Australian Society of CytologyCase 1 - 2009 - Schistosomiasis
Answer
Cytology:
No malignant cells were seen. A single parasite egg was identified. The egg was ellipsoid and had a rounded terminal spine. It measured approximately 120 microns in length.
The appearances were in keeping with an egg of Schistosoma haematobium.
Further History: The patient had travelled to Malawi a few months prior to presentation where he had been swimming in fresh water lakes.
Diagnosis:
Schistosomiasis
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Discussion
This is an interesting case depicting parasitic infection of the urinary tract diagnosed on urine cytology. Schistosomiasis, also known as bilharziasis, is the commonest tropical parasitic infection to affect the urogenital system.[1] Schistosomiasis infestation is of major clinical significance as it is considered to be a risk factor in the development of squamous cell carcinoma of the bladder.
The main aetiological agents are Schistosoma japonicum, S. mansoni and S. haematobium. S. haematobium and S. mansoni are both endemic to the Middle East and subtropical and tropical regions of Africa, with S. mansoni also being endemic to South America. The endemic areas for S. japonicum include China, Japan and countries in South-East Asia such as Taiwan, the Philippines and Thailand. An estimated 500-600 million people worldwide are exposed to Schistosomiasis with a particularly high prevalence in sub-Saharan Africa at 98 million.[2,3] The mortality rate is estimated to be 1 to 7 per 1000 infected individuals annually.[3]
Schistosoma is classified as a trematode within the phylum Platyhelminthes. Interestingly, trematodes are also commonly referred to as flukes, a term which can be traced back to the Old English description for flounder, referring to the flattened rhomboidal shape of the worms.
Nearly all trematodes are parasites of molluscs and vertebrates, having complex life cycles. The various schistosome species have different freshwater snail hosts with Bulinus being the host for S. haematobium, Biomphalaria for S. mansoni and Oncomelania for S. japonicum. Schistosomes are dioecious, having separate sexes. The female worm lies in the gynecophoral canal, a ventral groove running through the male fluke, on whom it is dependent on transportation.
Adult schistosome worms copulate in humans, their definitive hosts and shed their ova in faeces or urine. The ova hatch in water into free-swimming transparent larvae, miracidia, with a lifespan of about 1 day. The miracidia subsequently infect freshwater snails which act as intermediate hosts, before passing through the sporocyst and cercaria stages of development. These are the larval stages within the snail. After about a month, free-swimming cercaria are released, which will then penetrate the skin of a definitive host. When cercariae penetrate the skin, a delayed hypersensitivity reaction is observed in response to a protein they release. This is known as cercarial dermatitis or swimmer’s itch.[4] At this point, the cercariae lose their tails and become schistosomes, migrating through the host’s lymphatic and systemic circulation. Schistosomes will mature within the portal circulation and subsequently migrate to the vesical or mesenteric veins where the female flukes will hatch their eggs, 1-3 months after infection. The main organs involved will ultimately be the bladder/ureters, the intestines or liver. All schistosome species may involve the intestines or liver but the commonest cause of urinary tract schistosomiasis is S. haematobium, also known as the ‘human bladder fluke’.
Since ova are deposited in the urinary bladder and ureters, this disease commonly presents with haematuria, dysuria and frequency. These clinical manifestations result from a common pathological process – a host response to schistosome eggs in which eosinophils play a major role, resulting in cellular infiltration and granuloma formation. This is followed by the onset of fibrosis, hyalinization and scar formation. The eggs subsequently calcify and are destroyed.
Cytological examination of urine is a relatively simple, safe and inexpensive diagnostic tool, allowing detection of lesions before they can be detected cystoscopically, although it is often complemented by cystoscopy and biopsy in reaching a diagnosis. In our case, a definite diagnosis was made solely on the basis of urine cytology.
If schistosomiasis is suspected, the terminal urine fraction which contains the largest number of eggs expelled by bladder contraction, should be examined. Urine smears are Papanicolau stained, showing up S. haematobium ova as ‘lemon drop’ crystals, having an ellipsoid shape with a thick semitranslucent shell and a characteristic terminal spine. (Figs. 1 & 2) The average size of an ovum is approximately 140mm in length and 60-70mm wide. Schistosome ova are nonoperculate and have a ciliated miracidium. The background may show a purulent exudate with prominent eosinophils.[5,6,7]
Praziquantel is the treatment of choice of schistosomiasis.
References
1. Rathert P., Soloway S.M., Roth S. Urinary Cytology Manual and Atlas 2nd
Edition, Springer-Verlag 1993. 141
2. WHO – Global Schistosomiasis Atlas 1987 www.who.int/wormcontrol
3. The Global Infectious Disease and Epidemiology Network - Gideon
Informatics, Inc. 1994-2009 www.gideononline.com
4. Sun T. Parasitic Disorders 2nd Edition, Williams & Wilkins 1999. 309-321
5. DeMay R.M. Practical Principles of Cytopathology Revised Edition,
American Society for Clinical Pathology 2007. 99-101
6. DeMay R.M. The Art and Science of Cytopathology – Exfoliative Cytology,
American Society of Clinical Pathologists 1996. 386-388, 407
7. Koss L.G., Melamed M.R Ed. Koss’ Diagnostic Cytology 5th Edition,
Lippincott Williams & Wilkins 2006. Vol 1: 275-277, 762
8. Rosai J Ed. Rosai and Ackerman’s Surgical Pathology 9th Edition, Mosby
2004. Vol 1: 1327
