Parasitic Infections

Introduction

Parasitic infections are caused by parasites that are found in our environment. Infection often occurs through ingestion of contaminated food or drinking water (eg Cryptosporidiosis, Giardiasis, helminthiasis, cysticercosis ). Other parasite infections are acquired through bites by insect vectors eg. Filariasis (mosquito), Leishmaniasis(sandfly), trypanosomiasis (Tsetse fly), etc and yet others are through direct penetration of skin through contaminated water( Schistosomiasis ) and direct contact with soil or sand (cutaneous larva migrans).

Epidemiology

The distribution of parasites often follow a geographic pattern and some are endemic in certain parts of the world eg African Sleeping illness ( Trypanosomiasis ), Chagas disease (South America ) Leishmaniasis ( Old World and New World ), filariasis (rural Asia, Africa, Caribbean) etc.

Other parasites are ubiquitous eg round worms, hookworms and giardia. Countries that have an agricultural industry also have zoonoses ( diseases that affect animals ) and parasites from animals may sometimes enter the human host, infecting them as an accidental host or an end host. Lifestyle habits also predispose to acquisition of parasitic disease eg. adventure travel, swimming in infested lakes, freshwater sports and ingestion of raw or undercooked meats. In certain parts of Asia, walking barefoot on the beach may predispose to cutaneous larva migrans.

Clinical Presentation

Parasitic infections can manifest in the following manner :

  1.  Pyrexia or febrile illness : Strongyloidiasis superinfection, Katayama fever, malaria (details of infection by malaria are covered in a separate section in this website)
  2. Skin itch, rash /urticaria : Scabies, hookworms, schistosomiasis (swimmer’s itch)
  3. Diarrhoeal disease, gastrointestinal syndrome (Helminthiasis, amoebiasis, giardiasis
  4. Systemic infections : Brain syndrome (Echinococcosis, Amoebic Meningo-encephalitis), Liver lesions (Clonorchis, Fascioliasis, Echinococcosis), myalgia/myositis (Sarcocystis, Trichinellosis), eye syndrome (Toxocariasis, Gnathostomiasis , Microsporidiosis), lung syndrome (Strongyloidiasis), anaemia (Helminthiasis), eosinophilic syndromes

History taking is important in diagnosing parasitic infections. An understanding of the patient’s risk factors, travel history, epidemiological exposure and lifestyle behaviour including dietary pattern are all important to pin down the diagnosis.

The most common presentation is “worms in stools” and anal itching especially at night. This may be associated with diarrhoea, abdominal pain or skin itch. In severe cases, the patient may present with malabsorption or anaemia.

Diagnosis

Laboratory investigations for workup of parasitic disease may include :

  1. Haematology : For the presence of anemia and eosinophilia
  2. Stools microscopy for ova, cysts, parasites
  3. Liver function test derangement
  4. Specific parasitic serology can be performed if suspected : Cysticercosis, Echinococcosis, Schistosomiasis, Fascioliasis, Trichinellosis, etc
  5. Biopsy of the affected tissue or organ for microscopic examination by the pathologist (histology)
  6. CT/ MRI imaging of affected organs (when required) to look for typical lesions eg water lily sign (Hydatid cyst)
  7. DNA amplification tests (e.g. PCR) on stool, tissue and blood samples may also be used in selected cases

Certain occupations/hobbies/recreational activities are associated with increased risk of acquiring parasitic infections:

  • Miners (Strongyloidiasis)
  • Slaughterhouse workers (Ascariasis)
  • River Rafters (Schistosomiasis)
  • Fishermen / Raw fish consumption(Diphyllobothrium latum/fish tapeworm)
  • Mountain/terrain bikers (Microporidiosis)
  • Frog/Snake handlers (Sparganosis)

Treatment

There are several anti-parasitic agents that are commonly used. “Deworming” medicines such as Albendazole and Mebendazole are familiar to many and can be bought over the counter. Others like Praziquantel, Ivermectin and Triclabendazole are prescription drugs which are used for more severe infections. Cutaneous larva migrans is treated by freezing the track of the migratory larva with ethyl chloride spray together with a short course of Albendazole.

In invasive parasitic disease involving vital organs like lungs, brain, heart, treatment often involve administering a short course of systemic steroids together with anti-parasitic agents to prevent side effects of tissue swelling that may occur from release of cytokines following parasite killing. Strongyloidiasis with hyperinfection ( seen in HIV patients ) may require the concomitant use of antibiotics as well. Filariasis sometimes respond to antibiotics like Doxycycline.

Ocular parasite syndrome will require referral to an ophthalmologist. Toxoplasmic chorioretinitis require a course of sulfadiazine or clindamycin and Pyrimethamine to prevent disease progression and blindness.

There is a small group of people suffering from a syndrome known as delusional parasitosis or Morgellon’s Disease where they begin to experience fibres and dirt coming out of their skin and have the sensation of crawling parasites in their skin. In this syndrome, blood, tissue and other samples do not demonstrate any parasitic infection.

Prevention

Maintaining good personal hygiene is the key to preventing parasitic infection. Some useful tips include :

  1. Minimise consumption of raw or uncooked meats, including sashimi. Recent reports of Anisakiasis in raw fish have scared off many raw fish aficionados
  2. Handwashing after contact with soil, pets, raw meat, fish or litter boxes (used for cats and dogs)
  3. Drink bottled/boiled water when in rural areas or in the wilderness. Runoff mountain water has been known to be contaminated with Giardia and cryptosporidium. The water may also be filtered (certified filters, NSF 53/58)
  4. Do not eat salads in underdeveloped countries as the vegetables may be contaminated by human/animal waste used as fertilizer. The water used to wash the vegetables may also not be clean
  5. Do not swim in stagnant lakes as they may harbour snail parasites (schistosomiasis). Lake Malawi in Africa is well known for Bilharziasis (Schistosoma haematobium)
  6. Avoid sandfly and mosquito bites in certain area known to be endemic for filariasis and leishmaniasis

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