of the 1950s to the 1980s.
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During the same period, an epidemiologist at the London School of
Hygiene and Tropical Medicine
first turned
attention to the use of mathematical models to study the transmission of
helminths in a pioneering paper on the schistosome parasites.
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Reviews
of the history of model development for helminth infections in both
ecology and epidemiology are provided by Anderson and May
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and
Bas
´
˜ ez et al.
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Macdonald used a simple deterministic differential equation to denote
changes over time in the mean worm burden per person and highlighted
George Macdonald
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