Microbiology, Group 4
Life cycles, epidemiology, pathogenicity and clinical symptoms of soil-transmitted
helminths.
Introduction
• Soil-transmitted helminth (STH) infections are among the most prevalent of
chronic human infections worldwide.
• There are four main nematode species of human soil-transmitted helminth
(STH) infections, also known as geohelminths: Ascaris lumbricoides
(roundworm), Trichuris trichiura (whipworm), Ancylostoma duodenale
and Necator americanus (hookworms).
• These infections are most prevalent in tropical and sub-tropical regions of
the developing world where adequate water and sanitation are lacking, with
recent estimates suggesting that A. lumbricoides infects 1221 million
people, T. trichiura 795 million and hookworms 740 million (de Silva et al.,
2003). The greatest numbers of STH infections occur in sub-Saharan Africa
(SSA), East Asia, China, India and South America.
• The life cycles of STH infection follow a general pattern. The adult parasite
stages inhabit some part of the host intestine (A. lumbricoides and
hookworm in the small intestine; T. trichiura in the colon), reproduce
sexually and produce eggs, which are passed in human faeces and deposited
in the external environment. Adult worms survive.
Ascaris Lubricoides
• Ascaris lumbricoides are very large (adult females: 20 to 35 cm; adult
males: 15 to 30 cm) nematodes (roundworms) that parasitize the human
intestine and the primary species involved in human infections globally.
• Hosts : Humans and swine are the major hosts for Ascaris; see Causal
Agents for discussion on species status of Ascaris from both hosts. Natural
infections with A. lumbricoides sometimes occur in monkeys and apes.
• Occasionally, Ascaris sp. eggs may be found in dog feces. This does not
indicate true infection but instead spurious passage of eggs following
coprophagy.
Life cycle
Adult worms (1) live in the lumen of the small intestine. A female may produce
approximately 200,000 eggs per day, which are passed with the feces (2) .
Unfertilized eggs may be ingested but are not infective. Larvae develop to
infectivity within fertile eggs after 18 days to several weeks (3) , depending on the
environmental conditions (optimum: moist, warm, shaded soil). After infective
eggs are swallowed (4) , the larvae hatch (5) , invade the intestinal mucosa, and are
carried via the portal, then systemic circulation to the lungs (6) . The larvae mature
further in the lungs (10 to 14 days), penetrate the alveolar walls, ascend the
bronchial tree to the throat, and are swallowed (7) . Upon reaching the small
intestine, they develop into adult worms. Between 2 and 3 months are required
from ingestion of the infective eggs to oviposition by the adult female. Adult
worms can live 1 to 2 years.
Epidimiology and pathogenicity
• Ascariasis is most prevalent in tropical and subtropical regions with poor
sanitation, particularly affecting children aged 2 to 10 years. It is estimated
that around 500 million people are infected globally, with the disease
contributing to malnutrition and causing approximately 2,000 to 10,000
deaths annually, primarily due to bowel or biliary tract obstruction in
children. In the United States, most cases occur among refugees,
immigrants, or travelers returning from endemic areas.
• Research indicates that 45% of infected individuals shed only fertilized eggs,
40% shed both fertilized and unfertilized eggs, and 20% shed only
unfertilized eggs, with unfertilized eggs accounting for only 6-9% of total
egg shedding. Fertilized eggs can become infectious within 5-10 days when
released into suitable soil and can remain viable for up to 10 years.
Clinical presentation and medication
• Although heavy infections in children may cause stunted growth via
malnutrition, adult worms usually cause no acute symptoms. High worm
burdens may cause abdominal pain and intestinal obstruction and potentially
perforation in very high intensity infections. Migrating adult worms may
cause symptomatic occlusion of the biliary tract, appendicitis,
nasopharyngeal expulsion, particularly in infections involving a single
female worm. Fever from other illnesses or certain medications, such as
albendazole or mebendazole, may trigger this aberrant migration.
Laboratory Diagnosis
Microscopic identification of eggs in the stool is the most common method for
diagnosing intestinal ascariasis. The recommended procedure is as follows:
• Collect a stool specimen.
• Preserve the specimen in formalin or another fixativ
• Concentrate using the formalin–ethyl acetate sedimentation technique
• Examine a wet mount of the sediment.
Image Gallery
Figure A: Unfertilized egg of A. lumbricoides. Note the prominent mammillations
on the outer layer.
Figure B: Unfertilized egg of A. lumbricoides in an unstained wet mount of stool
Figure C: Infertile, decorticated egg of Ascaris lumbricoides. Image courtesy of
The Leiden University Medical Center, The Netherlands.
Figure D: Fertilized egg of A. lumbricoides in unstained wet mounts of stool, with
embryos in the early stage of development.
Figure E: Fertilized egg of A. lumbricoides in an unstained wet mount of stool,
undergoing early stages of cleavage.
Figure F: Larva of A. lumbricoides hatching from an egg.
Figure G: Larva of A. lumbricoides hatching from an egg.
Figure H: Adult female A. lumbricoides.
Figure H: Close-up of the anterior end of an adult A. lumbricoides. Note the three
'lips.' Image courtesy of the Orange County Public Health Laboratory, Santa Ana,
CA
The End
