While the average Canadian physician deals very little with worm infestations, disease caused by helminths are the cause of considerable morbidity and mortality in large areas of the world, in particular the developing world. Increases in international travel and immigration make it increasingly important to be aware of the common helminths. The US Center for Diseases Control has a superb general helminth website called DPDx – Identification and Diagnosis of Parasites of Public Concern that will be linked to in this section.
As with other biological classifications, the taxonomy of the Helminthes is complicated. Worms of medical interest are broadly divided into Phylum Nematoda – the roundworms, and Phylum Platyhelminthes – the flatworms. Flatworms are further divided into trematodes (flukes) and cestodes (tapeworms).
Worms Most Likely to be Considered by Canadian Physicians
The pinworm is the only worm that a Canadian primary care physician will be confronted with on a regular basis. It is a relatively common infestation of small children especially in daycare settings and elementary school. These small (5 – 10 mm long) white worms reside in the proximal colon. The females migrate onto the perianal skin at night and deposit eggs. They hatch and develop very quickly into infective larvae which are ingested completing the cycle.
The only symptom in the vast majority of infested individuals is pruritis, sometimes intense. Transmission is promoted by scratching with larvae deposited under the nails. In the pre-school setting, sand or rice play tables have been implicated in outbreaks. Occasionally, obstruction of the appendix by worms can result in acute appendicitis.
Further Reading — DPDx Enterobiasis
This worm has a distribution and life cycle that is similar to the hookworms with one very important exception. Auto infection is possible and in immunocompromised individuals a very serious “hyper-infection syndrome” can be fatal. While S. stercoralis is not endemic in Canada, it is important that all patients that will be undergoing transplantation be screened for this helminth especially if they have ever resided in an endemic area.
Further Reading — DPDx Strongyloidiasis
Cutaneous Larva Migrans
Another example of humans as accidental hosts, cutaneous larva migrans or “creeping eruption” is caused by the canine hookworm Ancylostoma braziliense. Larva penetrate the skin, usually of the feet and then cause intense inflammation and itching as they migrate. This is one of the primary reasons dogs are not allowed on the beach in many areas. With the large amount of travel to the beaches of the Caribbean and Mexico, this is a condition seen with increasing frequency in Canada.
Worms of Extreme World Import Encountered Rarely in Canada
This large roundworm is, perhaps, the most common parasite of human’s inhabiting as many as one quarter of all people. The life cycle is a typical one for intestinal nematodes. Eggs are passed in the faeces and after a period of maturation in the soil are ingested. Larvae hatch, penetrate the intestinal wall and reach the venous circulation by which they are deposited in the lungs. At this time patients may present with pneumonitis. Larvae are then coughed up, swallowed and then mature into adults that reside in the small intestine without attaching. A single mature female can lay up to 200,000 eggs per day.
Most often infestations are asymptomatic, however large worm burdens can be associated with intestinal obstruction. Obstruction of the common bile duct is another well-recognized complication and appendix obstruction is also possible. Now rare in Canada, occasional large worms are brought to family physicians by very concerned individuals!
Further Reading — DPDx Ascariasis
The whipworm. The third most common helminth infestation worldwide. Most often asymptomatic, heavy worm burdens can cause anemia.
Further Reading – DPDx Trichuriasis
Human hookworms include Ankylostoma duodenale the “old world” hookworm and Necator americanus the “new world” hookworm. Very common in warm climes hookworm larvae penetrate intact skin of the feet, migrate to lungs in veins, are coughed up, swallowed and attach as adults to small intestinal mucosa. Large infestations lead to iron deficiency anemia and are a particular problem for young children where they may contribute to developmental delay.
Further Reading — DPDx Hookworm
Visceral Larva Migrans (Toxocariasis)
The canine and feline Ascarids Toxocara canis and Toxocara cati have the ability to infect humans but cannot complete their life cycle. After ingestion of eggs excreted by dogs or cats humans are “accidental hosts” and the larval forms penetrate the intestinal mucosa and “wander aimlessly” causing local inflammatory reaction. Clinical disease occurs most frequently in small children with an acute febrile illness with hepatosplenomegally. The diagnosis is strongly suggested by the finding of very high levels of eosinophilic polymorphonuclear leukocytes on routine CBC testing. The finding of Toxocara serum antibodies confirms the diagnosis.
Further Reading — DPDx Toxocariasis
Trichinellosis (trichinosis) is a disease with great historical import. It is the primary reason that pork is not eaten by some large groups within the world and that recommendations for thorough cooking of pork are made. Today, great efforts are made to keep food animals free of this helminth and clinical cases are very rare in Western countries. Other species of Trichinella have been described to cause disease, most notably T. nativa found in Polar bears. Cases of trichinellosis in the Canadian north have been attributed to the consumption of bear meat.
Clinical disease is caused by the inflammatory response to encysted larval forms in tissue. It can be very severe or even fatal. Classically, patients present with a triad of periorbital edema, myalgia and extreme eosinophilia.
Further Reading — DPDx Trichinellosis
The Guinea worm. Disease is referred to as dracunculiasis or dracontiasis. After ingestion of drinking water containing copepods infected with larvae, the larvae penetrate gastric mucosa. In the retroperitoneum maturation into adults and copulation occurs and females then migrate down into the subcutaneous tissues of the legs. A sore appears on the skin through which the head emerges and discharges eggs into water during bathing.
Secondary infections are common and are the cause of most pathology. Cellulitis is a life-threatening condition in many areas where Dracuncula infections are prevalent.
Further Reading — DPDx Dracunculiasis
The term filaria describes several species of small nematodes that infect humans through the bite of an insect. Most including Wucheria spp. and Brugia spp. are “Lymphatic” filaria that cause disease by inhabiting and disrupting the function of lymphatics. They are the most common cause of “elephatiasis” – gross swelling and thickening of skin secondary to chronic obstruction of lymphatics. Onchocerca volvulus has a predilection for the eye and is the cause of “river blindness”.
Further Reading — DPDx Filariasis
Schistosomiasis is an extremely important disease in the developing world with more than 200 million people affected. It is caused by several species of “blood flukes” the most important of which are S. hematobium and S. mansoni. Please look at DPDx Schistosomiasis for a description of the life cycle and snail vectors.
S. hematobium is endemic in the Nile Delta and is found in parts of the middle east. The adult worms inhabit the veins of the bladder and result in ureteric obstruction. This is a very common cause of renal failure. Transitional cell carcinoma is also more common in people infected with S. hematobium.
S. mansoni is found in Africa and parts of Central and South America including some Caribbean islands. Worms inhabit portal veins and disease is caused principally by egg deposition in the liver causing inflammation and fibrosis with ultimate obstruction of portal venous flow and resultant ascites and portal-systemic varices.
Further Reading — DPDx Schistosomiasis
Human tape worms include Taenia saginata – the beef tapeworm, Taenia solium – the pork tapeworm, Dyphillobothrium latum – the fish tapeworm and Hymenolepis nana – the dwarf tapeworm. Infestations with adult tapeworms are usually asymptomatic.
However, severe disease can be caused by larval forms of some tapeworms. Humans can be “accidental intermediate hosts” for several species of tape worm, most importantly T. solium the cause of cysticercosis and the canine tapeworms Echinococcus granulosis and Echinococcus multilocularis.
When humans ingest the eggs of Taenia solium, larval forms may encyst in many kinds of tissue. The most serious manifestation is in brain referred to as neurocysticercosis. This is the most common cause of adult onset epilepsy in the world and very common reason for neurosurgical intervention in many countries.
Further Reading — DPDx Cysticercosis
The larval stages of the canine tapeworms Echinococcus granulosis and E. multilocularis form cysts in many organs, most commonly the liver. Cysts cause by E. granulosis may be large and are most often treated by excision. Great care must be taken not to spill the contents of the cysts as many “daughter” cysts will form and be extremely difficult to treat. E. granulosis is most common in areas where sheep are grazed. Active worm eradication programs in sheep dogs is the most effective means of limiting human infections and is a priority in places such as New Zealand.
E. multilocularis is found in the North, wolves are the primary definitive hosts. Cases are occasionally seen in Northern Canadian residents. Liver disease is usually more diffuse, hence the name, and more difficult to excise.
Further reading — DPDx Echinococcosis