The Human Pathogens and Toxins Act Application to Helminths

This Statement of Administrative Intent is intended to clarify when helminths are within the scope of the Human Pathogens and Toxins Act (HPTA).

  1. Helminths

    Helminths are eukaryotic, worm-like parasites belonging to phylum Platyhelminthes (i.e., flatworms, including flukes and tapeworms) and phylum Nematodes (i.e., round worms). More than 340 species of helminths are capable of infecting humans.

    While helminths are quite large in their adult form (sometimes metres long) the egg and larval stages of development for certain species can be classified as micro-organisms.1

  2. When is a Helminth a Human Pathogen?
    • If the infectious form of a helminth is a micro-organism and the helminth meets the definition of a Risk Group 2, Risk Group 3 or Risk Group 4 human pathogen as defined in the HPTA, then it is considered a human pathogen.
    • If the infectious form of a helminth is a micro-organism, and is not listed in any of the Schedules to the HPTA and it is unclear if the helminth meets the definition of Risk Group 2, Risk Group 3 or Risk Group 4 human pathogen, then a risk assessment should be conducted and the Agency should be consulted.
      • Note that the risk assessment should be more precautionary as the level of potential risk increases.

    For additional information on the stages of helminth development commonly responsible for pathologic changes in humans and the species of helminths most commonly reported from human hosts, including size of the infectious stage to humans, please consult Table 1 and Table 2.

Table 1: Stages of Helminths Commonly Responsible for Pathologic Changes in Humans.
Helminth Egg Larva Adult
Flukes Infective InfectiveTable note * Infective
Tapeworms Non-infective Infective Infective
Roundworms Non-infective Infective Infective

* Migrating and developing larva forms may cause transient pathologic responses in the host

Table 2: Helminths most commonly reported from human hosts, including size of the infectious stage to humans.
Parasitic worms Infectious
stage to humans
Life cycle
* denotes
infections stage
Sources Mode of transmission Clinical
Symptoms
Ancylostoma duodenale Third-stage larva (500-600 μm long)
  1. eggs in feces
  2. rhabditiform larva hatches
  3. filariform larva (third-stage larva)*
  4. adults in small intestines
Soil, feces, or contaminated surfaces Penetration of intact skin / oral route
  • Iron deficiency anemia
  • Can be accompanied by cardiac complications
  • Gastrointestinal and nutritional/metabolic symptoms
  • Local skin manifestations during penetration of filariform larvae
Angiostrongylus cantonensis Third-stage larvae (375 to 420 μm in length)
  1. eggs hatch, and first-stage larvae are passed in rodent feces
  2. slugs and snails are intermediate hosts and after 2 months, the larvae reach the infective (third) stage
  3. humans become infected through food containing third-stage larvae*
Food items: uncooked snails or slugs, vegetables contaminated with snails, slugs or mollusk secretions, or infected paratenic hosts (i.e. crabs, freshwater shrimp)
Humans do not transmit A. cantonensis or A. constaricensis
Ingestion
  • Clinical symptoms of eosinophilic meningitis are caused by the presence of larvae in the brain and by local host reactions. Symptoms include: severe headaches, nausea, vomiting, neck stiffness, seizures, and neurologic abnormalities. Occasionally, ocular invasion occurs. Eosinophilia is present in most of cases. Most patients recover fully. Abdominal angiostrongyliasis mimics appendicitis, with eosinophilia.
Ascaris spp. Fertilized eggs (45-75 x 15 μm)
  1. fertilized egg (unfertilized egg may be ingested but is not infective)
  2. 18 days to several weeks later fertile eggs become infective*
  3. infective eggs are swallowed
  4. larvae hatch
  5. larvae mature in lungs
  6. reach intestines and develop into adult worms
Soil contaminated with human or pig feces, or contaminated water, food, agricultural products. Ingestion or inhalation of mature eggs
  • adult worms usually cause no acute symptoms
  • may cause stunted growth
  • abdominal pain and intestinal obstruction
  • occlusion of the biliary tract or oral expulsion possible
  • pulmonary symptoms can occur (cough, dyspnea, hemoptysis, eosinophilic pneumonitis - Loeffler's syndrome)
Brugia spp. L3 larvae (third-stage larvae)
B. tupaiae: 1mm x 21 μm
B. malayi: 1.5mm x 25 μm
B.pahangi: 1.6mm x 25 μm
  1. mosquito takes a blood meal (L3 larvae enter skin)*
  2. adults in lymphatics
  3. adults produce sheathed microfilariae that reach the blood stream
  4. mosquito takes a blood meal (ingests microfilariae)
  5. microfilariae shed sheaths, penetrate mosquito's midgut and migrate to thoracic muscles
  6. L1 larvea
  7. L3 larvea
  8. migrate to head and mosquito's proboscis
Mosquito Mosquito bite
  • infections involving small numbers of worms are often asymptomatic
  • Early symptoms of lymphatic filariasis consist of intermittent fever and enlarged, tender lymph nodes
  • In a small number of chronic cases, permanent lymphatic dysfunction caused by repeated exposure to infection over a number of years results in the massive lymphedema and accumulation of excess tissue known as elephantiasis.
Clonorchis sinensis Metacercariae (166 μm x 148 μm)
  1. embryonated eggs in feces
  2. eggs are ingested by snails
  3. free-swimming cercariae encyst in the skin or flesh of fresh water fish
  4. metacerariae in flesh or skin of fresh water fish are ingested by human host*
  5. excyst in duodenum
  6. adults in biliary duct
Fresh water fish containing metacercariae in flesh or skin Ingestion
  • inflammation and intermittent obstruction of the biliary ducts. 
  • acute phase:
  • abdominal pain, nausea, diarrhea, and eosinophilia can occur
  • long-standing infections: cholangitis, cholelithiasis, pancreatitis, and cholangiocarcinoma can develop, which may be fatal
Echinococcus spp. Embryonated eggs
(30 x 34 μm up to 90 μm)
  1. embryonated eggs in feces*
  2. oncosphere hatches, penetrates intestinal wall
  3. hydatid cyst in liver, lungs
  4. protoscolices from cyst
  5. scolex attaches to intestines
  6. adult in small intestines
Eggs from infected dogs Ingestion E. granulosus infections remain silent for years before the enlarging cysts cause symptoms in the affected organs: abdominal pain, a mass in the hepatic area, and biliary duct obstruction
  • chest pain, cough, and hemoptysis
  • rupture of the cysts can produce fever, urticaria, eosinophilia, and anaphylactic shock, cyst dissemination
  • brain, bone, heart can also be involved
  • E. multilocularis affects the liver as a slow growing, destructive tumor, with abdominal pain, biliary obstruction, and occasionally metastatic lesions into the lungs and brain
  • E. vogeli affects mainly the liver, where it acts as a slow growing tumor; secondary cystic development is common.
Enterobius vermicularis Embryonated eggs (60 μm x 27 μm)
  1. eggs are deposited on perianal folds
  2. larvae inside the eggs mature within 4 to 6 hours
  3. self-infection occurs by transferring infective eggs to the mouth
  4. larvae hatch in small intestine
  5. adults in lumen of cecum
Food, or contaminated hands
Contaminated clothes or bed linens.
Small number of eggs may become airborne
Direct contact (hand to mouth), Inhalation
  • frequently asymptomatic
  • Perianal pruritus
  • excoriations and bacterial superinfection possible
  • invasion of the female genital tract with vulvovaginitis and pelvic or peritoneal granulomas possible
  • anorexia, irritability, abdominal pain
Fasciola hepatica and Fasciola gigantica Metacercariae
(Eggs: 130-150 μm long by 60-90 μm wide)
  1. unembryonated eggs in feces
  2. embryonated eggs in water
  3. miracidia hatch, penetrate snail
  4. free-swimming cercariae encyst on water plants
  5. metacercariae on water plant ingested by human, sheep or cattle*
  6. excyst in duodenum
  7. adults in hepatic biliary ducts
Freshwater plants (watercress) Unboiled or unfiltered surface water Ingestion
  • acute phase: abdominal pain, hepatomegaly, fever, vomiting, diarrhea, urticaria and eosinophilia, and can last for months
  • chronic phase: intermittent biliary obstruction and inflammation.  Occasionally, ectopic locations of infection (such as intestinal wall, lungs, subcutaneous tissue, and pharyngeal mucosa)
Necator americanus Third-stage larva (60 μm x 35 μm)
  1. eggs in feces
  2. rhabditiform larva hatches
  3. filariform larva (third-stage larva)*
  4. adults in small intestines
Soil, contaminated surfaces, feces Penetration of intact skin / oral route
  • Iron deficiency anemia
  • can be accompanied by cardiac complications
  • Gastrointestinal and nutritional/metabolic symptoms
  • local skin manifestations during penetration of filariform larvae
Onchocerca volvulus L3 larvae (third-stage larvae; 500 μm by 25 μm)
  1. blackfly takes a blood meal (L3 larvae enter skin)*
  2. adults in subcutaneous nodule
  3. adults produce unsheathed microfilariae found in skin and lymphatics or connective tissue; occasionally in peripheral blood, urine, sputum
  4. blackfly takes a blood meal (ingests microfilariae)
  5. microfilariae shed sheaths, penetrate backfly's midgut and migrate to thoracic muscles
  6. L1 larvea
  7. L3 larvea
  8. migrate to head and blackfly's proboscis
Blakcfly Bite
  • Onchocerciasis is a chronic systemic illness associated with extensive and disfiguring skin changes, musculoskeletal complaints, weight loss, and changes in the immune system.
  • The principal organ affected in onchocerciasis is the skin; however, infection with O. volvulus can lead to severe visual impairment and blindness
Opisthorchis spp. Metacercariae (201 μm x167 μm)
  1. embryonated eggs in feces
  2. eggs ingested by snails
  3. free-swimming cercariae encyst in the skin or flesh of fresh water fish
  4. metacercariae in flesh or skin of fresh water fish are ingested by human*
  5. excyst in duodenum
  6. adults in biliary duct
Freshwater fish Ingestion of undercooked fish
  • frequently asymptomatic
  • dyspepsia, abdominal pain, diarrhea or constipation
  • longer duration infections: hepatomegaly and malnutrition
  • rarely: cholangitis, cholecystitis, cholangiocarcinoma
  • fever, facial edema, lymphadenopathy, arthralgias, rash and oesinophilia
Schistosoma spp Cercariae (380-496 x 50 mm, bird schistosoma;  ~ 225 mm long, S. mansoni) (Eggs size: 40-240 μm x  45-70 μm)
  1. eggs in feces or urine
  2. eggs hatch releasing miracidia
  3. miracidia penetrate snail tissue
  4. sporocysts in snail
  5. cercariae released by snail into water and free swimming*
  6. penetrate skin
  7. cercariae lose tail during penetration and become schistosomulae
  8. mature into adults
Water Penetration of skin
  • frequently asymptomatic
  • Acute schistosomiasis (Katayama's fever) may occur: fever, cough, abdominal pain, diarrhea, hepatosplenomegaly, and eosinophilia) -Occasionally central nervous system lesions occur: cerebral granulomatous disease
  • colonic polyposis with bloody diarrhea;portal hypertension with hematemesis and splenomegaly; cystitis and ureteritis with hematuria, which can progress to bladder cancer; pulmonary hypertension; glomerulonephritis; and central nervous system lesions.
Strongyloides stercoralis Filariform larvae (55 μm by 30 μm)
  1. rhabditiform larvae in the intestine area excreted in stool
  2. development into free-living adult worms
  3. eggs are produced by fertilized female worms
  4. rhabditiform larvae hatch from embryonated eggs
  5. rabditiform larvae develop into infective filariform
  6. infective filariform larvae penetrate the intact skin initiating the infection*
  7. filariform larvae enter the circulatory system
  8. adult worm in intestine
Soil, feces Penetration of skin
  • frequently asymptomatic
  • abdominal pain and diarrhea
  • Loeffler's synodrome
  • urticarial rashes in the buttocks and waist area
  • in immunosuppresssed patients: distension, shock, pulmonary and neurologic complications and septicaemia, and is potentially fatal
Taenia saginata Cysticerci (7.5 - 10mm wide by 4 - 6 mm in length)
  1. eggs or gravid proglittids in feces and passed into environment (not infectious to humans)
  2. cattle becomes infected by ingesting vegetation contaminated by eggs of gravid prolottids
  3. oncospheres hatch, penetrate intestinal wall and circulate to musculature
  4. oncospheres develop into cysticerci in muscle*
  5. humans infected by raw or uncooked infected meat
Intermediate host (cattle) Ingestions of undercooked meat
  • mild abdominal symptoms
  • occasionally appendicitis or cholangitis can result from migrating proglottids.
Taenia solium Cysticerci (6- 18mm wide by 4 - 6mm in length) OR eggs (30-40 μm in length)
  1. eggs or gravid proglittids in feces
  2. embryonated eggs and/or gravid prolottids ingested by pigs
  3. oncospheres hatch, penetrate intestinal wall and circulate to musculature
  4. oncospheres develop into cysticerci in pig muscle. Humans infected by ingesting raw or undercooked infected meat*  
  5. scolex attached to intestine
  6. adults in small intestine
  7. Alternatively, embryonated eggs (from feces/environment)ingested by human host
Intermediate host (pigs), water, contaminated hands Ingestions of undercooked meat, fecally contaminated water, direct contact (hand-to-mouth)
  • less frequently symptomatic than T. saginata
  • risk of development of cysticercosis (formation of cysts in brain and muscles that persist for years; the cysts will eventually cause an inflammatory reaction presenting as painful nodules in the muscles and seizures when the cysts are located in the brain).
Toxocara canis
Toxocara cati
Embryonated eggs with larva (85 by 75 μm for T. canis and 75 by 65 μm for T.cati)
  1. eggs passed in dog's feces
  2. eggs embryonate and become infective in environment
  3. humans are accidental hosts that become infected by ingesting infective eggs in contaminated soil*
  4. eggs hatch larvae
Contaminated soil Ingestion
  • usually asymptomatic
  • two main clinical presentations of toxocariasis: visceral larva migrans (VLM) and ocular larva migrans (OLM).  In VLM, which occurs mostly in preschool children, the larvae invade multiple tissues (liver, heart, lungs, brain, muscle) and cause various symptoms including fever, anorexia, weight loss, cough, wheezing, rashes, hepatosplenomegaly, and hypereosinophilia.
  • Death occurs rarely, by severe cardiac, pulmonary or neurologic involvement.
  • In OLM, the larvae produce various ophthalmologic lesions, which in some cases have been misdiagnosed as retinoblastoma, resulting in surgical enucleation. OLM often occurs in older children or young adults, with only rare eosinophilia or visceral manifestations.
Trichinella spp. Encysted larvae (100 μm x 5 μm)
  1. ingestion of undercooked meat containing encysted larvae*
  2. larvae are released from cysts
  3. adults are developed
  4. lavra deposited in mucosa
  5. encysted larva in muscle
Undercooked meat Ingestion
  • Light infections may be asymptomatic
  • Intestinal invasion can be accompanied by gastrointestinal symptoms (diarrhea, abdominal pain, vomiting).
  • Larval migration into muscle tissues (one week after infection) can cause periorbital and facial edema, conjunctivitis, fever, myalgias, splinter hemorrhages, rashes, and peripheral eosinophilia.  --Occasional life-threatening manifestations include myocarditis, central nervous system involvement, and pneumonitis.
  • Larval encystment in the muscles causes myalgia and weakness, followed by subsidence of symptoms.
Trichuris trichiura Embryonated eggs (50 to 55 μm x 20 to 25 μm).
  1. unembryonated eggs in feces
  2. 2-cell stage
  3. advanced cleavage
  4. embryonated eggs are ingested*
  5. larvae hatch in small intestine
  6. adults in cecum
Soil-contaminated food Ingestion
  • frequently asymptomatic
  • heavy infections (especially in small children) causes gastrointestinal problems such as: abdominal pain, diarrhea, rectal prolapse) and possible growth retardation
Wuchereria bancrofti L3 larvae (260 μm x 10 μm)
  1. mosquito takes a blood meal (L3 larvae enter skin)*
  2. adults in lymphatics
  3. adults produce sheathed microfilariae that reach the blood stream
  4. mosquito takes a blood meal (ingests microfilariae)
  5. microfilariae shed sheaths, penetrate mosquito's midgut and migrate to thoracic muscles
  6. L1 larvea
  7. L3 larvea
  8. migrate to head and mosquito's proboscis
Mosquito Bite
  • infections involving small numbers of worms are often asymptomatic
  • Early symptoms of lymphatic filariasis consist of intermittent fever and enlarged, tender lymph nodes
  • In a small number of chronic cases, permanent lymphatic dysfunction caused by repeated exposure to infection over a number of years results in the massive lymphedema and accumulation of excess tissue known as elephantiasis.
  • 1. For the purpose of HPTA, micro-organism means any organism that cannot be reasonably detected by the naked human eye, where reasonably detected means: a) any dimension of the organism is less than 100 μm; or b) all dimensions of the organism are more than 100 μm, but the organism can only be seen under ideal conditions of lighting and/or contrasting surface: Public Health Agency of Canada, Statement of Administrative Intent, "Defining the term Micro-organism in the context of the Human Pathogens and Toxins Act".
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