[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

214
CHAPTER 8 The Nematodes
TABLE 8-19
Dracunculus medinensis
Adults: Typical
Characteristics at a Glance
into their third-stage infective form then occurs.
Ingestion of the infected copepod begins the
cycle again.
Characteristic Adult Female Adult Male
Size 840 by 1.5 mm 21 by 0.4 mm
Other features Prominent
rounded
anterior end
Anterior end
coils itself at
least once
The anterior end of the male characteristically
coils on itself a minimum of one time.
Laboratory Diagnosis
Adult D. medinensis worms may be recovered by
observing infected ulcers for the emergence of the
worms. Induced rupture of the infected ulcers by
immersing in cool water reveals the first-stage
larvae.
Life Cycle Notes
Ingestion of drinking water contaminated with
infected copepods (freshwater fleas) initiates
human infection. These copepods contain infective
D. medinensis third-stage larvae, which
upon human ingestion emerge into the intestine.
The larvae mature into adult worms, penetrate
the intestinal wall, and proceed to connective
tissues or body cavities. Following mating, the
gravid female worms migrate into the subcutaneous
tissue, especially in the skin of the extremities,
where they lay live first-stage larvae. On
release of all their larvae, the adult females may
escape from the body at the larvae deposit site
or migrate back into deeper tissues, where they
eventually become absorbed. The fate of the
adult males is unknown. An infected ulcer
results at the site of the larvae deposit. Under
appropriate conditions, such as contact with
cool freshwater, the ulcer ruptures and releases
the larvae into the water. Copepods living in the
water consume the first-stage larvae, serving as
its intermediate host. Maturation of the larvae
Epidemiology
Guinea worm is found in parts of Africa, India,
Asia, Pakistan, and the Middle East. Copepods
reside in fresh water, located particularly in areas
called step wells, from which people obtain
drinking water and bathe. First-stage D. medinensis
larvae escape from the ulcers of infected
persons who come into contact with this water.
Ponds, human-made water holes, and standing
water may also serve as sources of infection.
There are a number of known reservoir hosts,
including dogs. Like humans, these animals
become infected via contaminated drinking
water.
Clinical Symptoms
Guinea Worm Infection: Dracunculosis, Dracunculiasis.
Patients infected with guinea worm
typically experience symptoms associated with
allergic reactions as migration of the organism
occurs. Secondary bacterial infections may also
develop, some of which may cause disability or
even death. Once the gravid female settles into
the subcutaneous tissues and lays her larvae, a
painful ulcer develops at the site. Unsuccessful
attempts to remove an entire adult female worm
may result in a partial worm being left at the site
and subsequent toxic reactions in the ulcer. Additional
allergic reactions and nodule formation
may develop on the death and calcification of an
adult worm.
Treatment
Because there are no specific dracunculiasis
medicines available, successful treatment typically
consists of total worm removal. The process
of removal usually takes place in five steps:
Step 1. This step consists of placing the affected
body part, in the form of a blister, in cool water.