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Schistosomiasis
Description
Schistosomiasis is caused by flukes, which have complex
life cycles involving specific fresh-water snail species as intermediate
hosts. Infected snails release large numbers of minute, free-swimming
larvae (cercariae) that are capable of penetrating the unbroken skin
of the human host. Even brief exposure to contaminated fresh water,
such as wading, swimming, or bathing, can result in infection. Human
schistosomiasis cannot be acquired by wading or swimming in salt
water (oceans or seas). The cercariae of birds and aquatic mammals
can penetrate the skin of human beings who enter infested fresh or
salt water in many parts of the world, including cool temperate areas.
The cercariae die in the skin but may elicit a puritic rash (“swimmer's
itch” or “clam-digger's itch”).
Occurrence
This infection is estimated to occur worldwide,
affecting some 200 million persons. Schistosomiasis is most prevalent
in sub-Saharan Africa, southern China, the Philippines, and Brazil.
Risk for Travelers
Exposure to schistosomiasis is a health hazard for
U.S. citizens who travel to endemic areas (see Map
3–8). Outbreaks of schistosomiasis have
occurred among adventure travelers on river trips in Africa, as well
as among resident expatriates, such as Peace Corps volunteers in
high-risk areas. Those at greatest risk are travelers who wade or
swim in or bathe with fresh water in areas where poor sanitation
and appropriate snail hosts are present.
Clinical Presentation
Clinical manifestations of acute infection can occur
within 2–3 weeks of exposure to cercariae-infested water, but
most acute infections are asymptomatic. The most common acute syndrome
is Katayama fever. Symptoms, which include fever, lack of appetite,
weight loss, abdominal pain, hematuria, weakness, headaches, joint
and muscle pain, diarrhea, nausea, and cough, may develop several
weeks after exposure. Rarely, the central nervous system can be involved,
producing seizures or transverse myelitis as a result of mass lesions
of the brain or spinal cord. Chronic infections can cause disease
in the liver, intestinal tract, bladder (including bladder cancer),
kidneys, or lung. Many persons with chronic infections recall no
symptoms of acute infection. Diagnosis of infection is usually confirmed
by serologic studies or by finding schistosome eggs on microscopic
examination of stool or urine. Schistosome eggs can be found as soon
as 6–8 weeks after exposure, but are not invariably present.
Prevention
No vaccine is available, nor are any drugs recommended
as chemoprophylactic agents at this time. Because there is no practical
way for the traveler to distinguish infested from noninfested water,
travelers should be advised to avoid fresh-water wading or swimming
in rural areas of endemic countries. In such areas, heating bathing
water to 50° C (122° F) for 5 minutes or treating it with
iodine or chlorine in a manner similar to the precautions recommended
for preparing drinking water will destroy cercariae and make the
water safe. Thus, swimming in adequately chlorinated swimming pools
is virtually always safe, even in endemic countries. Filtering water
with paper coffee filters can also be effective in removing cercariae
from bathing water. If these measures are not feasible, travelers
should be advised to allow bathing water to stand for 3 days because
cercariae rarely survive longer than 48 hours. Vigorous towel drying
after accidental exposure to water has been suggested as a way to
remove cercariae in the process of skin penetration; however, this
may only prevent some infections and should not be recommended to
travelers as a preventive measure.
Upon return from foreign travel, those who might
have been exposed to schistosome-infested fresh water should be advised
to undergo screening tests. Because serologic tests are often more
sensitive than microscopic examination of stool and urine for eggs,
previously uninfected but potentially exposed travelers should be
tested for antibodies to schistosomes if microscopic examination
of stool and urine for eggs is negative or not available. CDC performs
a screening ELISA that is 99%, 90%, and 50% sensitive for Schistosoma
mansoni, haematobium, and japonicum, respectively,
and a confirmatory, species-specific immunoblot that is at
least 95% sensitive and
99% specific for all three species. Serologic tests performed in
commercial laboratories may not be as sensitive or specific.
Treatment
Safe and effective oral drugs are available for the
treatment of schistosomiasis. Praziquantel is the drug of choice
for all species of Schistosoma. Oxamniquine has been effective
in treating infections caused by S. mansoni in some areas
where praziquantel is less effective. Travelers should be advised
to contact an infectious disease or tropical medicine specialist.
— James
Maguire
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