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SCHISTOSOMIASIS

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 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
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.