Cutaneous schistosomiasis

1 - HUMAN SCHISTOSOMIASIS

1.1 - GENERAL

Also known as bilharziasis, classic human schistosomiasis are helminthiases caused by flatworms of the trematodes class. In their adult state, these worms live in the perirectal and perivesical venous plexus in man. Five species are known to infect humans, i.e. Schistosoma haematobium (genitourinary schistosomiasis), Schistosoma mansoni and Schistosoma intercalatum (intestinal and rectal schistosomiasis), Schistosoma japonicum and Schistosoma mekongi (arteriovenous schistosomiasis). These parasite infections are tropical diseases prevalent in Africa (S. haematobium, S. mansoni, S. intercalatum), South America and the Caribbean (S. mansoni), the Middle East (S. haematobium, S. mansoni) or in the Far East (S. japonicum, S. mekongi). Schistosomiasis is a water-related disease whose incidence is growing rapidly in the endemic countries, encouraged by the construction of dams and irrigation systems.

For the cycle to take place, the embryos from the eggs eliminated in the excreta must multiply in an intermediate host, in this case a freshwater snail. This results in the formation of cercariae, the actual infectious agent, which are released by the snail into the water. Humans become contaminated through the skin (early phase). The larvae then migrate via the bloodstream to the portal system where they become adults which then lay their eggs in their preferred territory (acute phase). It is at least 6 to 8 weeks after contact with the contaminated water before the eggs are eliminated in the excreta. Other eggs remain in the tissues where they form granulomas which cause most of the clinical signs of schistosomiasis (chronic phase). These granulomas tend to form in the target organs (genitourinary apparatus, liver and colon). Borne by the venous system, some eggs may migrate to portacaval anastomoses in the organs linked to the major circulation system, creating ectopic locations. The anastomoses between the branches of the para-umbilical vein and the subcutaneous abdominal and internal mammary veins explain the predilection of extragenital cutaneous schistosomiasis for the chest and umbilical regions.

Cutaneous manifestations may be present during the three active stages of schistosomiases in humans. These signs can be minor and self-limiting and are often overlooked or misdiagnosed during the first two stages. Cutaneous signs are rare during the status stage but their incidence is undoubtedly underestimated. In most of the cases reported, they were diagnosed after their chance discovery in pathology samples.

In the early phase, transcutaneous penetration of the cercariae after contact with the contaminated water may cause an allergic reaction manifesting as a pruritic, maculopapular rash located at the larvae penetration sites, known as cercarian dermatitis [1]. These signs regress spontaneously within a few days. The symptoms and signs are less pronounced when the infectious agent is S. haematobium, the species most adapted to man. In endemic regions, these signs do not arise in the event of reinfection.

The acute phase is symptomatic in non-immune subjects and arises 4 to 6 weeks after contamination. The clinical signs are attributed to an immune complex disease. In Europe, the cases observed concerned rash or uninformed travellers who had bathed in freshwater in endemic zones [1]. A fever is ubiquitous (safari fever, Katayama fever) and an infected person may also experience prostration, headache, myalgia and night sweats. A variable set of skin symptoms may also be present, including rash, urticaria and facial oedema. Some patients may also present with hepatosplenomegaly. The clinical signs are more pronounced when the infection is caused by S. mansoni and S. japonicum. The total blood count may show hyperleukocytosis with hypereosinophilia, indicative of ongoing helminthiasis. It can be difficult to obtain concrete proof at this stage since the patient may not yet be excreting eggs and serology test results can be inconclusive. The examinations must be repeated. This phase is of variable duration. The course tends towards an improvement in the clinical signs, even when no treatment is given, or progression towards the chronic phase which starts about three months after the contaminating event.

The cutaneous lesions observed during the chronic phase of schistosomiasis consist of clusters of bilharzian granulomas located in the dermis; they are most frequently caused by S. haematobium. In most cases, it is these cutaneous signs that give away the underlying disease in a patient with minimal or no clinical symptoms. In a few cases, these lesions have appeared a few months after massive infestation resulting in myelitis.

The extragenital cutaneous forms have a singular appearance, with small, firm, sometimes hyperpigmented and itchy papules arising singly or in clusters [2]. They are located on the trunk or may have a zoniform distribution in the peri-umbilical, iliac fossae and lumbar regions. The cases reported concern children living in endemic zones or recently infected adults.

Genital and peri-anal involvement [2, 3] is more common in women and presents as pseudo-tumoural lesions of the labia or vegetating, papillomatous perineal lesions. They can be confused with other conditions such as genital warts, secondary syphilis, donovasosis or an epithelioma. In men, it is the scrotum and penis that are affected. The diagnosis is made on the basis of a skin biopsy showing egg-containing granulomas. It is not always easy to identify the species responsible from a histology sample.

This can be achieved by a parasitology work-up with analysis of the urine and/or stools, a superficial biopsy of the rectal mucosa if the former examination is negative, and serology.