The epidemiology of Necator americanus Infection in rural Thailand

266 Pages·1997·8.6 MB·English

by  Kiang, Karen Minnie

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The Epidemiology of Necator americanus Infection
in Rural Thailand
A Thesis Submitted to the
Yale University School of Medicine
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Medicine
by
Karen Minnie Kiang
1997
6
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THE EPIDEMIOLOGY OF NECATOR AMERICANUS INFECTION IN
RURAL THAILAND.
Karen M. Kiang, Jeffrey Bethony, and Peter J. Hotez. Section of Infectious
Disease, Departments of Pediatrics and of Epidemiology and Public Health,
Yale University, School of Medicine, New Haven, CT.
We determined the prevalence, intensity, and distribution of human
hookworm infection (as well as degree of co-infection by Ascaris lumbricoides
and Trichuris trichiura) along the central Thai-Myanmar border. 660
individuals from 3 villages underwent stool examination for fecal egg counts
using the McMaster's counting technique. Age of participants ranged from 0-
85 years old. Hookworm larvae were cultured and brought back to the United
States for species determination using a PCR-RFLP technique. Using PCR-
RFLP, the predominant species was found to be Necator americanus, with
very little if any evidence for Ancylostoma duodenale in our population.
Hookworm distribution was highly aggregated, with 65.8% of those infected
having light infections (<1000 eggs per gram stool). On the other end of the
intensity spectrum, 14.8% of infected subjects accounted for 55.8% of the total
hookworm egg production. Overall hookworm prevalence was found to be
57.2%. Prevalence and mean intensity varied co-linearly and showed a strong
age-dependence. They climbed steadily throughout childhood and
adolescence before plateauing in the 26-30 year old age group, where they
were maintained through to the oldest age group. They did not increase or
decrease with age. Hookworm levels also varied by village, with Village 2
showing significantly lower prevalence and mean intensities of Necator than
Villages 1 and 3. Sex was not a statistically significant factor.
Co-infection by Ascaris and Trichuris was rare. Overall Ascaris
prevalence was 6.4% (data for Trichuris was unavailablee). Only 5.8% of
those infected with Necator were co-infected with Ascaris. Ethnically, the
Mon group had a higher risk of being infected than the Karen group, and
were infected at higher levels. However, since Village 2 is made up of mostly
Karen people and Villages 1 and 3 by Mon people, it is impossible to
distinguish between “village" and "ethnic group" as separate risk factors.
Other risk factors proved to be both the lack of a latrine, and the types of
bathroom floor. Dirt and bamboo floors carried a higher risk for infection,
while cement floors were protective. Shoe-wearing as a risk factor was
eliminated during statistical analysis due to reporting inaccuracies. In
comparing the frequency of hookworm-infected mother-infant pairs to pairs
in which one or neither were infected, no evidence for vertical transmission
was found.