Helminth Infections

Helminth infections are a broad category of infections caused by multicellular parasitic worms: nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes). Each has distinct life cycles, transmission routes and tissue tropism.

Collectively they are among the most prevalent infections worldwide, particularly in resource limited settings.

The shared clinical principle is that eosinophilia is a characteristic, though not universal, clue to this category of infection, most prominent during tissue migratory phases of the parasite life cycle.

Specific individual conditions within this group include schistosomiasis, echinococcosis, strongyloidiasis, and gnathostomiasis.

Classification

• Soil transmitted helminths include Ascaris lumbricoides, Trichuris trichiura and hookworm species (Ancylostoma duodenale, Necator americanus). Ascaris and Trichuris are acquired through ingestion of contaminated soil or food. Hookworm is acquired through direct skin penetration by infective larvae from contaminated soil.
• They carry an enormous global burden of chronic, often subclinical infection. They contribute significantly to childhood malnutrition and growth impairment. Hookworm specifically causes iron deficiency anaemia from chronic intestinal blood loss.
• Filarial nematodes include Wuchereria bancrofti, Brugia species and Onchocerca volvulus. Onchocerca volvulus is transmitted by blackflies and causes onchocerciasis, with characteristic skin and, critically, ocular involvement leading to river blindness.
• Filarial nematodes are transmitted by specific arthropod vectors. Disease reflects the tissue tropism of the adult worm and migrating microfilariae.
• Cestodes include Taenia species and Echinococcus species. Taenia causes intestinal taeniasis after consumption of undercooked meat containing larval cysts. It also causes cysticercosis, which is distinct and more serious: ingestion of Taenia solium eggs leads to larval migration to tissue, including the central nervous system.
• Trematodes include Schistosoma species and various food borne flukes. These are liver flukes (Fasciola hepatica and Clonorchis sinensis) and lung flukes (Paragonimus species). They are acquired through consumption of contaminated freshwater vegetation or undercooked freshwater fish or crustaceans.

• Many soil transmitted helminth infections are asymptomatic or produce only mild, non specific gastrointestinal symptoms in light infection.
• Heavier infection burden has more significant consequences. These include abdominal pain, and malnutrition and growth impairment in children (particularly relevant for Ascaris and Trichuris). Hookworm characteristically causes iron deficiency anaemia from chronic intestinal blood loss.
• A pulmonary migratory phase occurs during larval migration through the lungs in Ascaris and hookworm infection. It produces transient cough, wheeze and eosinophilic pulmonary infiltrates (Loeffler syndrome). It typically precedes established intestinal infection by one to two weeks.
• Neurocysticercosis results from larval Taenia solium migration to the central nervous system after ingestion of eggs. This contrasts with intestinal taeniasis, which results from ingestion of larval cysts in undercooked meat.
• Neurocysticercosis is a leading global cause of acquired epilepsy. It presents with seizures and headache. Depending on the number and location of cysts, there may be focal neurological deficit or hydrocephalus.
• Onchocerciasis presents with pruritic dermatitis and subcutaneous nodules (containing adult worms). Most seriously, it causes progressive ocular involvement (punctate keratitis progressing to sclerosing keratitis and chorioretinitis). This leads to irreversible blindness if untreated. It is a major cause of preventable blindness in endemic regions of sub Saharan Africa and parts of Latin America.
• Liver and lung flukes present with symptoms reflecting their tissue tropism. Liver flukes cause right upper quadrant pain and biliary symptoms. Chronic infection carries an elevated risk of cholangiocarcinoma for certain species, including Clonorchis sinensis and Opisthorchis viverrini.
• Lung flukes cause chronic cough with haemoptysis, sometimes mimicking tuberculosis.

• Stool microscopy for ova and parasites remains the primary diagnostic approach for intestinal helminth infections. Multiple samples improve sensitivity given intermittent egg shedding. Egg morphology allows species identification for the great majority of intestinal nematodes and cestodes.
• Serological testing is particularly valuable for tissue dwelling helminths not readily accessible to direct stool examination. These include neurocysticercosis and onchocerciasis. They also include liver and lung fluke infection where specific serological assays are available.
• In neurocysticercosis, serology combined with characteristic neuroimaging supports the diagnosis. Imaging shows cystic lesions with or without a visible scolex and, in the involuting stage, calcification.
• Skin snip examination was historically the diagnostic reference standard for onchocerciasis. It identifies microfilariae migrating within the dermis. Polymerase chain reaction based testing is increasingly used given improved sensitivity.
• Full blood count, with attention to eosinophilia, supports the diagnostic evaluation. Add imaging where relevant to the suspected organism: neuroimaging for suspected neurocysticercosis, abdominal imaging for suspected biliary fluke infection. Guide the evaluation throughout by the geographical exposure and dietary history.

• Albendazole 400 mg as a single dose, or mebendazole 500 mg as a single dose, is standard, highly effective treatment for the great majority of soil transmitted helminth infections (Ascaris, Trichuris, hookworm).
• Mass deworming programmes using these agents are a major global public health strategy in highly endemic regions. Reducing infection burden at population level substantially benefits child health and development.
• Neurocysticercosis management requires careful individualisation. Antiparasitic treatment risks provoking a significant inflammatory response as larvae are killed.
• Treatment combines albendazole (15 mg per kg per day in divided doses, typically for eight to fourteen days) with corticosteroid therapy to mitigate this response, and antiepileptic medication for seizure control.
• Guide the approach by the number, location and stage (active, degenerating or calcified) of cysts on neuroimaging. This includes whether antiparasitic treatment is indicated at all, as inactive, calcified lesions generally do not warrant treatment. Ideally use specialist neurology and infectious disease guidance.
• Onchocerciasis is treated with ivermectin 150 micrograms per kg as a single dose, repeated every six to twelve months. The drug acts against microfilariae rather than adult worms. Repeated dosing is therefore required to control disease over the several year lifespan of the adult worm.
• Mass drug administration programmes using this approach are the basis of successful onchocerciasis control and elimination efforts in several previously highly endemic regions.
• Liver and lung fluke infections are treated with praziquantel (dosing specific to the organism) for most species. Triclabendazole is specifically preferred for Fasciola hepatica, as praziquantel is less effective against this organism.
• Prevention across this group centres on four measures.
  • Improved sanitation, reducing soil and water contamination with human waste. This is fundamental to breaking the transmission cycle for soil transmitted helminths.
  • Food safety: thorough cooking of meat, fish and freshwater vegetation, relevant to the regional risk.
  • Vector control, relevant to filarial infections.
  • In highly endemic settings, mass drug administration programmes targeting the locally prevalent organisms.