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2nd August 2023
Helminth Infections
Introduction
Helminth infections refer to infections caused by different species of parasitic worms and are the major infectious causes of a raised eosinophil count. They can be subcategorised into:
- Roundworms (nematodes)
- Tapeworms (cestodes)
- Flukes (trematodes)
The following considerations should be made to help narrow the likely helminth responsible for the clinical picture: travel history, incubation period, clinical syndrome, and main organ site affected. This will help decide the likely pathogen and allow for specific investigations to confirm the organism (as generic laboratory and stool testing is often insensitive). The majority of helminth infections are associated with foreign travel and are hence considered tropical diseases. However, immunocompromised patients are more susceptible to parasitic infections and can often present without an associated travel history.
Types of Helminth Infections
Roundworms
Also known as nematodes. They exist in nearly every ecosystem, including fresh and salt water, soil, the tropics and the polar regions, and hence can be found worldwide. They are most common in Northern Africa. The key features to distinguish different forms of nematodes are summarised in the two tables below.
| Organism | Geography | Incubation period | Transmission |
|---|---|---|---|
| Strongyloides stercoralis | Southeast Asia
Africa Western Pacific |
Years | Soil contaminated by larvae |
| Enterobius vermicularis (pinworm) | Worldwide | 1 to 2 months | Person to person – usually in crowded places |
| Loa loa | Central and West Africa | Minimum 4 to 6 months.
(Infection may occur years after first exposure) |
Deer fly bite |
| Ancylostoma duodenal
Necator americanus |
Asia
Africa Subsaharan Africa Latin America Western Pacific |
1 to 3 weeks | Larval penetration |
| Trichinella spiralis | Worldwide | 1 to 4 weeks | Ingestion of raw or undercooked meat |
| Onchocerca volvulus (river blindness) | Sub-Saharan Africa | 3 months to 3 years (median 18 months) | Female black fly |
| Wuchereria bancrofti | Asia
Africa Sub-Saharan Africa Western Pacific |
1 to 2 months | Female mosquito |
| Toxocara canis (dog roundworm) | Worldwide | 2 to 4 days | Faeco-oral |
| Ascaris lumbricoides (giant roundworm) | Latin America
Sub-Saharan Africa Asia Western Pacific |
2 to 3 weeks | Faeco-oral |
Table 1: Epidemiological features of common nematodes
| Organism | Organ site and clinical features | Diagnosis |
|---|---|---|
| Strongyloides stercoralis | GI tract, lung and skin
Papulovesicular lesions, pruritus and urticaria on soles/buttocks (main site of infection) Diarrhoea, bloating, abdominal pain Dry cough |
Stool microscopy revealing larvae ( low sensitivity) |
| Enterobius vermicularis (pinworm) | Usually asymptomatic
Nocturnal perianal itching and vulval itching |
Adhesive tape test – applied to the perianal region (worms will stick to tape, are visible) |
| Loa loa | Subcutaneous tissue and eyes
Calabar swellings – self-resolving, non-pruritic, non-tender subcutaneous limb or facial swellings Transient conjunctivitis Pruritus and urticaria |
Serology for microfilariae |
| Ancylostoma duodenale
Necator americanus |
GI tract and lung
Rash Cough Nausea/vomiting, diarrhoea, and abdominal pain Malnutrition (if chronic) |
Stool microscopy |
| Trichinella spiralis | GI tract
Fever and myalgia (precedes diarrhoea) Diarrhoea Periorbital oedema |
Serology – antibodies become after three to four weeks
Muscle biopsy |
| Onchocerca volvulus (river blindness) | Eyes and skin
Blindness Pruritus Papular dermatitis Subcutaneous nodules |
Skin snips |
| Wuchereria bancrofti | Limb extremity swelling –known as “elephantiasis.”
Scrotal swelling |
Serology and antigen testing |
| Toxocara canis (dog roundworm) | Liver, eyes, lungs
Visceral larvae migrans – leading to hepatitis/pneumonitis as the larvae migrate through the liver and lungs. Ocular larvae migrans – leading to unilateral visual impairment |
Serology |
| Ascaris lumbricoides (giant roundworm) | GI tract and lungs
Loeffler syndrome (dry cough, wheeze, fever, dyspnoea) Abdominal pain but may also cause acute abdomen – i.e. obstruction/ appendicitis/cholangitis |
Stool microscopy |
Table 2: Clinical features, diagnosis and management of common nematodes
Tapeworms
Also known as cestodes. Tapeworm infections are acquired in humans through ingesting undercooked food, typically pork, beef and fish. They attach themselves to the lining of the GI tract in the host organisms and hence generally cause GI symptoms. The key features of tapeworm infections are summarised in the two tables below.
| Organism | Geography | Incubation period | Transmission |
|---|---|---|---|
| Echinococcus granulosus | South America
Middle East South Asia Western China |
Years to decades | Faeco-oral transmission
Often seen in farmers Dogs can ingest hydatid cysts from sheep faeces |
| Taenia solium | Worldwide | Days to months | Uncooked pork |
| Hymenolepiasis nana | Egypt
Sudan Thailand India Latin American countries |
20 to 30 days | Person to person spread
Contaminated food and water |
Table 3: Epidemiological features of common cestodes
| Organism | Organ site and clinical features | Diagnosis |
| Echinococcus granulosus | Lung and liver
In severe cases, liver cysts present with right upper quadrant pain, nausea, vomiting, and obstructive jaundice. Lung cysts present as a respiratory illness of cough, chest pain and dyspnoea. |
Ultrasound
Serology |
| Taenia solium | Cysticercosis and associated organ complications. Cysticercosis is a disease caused by larval infection of various tissues, forming cysts.
Neurocysticercosis is most common – presents with seizures |
Proglottid is visible in the stool |
| Hymenolepiasis nana | GI tract
Abdominal pain, diarrhoea, pruritus ani |
Stool microscopy |
Table 4: Clinical features, diagnosis and management of common cestodes
Flukes
A type of flatworm, also known as trematodes. The key features of fluke infections are summarised in the two tables below. They are also often categorised as either lung, liver or blood flukes, depending on the system each species predominates in.
| Organism | Geography | Incubation period | Transmission |
|---|---|---|---|
| Schistosoma haematobium | South America
Middle East |
2 weeks to 3 months | Swimming in water containing cercariae (the active form of the parasite) –which penetrates the skin |
| Paragonimus westermani | East Asia
West Africa Americas |
2 to 3 months | Ingestion of undercooked crab meat |
| Clonorchis sinensis | East Asia | 2 to 3 weeks | Ingestion of undercooked fish |
| Fasciola hepatica | South America
Europe Asia Egypt |
3 months | Ingestion of contaminated aquatic vegetation grown near farmland |
Table 5: Epidemiological features of common trematodes
| Organism | Organ site and clinical features | Diagnosis |
|---|---|---|
| Schistosoma haematobium | Genitourinary tract
Associated with ‘swimmer’s itch’ and rash, urinary frequency and haematuria Can cause “Katayama syndrome” – fever, urticaria, angioedema, diarrhoea, abdominal pain, myalgia Risk factor for squamous cell bladder cancer |
Urine microscopy
Serology |
| Paragonimus westermani | Lung, CNS, GI tract
Lung –pleuritic chest pain, cough, dyspnoea CNS –meningitis, encephalitis GI tract –fever, bloody diarrhoea, abdominal pain |
Serology
Sputum or stool microscopy |
| Clonorchis sinensis | Hepatopancreaticobiliary tract
Presents as right upper quadrant pain, diarrhoea, nausea/vomiting, weight loss Risk factors for cholangiocarcinoma |
Stool microscopy
Serology |
| Fasciola hepatica | GI tract
Fever, right upper quadrant pain, hepatomegaly This can eventually lead to biliary obstruction |
Serology
A CT scan shows characteristic tortuous tracks in the liver (migration of larvae) Stool microscopy is only positive after the acute infection |
Table 6: Clinical features, diagnosis and management of common trematodes
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