This is a brief introduction to the epidemiology of soil transmitted helminth parasites.

1. Geohelminthes
Lecture 10
Dr. Harriet Angwech
Gulu University

2. Introduction
 Soil-transmitted helminths refer to the intestinal worms
infecting humans that are transmitted through
contaminated soil.
 Soil-transmitted helminthic infections are of two types:
Ø The hookworms, which undergo a cycle of
development in the soil; the larvae being infective
and;
Ø A group of nematodes which survive in the soil
merely as eggs.

3. Introduction
 More than one billion people are infected with
at least one species.
 Recent estimates suggest that:
 A. lumbricoides infects 1.221 million,
 T. trichiura 795 million, and
 hookworms 740 million people.
 85% of infected people in the world are
estimated to be in sub-Saharan Africa.
Continued

4. Transmission
Ø Soil-transmitted helminths are transmitted by
eggs that are passed in the faeces of infected
people.
Ø Adult worms live in the intestine where they
produce thousands of eggs each day.

5. Transmission cont’d
Ø In areas that lack adequate sanitation, eggs
contaminate the soil.
Ø Transmission can happen in several ways:
Ø eggs that are attached to vegetables are ingested
when the vegetables are not carefully cooked,
washed or peeled;
Ø eggs are ingested from contaminated water sources;
Ø eggs are ingested by children who play in soil and
then put their hands in their mouths without washing
them.

6. Transmission
Ø Hookworm eggs hatch in the soil, releasing
larvae that mature into a form that can actively
penetrate the skin.
Ø Infection with hookworms occur primarily by
walking barefoot on the contaminated soil.
Continued

7. Transmission
Ø There is no direct person-to-person
transmission, or infection from fresh faeces,
Ø because eggs passed in faeces need about 3 weeks
to mature in the soil before they become infective.
Ø Since these worms do not multiply in the human
host, re-infection occurs only as a result of
contact with infective stages in the environment.
Continued

8. Transmission cycle
 Adult Necator and Ancylostoma : the upper
part of small intestine.
 Ascaris (roundworms): parasites of the entire
small intestine.
 Adult Trichuris (whipworms): large intestine,
especially the caecum.

9. Transmission cycle cont’d
 Man - only major definitive host for these
parasites, although Ascaris infections can also
be acquired from pigs.
 After mating, each adult female produces
thousands of eggs per day, which leave the
body in the faeces.

10. Transmission cycle cont’d
 People become infected with T. trichiura and A.
lumbricoides by ingesting the fully developed
(embryonated)eggs.
 The released larvae of T. trichiura moult and
travel to the colon where;
 they burrow into the epithelia and develop into adult
whipworms within 12 weeks.

11. Transmission cycle cont’d
 Ascaris larvae penetrate the intestinal mucosa
and enter the liver, the lungs, before passing over
the epiglottis to re-enter the gastrointestinal tract and
develop into egg-laying adult worms.
 This takes about 9-11 weeks after egg
ingestion.

12. Transmission cycle cont’d
 N. americanus and A. duodenale (hookworm) eggs hatch in
soil.
 The larvae moult twice to become infective third-stage
larvae, which are non-feeding but motile organisms.
 After skin penetration, they enter the blood stream and
ultimately the larvae become trapped in pulmonary
capillaries, enter the lungs, pass over the epiglottis, and
migrate into the gastrointestinal tract.
 About 5-9 weeks -from skin penetration to development of
egg-laying adults.

13. Transmission cycle cont’d
 A. duodenale larvae are also orally infective.
 Eggs are thus released together with faecal
material in to the soil hence completing the cycle.

14. Epidemiology of geohelminth infections
 The geographical distribution of the hookworms
is limited by the requirements of the developing
larvae for:
 warmth and
 humidity.
 Generally speaking, they can occur not only in
the tropics and subtropics, but also in temperate
regions.

15. Epidemiology of geohelminth
infections
 In tropical and subtropical areas, wet soil (such
as that found at the edges of rice fields, rubber
plantations and the surroundings of villages in
areas of high rainfall)
 supports the maturation of hookworm larvae from
eggs, deposited by indiscriminate defaecation.
 In contrast, the well-protected eggs of Ascaris
lumbricoides and Trichuris trichiura) can survive
in drier conditions.
Continued

16. Ø Equally important risk factors are:
Ø poverty,
Ø inadequate water supplies and
Ø sanitation.
Ø All these helminthiases provide an index of the
level of personal hygiene and sanitation in a
community,
Epidemiology of geohelminthes
infections
Continued

17. Epidemiology of geohelminthes
infections
Ø Since they depend for their dispersal on:
Ø the indiscriminate deposition of faecal material,
Ø the use of untreated night soil as an agricultural
fertilizer and similar unsophisticated human habits,
Ø The provision of adequate sewage disposal
facilities virtually excludes these diseases.
Continued

18. Ø Faecal contaminated soil in the neighbourhood
of human habitations or on farmland is the
source of hookworm infection for the barefoot
inhabitants.
Ø Conversely, the use of footwear greatly
reduces the prevalence of Necator americanus
infection, but not necessarily that of
Ancylostoma duodenale infection.
Epidemiology of geohelminthes
infections
Continued

19. Prevention
Ø Improved water supply and sanitation
Ø Deworming
Ø Mebendazole and albendazole are used for large-
scale prevention of morbidity in children living in
endemic areas
Ø Diagnosis :
Ø Stool examination
Ø Serology.

20. Fertilized Ascaris Egg
A fertilized Ascaris egg, still
at the unicellular stage, as
they are when passed in
stool.

21. Unfertilized egg
The chitinous layer and
albuminous coat a r e
thinner than those of the
fertilized eggs without
ascaroside and fertilizing
membrane. The content is
made of many refractable
granules of variable sizes.

22. Some pictures

23. Some pictures
n Trichuris trichuria -
These have opercular
plugs at either end of
the egg shell.
Hookworm Egg
Typical nematode egg

24. Pathogenesis
Symptoms associated with larval migration
Ø Migration of larvae in lungs may cause hemorrhagic/
eosinophilic pneumonia, cough (Loeffler's Syndrome)
Ø Breathing difficulties and fever
Ø Complications caused by parasite proteins that are
highly allergenic - asthmatic attacks, pulmonary
infiltration and urticaria

25. Pathogenesis Cont.
Symptoms associated with adult parasites in the
intestine
Ø Usually asymptomatic
Ø Abdominal discomfort, nausea in mild cases
Ø Malnutrition in host especially children in severe cases
Ø Sometimes fatality may occur when mass of worm
blocks the intestine