Schistosoma nasale infection

Etiology

• S. nasale, a digenetic trematode, is a member of genus Schistosoma, family Schistosomatidae.
• Inhabits the blood vessels of the nasal mucosa of cattle and occasionally buffaloes.
• Has a two host-life cycle with a sexual adult stage inside the mammalian vertebrate host and asexual stages in the intermediate host, a freshwater snail, Indoplanorbis exustus.
• Is transmitted to the animals through contact with water infested with cercariae and possibly also by ingestion of cercariae when drinking.
• Is genetically related to the African/Indian Schistosoma species rather than the East Asian Schistosoma species.

Predisposing factors

General

• Keeping animals on pasture with free access to fresh-water bodies, including pools, ponds and paddy fields where infected snails reside.

Specific

• The presence of fresh-water snails (Indoplanorbis spp) and direct skin contact with water infested with the infective stage cercariae of S. nasale worms.

Pathophysiology

• Transmission of infection to the susceptible animal occurs by penetration of the skin by cercariae of S. nasale when animals forage or drink in water.
• The cercariae grow into adult male and female flukes as they migrate in the circulation until they settle in the nasal vein.
• The female flukes lay their eggs in the nasal vein, which enter the nasal mucosa where they trigger an inflammatory reaction leading to rhinitis, hemorrhagic and/or mucopurulent nasal discharge and development of granulomatous cauliflower-like growths in the nasal cavity.
• These abnormal growths on the nasal mucosa cause partial obstruction of the nasal cavity, which leads to dyspnea and snoring sounds during breathing.
• The main pathogenic effects are attributed to the fluke eggs owing to their sharp terminal spine, which causes micro-abscesses and other pathologies in the nasal mucosa.
• The matured eggs containing fully developed miracidia are released in nasal discharges during sneezing or drinking.

Timecourse

• The pre-patent period ranges from 50-103 days.

Epidemiology

• S. nasale is widely distributed in various parts of India and Indian subcontinent.
• The prevalence rates reported in cattle from different regions range from 1-20.9%.
• Nasal schistosomiasis has been also reported in Bangladesh, Malaysia, Myanmar, Nepal, Pakistan, and Sri Lanka.
• Freshwater snails of the species Indoplanorbis exustus serve as intermediate hosts for S. nasale.
• The overall prevalence of S. nasale is highest during the rainy season and winter and is lowest during summer, but in monsoon zones, seasonality is less marked.
• Warm, moist climatic conditions are favorable for the growth of intermediate host snails and the development of immature fluke, hence more cercariae are shed by snails during the warm, rainy seasons.