Schistosomiasis Case File

Schistosomiasis Case File

Eugene C.Toy, MD, Cynthia Debord, PHD, Audrey Wanger, PHD, Gilbert Castro, PHD, James D. Kettering, PHD, Donald Briscoe, MD

CASE 49

A 40-year-old man presents for a routine examination. He is generally feeling well but complains of some mild dysuria and increasing urinary frequency. He has never had a urinary tract infection (UTI) and thought that the increasing urinary frequency was a normal part of aging. He has not seen any blood in his urine but says that the urine does appear darker than it used to look. He has no other complaints and his review of systems is otherwise entirely negative. He has no significant medical or family history. He smokes a pack of cigarettes a day and denies alcohol use. He is an immigrant from Egypt who has lived in the United States for 3 years. His vital signs and physical examination, including genital and prostate exams, are normal. A urinalysis shows many red blood cells, a few white blood cells, and oval-shaped parasite eggs with terminal spines.

◆ What organism is the likely cause of his hematuria?

◆ How does this organism gain entry into humans?

ANSWERS TO CASE 49: SCHISTOSOMIASIS

Summary: A 40-year-old Egyptian man has hematuria. A urinalysis shows many red blood cells, a few white blood cells, and oval-shaped eggs with terminal spines.

◆ Organism likely cause of his hematuria: Schistosoma haematobium

◆ Method organism gains entry into humans: Penetration through intact skin by the cercarial stage of the organism

CLINICAL CORRELATION

Schistosomiasis is a human disease syndrome caused by infection with one of several parasitic trematodes or flukes of the genus Schistosoma. These parasites are known commonly as blood flukes because the adult worms live in blood vessels of the definitive host. The human disease syndrome is characterized by dermatitis that is caused by entry of the infective stage and by acute and chronic systemic symptoms caused by host responses to eggs deposited by adult worms. Schistosoma haematobium, S. mansoni, and S. japonicum are the major species that infect over 200 million humans in Asia, Africa, the Middle East, and South America. Schistosoma japonicum is considered a zoonotic infection. In addition to these three major species, others, such as S. mekongi and S. intercalatum, also with potential zoonotic properties, rarely infect humans. Schistosomes with avian or nonhuman mammalian hosts can cause severe dermatitis or swimmer’s itch in humans, but not infection with adult worms.

The disease syndrome parallels the development of the parasite in the definitive host. Swimmer’s itch, an allergic dermatitis, is caused shortly after humans make skin contact with microscopic, infective larval forms called cercariae that live in an aquatic environment. Following exposure to human schistosomes, the dermatitis is mild and may go unnoticed. However, when exposed to cercariae of schistosomes that normally infect birds, swimmer’s itch can present as an itchy maculopapular rash. Cercariae penetrate the intact skin, enter the circulation and migrate to the liver where they mature into adult male and female worms. The adult worms migrate via the bloodstream to their final locations. Schistosoma mansoni and S. japonicum descend to the mesenteric veins and S. haematobium to the vesical plexus. Gravid female worms may release 300–3000 eggs per day over a 5–10 year life span. Eggs of S. haematobium can work their way through the wall of the urinary bladder into the lumen and are eliminated in urine, while eggs of S. mansoni and S. japonicum work their way through the walls of the small intestine and colon and are voided in feces. Acute schistosomiasis or Katayama syndrome develops one to two months after initial infection and includes fever, chills, abdominal pain, lymphadenopathy, and hepatomegaly and splenomegaly The pathophysiologic etiology of acute schistosomiasis is not known.

However, the association of its manifestations with heavy infection suggests that it is a form of serum sickness as a result of circulating antigen-antibody complexes. Chronic schistosomiasis results from the inflammatory response to eggs, with granulomas, fibrosis, and scar tissue occurring at the site where eggs are deposited in tissues. Chronic schistosomiasis results from the inflammatory response to the presence of eggs, with granulomas, fibrosis, and scarring around the eggs. Eggs in the bowel wall may result in symptoms of abdominal pain, diarrhea, and blood in the stool. Schistosomiasis of the bladder can cause hematuria, dysuria, frequent urination, and a reduction in bladder capacity. With intestinal schistosomiasis, the liver is frequently involved as a result of eggs being carried by the portal circulation and becoming trapped in the liver. Pathology involves inflammation and fibrosis, leading to cirrhosis with resulting portal hypertension, splenomegaly, ascites, and abdominal and esophageal varices. Ectopic lesions may rarely be associated with eggs reaching the brain and/or lungs.

APPROACH TO THE SUSPECTED SCHISTOME INFECTION

Objectives

1. Learn the life cycle of blood flukes and the epidemiology and clinical course of infection.

2. Be able to describe three basic aspects of infection: transmission, diagnosis, and treatment/prevention.

Definitions

Cercaria: Infective, aquatic larval form of schistosomes; characterized by a forked tail.

Miracidium: Ciliated larval form of schistosomes (and other flukes) that escapes from the egg and infects a snail intermediate host.

Intermediate host: The host in the life cycle of a helminth that harbors the larval stage(s) of the parasite.

Bilharziasis: A synonym for schistosomiasis.

Swimmer’s itch: Dermatitis in humans caused by cercariae penetrating the skin, commonly involving cercariae of schistosomes that parasitize birds or mammals but which cannot complete their life cycle in humans.

Dioecious fluke: A fluke that has separate sexes, as opposed to hermaphroditic flukes.

DISCUSSION

Characteristics of Schistosomes That Impact Transmission

The life cycle of all human schistosomes is similar, except in fine details. Eggs voided in feces or urine hatch in fresh water, releasing ciliated miracidia that penetrate a snail intermediate host. The species of snail varies with the species of schistosome. Miracidia undergo morphologic development through other larval stages, eventually reproducing asexually. The product is hundreds of cercariae with forked tails that emerge from the snail and swim freely. On contacting humans that enter their environment, the cercariae penetrate the intact skin, losing their tails in the process, and enter the circulation and are disseminated to all parts of the body.

The cercariae, now termed schistosomula, are carried via the portal vein into the intrahepatic portal system where they mature in approximately 3–4 weeks. After maturing, the worms migrate against the blood current and move into branches of veins that drain the urinary bladder (S. hematobium) or the lower ileum and cecum (S. japonicum) or the colon (S. mansoni). Female worms lay several hundred eggs per day. The eggs of each species have characteristic morphology. All eggs leaving the host contain a fully developed miracidium.

Diagnosis

Infections can be definitively diagnosed by finding characteristic eggs in the urine or feces. Eggs of S. hematobium are in the shape of an elongated oval with a terminal spine; S. mansoni eggs are also oval shaped but with a distinct lateral spine; and S. japonicum eggs are round to oval with a short lateral spine or knob that often is unseen. In suspected cases where stools are negative, eggs of S. mansoni may be seen in microscopic examination of a rectal biopsy.

Treatment and Prevention

Praziquantel is the drug of choice in the treatment of human schistosomiasis and is effective against all human species. Because the drug has been reported to be effective as a prophylactic, larval stages and adult forms are presumed to be susceptible. The treatment of cercarial dermatitis is symptomatic. Prevention is based on avoiding skin exposure to groundwater in endemic areas.

Synopsis

The synopsis of schistosomiasis is presented in Table 49-1.

Table 49-1

COMPREHENSION QUESTIONS

[49.1] Which of the following is a host in the life cycle of all trematodes that infect humans?

A. Flea

B. Mosquito

C. Mollusk

D. Flour weevil

E. Sand fly

[49.2] An oil field worker who has lived in Brazil for 10 years has mild gastrointestinal symptoms. Brazil is the only country ever visited by the patient outside of the United States. The patient is diagnosed by his physician of having S. mansoni because of which of the following?

A. Round eggs with a prominent terminal spine were observed in a rectal biopsy.

B. Blood was detected in the stool.

C. Nonoperculated eggs with a miracidium inside were observed in stool samples.

D. Eggs were found in a urine sample.

E. Symptoms were relieved by treatment with praziquantel.

[49.3] A 12-year-old boy reports feeling tingling and itching of his legs 30 minutes after swimming in a lake. Over the next day, small papules develop followed by blisters of the legs. Dermatitis due to schistosome infection is diagnosed. What larval stage most likely caused the infection?

A. Filariform larva

B. Cysticercus

C. Cercaria

D. Miracidium

E. Sparganum

Answers

[49.1] C. Snails are mollusks. All flukes have snails as intermediate hosts. Fleas, mosquitoes, flour weevils, and sand flies serve as intermediate or definitive hosts to various helminth and protozoan parasites, but not to flukes.

[49.2] C. Eggs with a miracidium inside is indicative of a fluke infection; nonoperculated eggs are characteristic of schistosomes and differentiate them from all other human flukes, which have eggs with opercula. Also, S. mansoni is the only human schistosome endemic to Brazil and to the western hemisphere. A round egg with a terminal spine (A) is characteristic of S. hematobium but would not be expected in a rectal biopsy; furthermore, S. hematobium is not endemic to the western hemisphere. Blood in the stool (B) may be a finding in S. mansoni infection but would not be a definitive diagnosis. Finding eggs in a urine sample (D) is consistent with S. hematobium but not S. mansoni infection; again, S. hematobium is not endemic to the western hemisphere. Vague gastrointestinal symptoms are not pathognomonic of schistosome infections. Praziquantel (E) is used to effectively treat all intestinal tapeworms of humans, as well as schistosomes. Symptoms could have been caused by adult tapeworms.

[49.3] C. Forked-tail cercariae are infective for humans. A filariform larva is the infective stage for hookworm and strongyloides, both nematode parasites. A cysticercus larva (B) is the infective stage of Taenia species of tapeworms. A miracidium (D) is the stage of flukes that infects snails, not humans. A sparganum larva (E) is involved in the life cycle of pseudophyllidean tapeworms, such as the broad fish tapeworm.

MICROBIOLOGY PEARLS ❖ Swimmer’s itch, a symptom of schistosomiasis, is also transmitted by bird schistosomes. ❖ Egg stages cause pathology in blood fluke infections and are critical in diagnosis of infection. ❖ Praziquantel is effective for all species of human schistosomes.

REFERENCES

Centers for Disease Control. DPDx. Laboratory identification of parasites of public concern. Schistosomiasis. 2007. http://www.dpd.cdc.gov/dpdx/HTML/ Schistosomiasis.htm. 

Centers for Disease Control. DPDx. Laboratory identification of parasites of public concern. Graphic Library. Schistosomiasis. 2007. http://www.dpd.cdc.gov/dpdx/ HTML/ImageLibrary/Schistosomiasis_il.htm. 

Medical Letter on Drugs and Therapeutics. Drugs for parasitic infections. New Rochelle, NY. 2004. http://www.medletter.com/freedocs/parasitic.pdf. This reference has been updated and “superseded by the special report Drugs for Parasitic Infections, which can be purchased (on-line) for $25.” This reference has been provided because it is a comprehensive and clinically useful reference that is regularly updated and could be of value to those who are involved in treating parasitic infections.

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