SCHISTOSOMIASIS
Schistosomiasis is a parasitic infection caused by Schistosoma haematobium, S. mansoni, S. japonicum, S. mekongi, or S. intercalatum. Worldwide, more than 207 million people are infected with schistosomes, and 700 million are at risk. The annual mortality is close to 100,000.
As S. haematobium is the species most
associated with renal and urologic disease, it is the main focus of this
section. This parasite is found throughout most of Africa and the Middle East,
and humans become infected when exposed to it in fresh water. Because eggs are
excreted in human waste, poor sanitation plays an important role in sustaining
fresh water reservoirs.
In endemic populations, infection generally occurs
during childhood. The prevalence of the disease, as well as the parasite
burden, peaks around 15 to 20 years of age. Because infection can occur during
even brief exposure, however, travelers to endemic regions are also
susceptible.
LIFE CYCLE OF SCHISTOSOMA HAEMATOBIUM
LIFE CYCLE
Fresh water snails are the intermediate hosts for the
larval forms of S. hematobium, whereas humans are the definitive hosts
for the adult worms.
Eggs are excreted in the urine of infected humans. The
eggs hatch into miracidia larvae upon contact with fresh water and then infect
fresh water snails. In this intermediate host, miracidia multiply asexually
into sporocysts and later into cercaria larvae. After 6 to 8 weeks, motile
cercariae are released back into the water by the thousands. When they come
into contact with humans, they penetrate the skin, aided by enzymatic
secretions.
Once cercariae have entered the human host, they lose
their tails, transform into schistosomula, and travel through the lymph and
venous systems to the right side of the heart. They then pass through the
pulmonary capillaries and left side of the heart and then travel through the
mesenteric vasculature until they reach the portal vein. In the liver, they
mature into adult worms over 6 weeks, then migrate through the venous system to
the pelvic venous plexus, located around the distal end of the ureter and the
bladder. Adult male worms enclose the females in a gynecophoric canal. The
average life span of the adult worm is 3 to 5 years, but some worms can live
for decades.
Mature female worms produce eggs throughout their
lifetime. Eggs are released into the vesical veins, migrate across the bladder
wall, and are excreted in urine, completing the life cycle.
PATHOPHYSIOLOGY
Adult worms evade the immune system and do not
typically produce symptoms.
In contrast, eggs cause irritation and microhemorrhage
as they pass through the bladder wall. More importantly, however, egg antigens
stimulate the immune system to mount a significant T-cell dependent response.
The granulomatous inflammation that ensues can lead to
the formation of large polypoid masses that project into the bladder lumen. The
eggs also promote calcification, sometimes involving the entire bladder. If
infection is chronic, the resulting inflammation can lead to squamous cell
carcinoma of the bladder. As entrapped eggs die and antigenic stimulation ceases, tissue fibrosis occurs, which leads to flat, tancolored
areas of mucosa known as “sandy patches.”
In some cases, these inflammatory processes involve the
ureteric orifices or urethra, causing stricture formation and obstruction. These
abnormalities may lead to hydronephrosis and, in a small percentage of cases,
renal failure. In addition, they increase the risk of recur- rent bacterial
urinary tract infections.
PRESENTATION
Acute. Shortly
after contact with cercariae, a small number of individuals develop local
urticaria followed by macular
rash. In previously unexposed persons, the rash is usually self-limited and
brief. In sensitized individuals, it may develop into a pruritic maculopapular
rash that persists for several days.
About 1 to 3 months after schistosomal infection, a
small number of previously unexposed individuals may develop a fulminant
febrile illness called Katayama fever. The syndrome coincides with the onset of
egg production, and it typically causes diarrhea, hepatosplenomegaly,
eosinophilia, pulmonary infiltrates, and (in rare cases) central nervous system
involvement. Signs and sympto s usually spontaneously resolve after 10 to 12 weeks.
Chronic. Chronic disease becomes evident months or even years after initial
infection, and the severity reflects the worm burden.
The most salient features typically are dysuria and
gross or microscopic hematuria. If severe, the hematuria may cause anemia. If
the ureters or urethra are involved, patients may have symptoms of obstruction
(see Plate 6-1).
In many female patients, deposition of the eggs in the
genital tract leads to sandy patches, mucosal bleeding, and occasionally
ulcerative or nodular lesions of the vulva, perineum, and cervix. In some male
patients, hematospermia can result from involvement of the prostate and seminal
glands (vesicles).
EFFECTS OF CHRONIC SCHISTOSOMA HAEMATOBIUM INFECTION
DIAGNOSIS
The diagnosis should be suspected in any patient with
dysuria and hematuria who has a history of travel to endemic areas. In women, a
pelvic examination may reveal sandy patches. Up to two thirds of infected
patients have nonspecific laboratory abnormalities, such as eosinophilia. A
superimposed bacterial urinary tract infection may also be noted.
Once S. haematobium infection is strongly
suspected, definitive diagnosis is accomplished using microscopic examination of
the urine to detect eggs, as well as serologic tests to detect the presence of
antischistosomal antibodies.
A single urine sample from a patient with light
infection may contain few to no eggs. Thus repeated urine examinations are
often required. With S. haematobium infections, timing the collection of
urine samples to midday, when egg excretion is highest, may be helpful. If
patients are strongly suspected of having infection but repeated urine samples
are negative, biopsy of the bladder mucosa may demonstrate the presence of
eggs.
Serologic tests generally have very high sensitivity
and specificity, but they may be negative in the acute stage of the disease. The
most significant problem with these tests is that they cannot differentiate past
from current infections. Thus they are primarily helpful in returning travelers
who would not be expected to have preexisting antischistosomal antibodies.
Intravenous pyelogram or CT may reveal focal
thickening of the bladder wall and polypoid bladder lesions. In more advanced
disease, bladder volume may be diminished, and ureteral strictures with
resulting hydroureteronephrosis may be seen. The bladder and ureteral walls may
appear calcified. Where the disease is endemic, these findings may be
pathognomonic for chronic urinary schistosomiasis, but tuberculosis, other
kinds of cystitis, and even bladder malignancies may produce bladder wall
calcification as well.
TREATMENT
Praziquantel is the therapy of choice for all five
species of schistosomes. It is relatively well-tolerated and has minimal
adverse effects. It has been shown to have an 85% cure rate with first
treatment. Patients with residual infection should be retreated. Resistance to
this agent is rare but has been reported in some travelers returning with S.
haematobium infections; however, there are currently no other agents as efficacious as
praziquantel for the treatment of schistosomiasis.
Praziquantel has been demonstrated to be safe in
pregnancy, and current World Health Organization (WHO) guidelines recommend
that symptomatic pregnant woman be treated. Some experts, however, advise
delaying treatment until after the first trimester.
Glucocorticosteroids can reduce the inflammation
associated with egg release, but they are recommended only in the setting of
neurologic disease or in acute infection.
PREVENTION
Travelers should be advised to avoid contact with
fresh water reservoirs when visiting endemic areas. When contact does occur,
travelers should aggressively dry off with a towel to prevent cercariae from
penetrating the skin. Medical evaluation, as outlined above, should be sought
upon return home.
Efforts aimed at improving water and sanitation infra-
structure in endemic countries may have a significant impact on infection rates.
In areas of high prevalence, the WHO recommends regular administration of
antischistosomal medications.
