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148 CHAPTER 6

LIFE CYCLE of—
Strongyloides stercoralis

Pharynx
Trachea
Swallowed
Lungs

Circulation
Adult in
mucosa of
Penetrates intestine small intestine
or perianal skin
Penetrates skin MAN
Eggs in
mucosa
(occasionally in feces)
Filariform larva

Filariform larva Rhabditiform larva in feces
(infective stage) DIRECT DEVELOPMENT (diagnostic stage)

Source: Centers for Disease Control and Prevention (CDC)

EXTERNAL ENVIRONMENT

Free-living adults
in soil

INDIRECT DEVELOPMENT
Rhabditiform larva in soil

Eggs in soil

FIGURE 6-9 Generalized life cycle of the hookworm

in the ground (Figure 6-9). Rainfall averages must be prevalence of the two species in certain areas of the
more than 1000 mm or 40 inches per year. These envi- world. Generally, the hookworm organisms live for only
ronmental conditions are certainly met in many areas of a few weeks at most under natural conditions and tend
the United States as well as in other parts of the world. to die almost immediately on exposure to direct sunlight
Especially conditions in the southeastern part of the or drying out of the larvae, which is the infective stage.
North American continent and other temperate zones in It is absolutely mandatory that these infective larvae
Latin and South America enable the organisms to thrive be deposited in an environment of damp dirt, particu-
during the warm seasons. Only if these conditions exist larly sandy and loamy soil in order to both survive and
can the eggs from fecally contaminated earth hatch to develop.
yield larvae. Infective larvae of Necator americanus can Once in the gut of the infected host, the hookworm
survive at higher temperatures than those of Ancylostoma Necator americanus is capable of causing a prolonged
duodenale which are better adapted for cooler tem- infection of several years. It has been observed that some
peratures. This fact accounts for the difference in the adult worms have been known to live for up to 15 years