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7.1 Unusual Tissue Dwelling Nematodes
Many species of nematodes are known to indirectly infect man causing an array of symptoms and diseases. Many of the larvae of these parasites cause serious zoonosis in man. These parasites are known to infect many intermediate hosts during their life cycle with a variety of animals being the definitive host, while man is normally an accidental definitive host.
Angiostrongylus (Parastrongylus) cantonensis
Angiostrongylus cantonensis was first described in 1944 in Taiwan from a boy with suspected meningitis. Since then it has been recognized throughout the Pacific areas with sporadic cases reported in other parts of the world including Central America and Cuba It is associated with eosinophilic meningitis. The species A. cantonensis and A. costaricensis have now been reclassified into the genus Parastrongylus.
Angiostrongylus cantonensis is also known as the rat nematodes where the main cause of eosinophilic meningitis is infection with the larvae of this nematode. Man and rats become infected by eating infected mollusks of the giant land snail (intermediate hosts) or food which is contaminated by the snails’ bodies.
Life Cycle and Transmission
Adult worms of A. cantonensis live in the pulmonary arteries of rats. The females lay eggs that hatch, yielding first-stage larvae, in the terminal branches of the pulmonary arteries. The first-stage larvae migrate to the pharynx, are swallowed, and passed in the feces. They penetrate, or are ingested by, an intermediate host (snail or slug). After two molts, third-stage larvae are produced, which are infective to mammalian hosts. When the mollusk is ingested by the definitive host, the third-stage larvae migrate to the brain where they develop into young adults. The young adults return to the venous system and then the pulmonary arteries where they become sexually mature. Of note, various animals act as paratenic (transport) hosts: after ingesting the infected snails, they carry the third-stage larvae which can resume their development when the paratenic host is ingested by a definitive host. Humans can acquire the infection by eating raw or undercooked snails or slugs infected with the parasite; they may also acquire the infection by eating raw produce that contains a small snail or slug, or part of one. There is some question whether or not larvae can exit the infected mollusks in slime (which may be infective to humans if ingested, for example, on produce). The disease can also be acquired by ingestion of contaminated or infected paratenic animals (crabs, freshwater shrimp). In humans, juvenile worms migrate to the brain, or rarely in the lungs, where the worms ultimately die. The life cycle of Angiostrongylus (Parastrongylus) costaricensis is similar, except that the adult worms reside in the arterioles of the ileocecal area of the definitive host. In humans, A. costaricensis often reaches sexual maturity and release eggs into the intestinal tissues. The eggs and larvae degenerate and cause intense local inflammatory reactions and do not appear to be shed in the stool.
Angiostrongylus cantonensis is a long slender worm measuring between 17 and 25 µm long by 0.26 to 0.34 µm wide.
Image 7-1. The adult worms of Angiostrongylus cantonensis. They measure between 17–25µm long and reside in the pulmonary arteries and arterioles of the definitive hosts. (x 3.5) (SOURCE: CDC)
The incubation period is usually about 20 days but can be up to 47 days. Infection in man is usually self-limiting but can result in fatalities. The main symptom is severe headache but other symptoms include convulsions, vomiting, facial paralyses, paresthesia, neck stiffness and fever. It is accompanied by moderate eosinophilia in the CSF (i.e. the cell count is 100 – 2000/µm3). Blood eosinophilia is also common. Eye involvement is accompanied by visual impairment, ocular pain, keratitis and retinal edema. Living worms have been reported and are removed surgically.
Presumptive diagnosis can be made on the basis of the patients’ symptoms (i.e. fever, meningitis, ocular involvement and severe headache). Eosinophilia in CSF and peripheral blood is also suggestive. Larvae or young adults can be recovered in the CSF. ELISA methods can also provide confirmation.
Angiostrongylus (Parastrongylus) costaricensis
Angiostrongylus costaricensis was first described in 1971. Human infections are most common in Costa Rica but have been reported in Mexico, Central and South America.
The life cycle is similar to A. cantonensis, (above) with the human being an accidental host by consumption of snails and salad vegetables which have become contaminated by infective larvae that have been shed by slugs in their mucus. Unlike A. cantonensis, the larvae of A. costaricensis penetrates the intestinal wall and results in inflammatory lesions of the bowel wall. Here the life cycle ends.
The eggs are 90mm, oval, thin shelled and can be embryonated. The adult female measures 42 x 300µm and the males measure 22 x 140µm.
Image 7-2. A: Eggs of Angiostrongylus costaricensis. B: Larva of A. costaricensis. In humans, eggs and larvae are not normally excreted, but remain sequestered in tissues. Both eggs and larvae (occasionally adult worms) of A. costaricensis can be identified in biopsy or surgical specimens of intestinal tissue. The larvae need to be distinguished from larvae of Strongyloides stercoralis; however, the presence of granulomas containing thin shelled eggs and/or larvae serve to distinguish A. costaricensis infections. (SOURCE: CDC)
Image 7-3. Picture showing the adult Angiostrongylus costaricensis worms in the natural host of a rat. The worms normally localize within the mesenteric arteries, especially those of the ileocecal region of the natural definitive host. (SOURCE: CDC)
The most common symptoms are pain and tenderness with fever, vomiting and diarrhea. A tumor-like mass is often palpable and can mimic malignancy. The symptoms of abdominal pain, vomiting and diarrhea, and anorexia, are often mistaken for those of appendicitis. Worms can be found in the regional lymph nodes and mesenteric arteries. They can also be found in the spermatic arteries causing testicular obstruction and necrosis. Occasionally the larvae and the ova reach the liver and symptoms may mimic visceral larvae migrans. Eosinophilia is also present.
Anisakiasis is a collective name for infections belonging to the genera of nematodes, Anisakis, Phokanema, Terranova and Contracaecum. The adult worms from the many species belonging to this family, Anisakidae, occur in various sea mammals, seals, dolphins, porpoises and whales, the larvae can cause a serious zoonosis, anisakiasis, in man. Anisakiasis was first reported in the Netherlands in 1960 and since then cases have been reported from Japan, North America, Canada, Chile, and the United Kingdom with the increase in popularity of ‘sushi’.
Two species of nematode have been especially implicated worldwide in causing human anisakiasis, Anisakis simplex and Pseudoterranova osculatum. The larvae of P. osculatum can be distinguished from A. simplex by the possession of a cecum.
These nematodes are parasites in a large number of marine animals including seals, sea-lions, whales, and dolphins. It is in these mammals that the adult worm is found. Eggs are passed out in the feces of these mammals, they embryonate and hatch to liberate the first stage larvae into the sea water. These larvae then become ingested by crustaceans and molt to become L2 and L3 larvae. If fish and squid eat the infected crustaceans, the third stage larvae become liberated and penetrate the intestine or muscles of that host. These larvae then encyst. Over 100 species of fish can act as intermediate hosts. The marine mammals ingest the fish and thus the cycle is completed.
Humans become infected by consuming raw or improperly cooked fish which contain the third stage larvae.
Illustration 7-2. Adult stages of Anisakis simplex or Pseudoterranova decipiens reside in the stomach of marine mammals, where they are embedded in the mucosa, in clusters. Unembryonated eggs produced by adult females are passed in the feces of marine mammals . The eggs become embryonated in water, and first-stage larvae are formed in the eggs. The larvae molt, becoming second-stage larvae , and after the larvae hatch from the eggs, they become free-swimming . Larvae released from the eggs are ingested by crustaceans . The ingested larvae develop into third-stage larvae that are infective to fish and squid . The larvae migrate from the intestine to the tissues in the peritoneal cavity and grow up to 3 cm in length. Upon the host's death, larvae migrate to the muscle tissues, and through predation, the larvae are transferred from fish to fish. Fish and squid maintain third-stage larvae that are infective to humans and marine mammals . When fish or squid containing third-stage larvae are ingested by marine mammals, the larvae molt twice and develop into adult worms. The adult females produce eggs that are shed by marine mammals . Humans become infected by eating raw or undercooked infected marine fish . After ingestion, the anisakid larvae penetrate the gastric and intestinal mucosa, causing the symptoms of anisakiasis. (SOURCE: CDC)
The larvae reach a length of 50µm with a diameter of 1-2µm. Classification of the Anisakids is made by the structure of the digestive tract.
Illustration 7-3. Line diagram representing the morphology of the third stage larvae of Anisakis simplex (from fish), it is the causative organism of anisakiasis in man. (SOURCE: Smyth, J.D, 1994)
After ingestion of the raw fish by humans, the larvae penetrate the intestinal wall resulting in inhabitation of the stomach or duodenum, but can be found in any part if the alimentary canal or outside the gut in various viscera. The end result is abdominal pain, nausea, and sometimes vomiting and diarrhea, often occurring within 6 hours of eating an infected meal. Abdominal irritation may mimic gastric ulcer, carcinoma, appendicitis or other conditions requiring surgery. Eosinophilic granulosus may result.
Transient anisakiasis which has been reported in North America is characterized by some vomiting and distress within a few hours of ingesting the fish and then quickly subsides. The larvae can be coughed up a few days later. Low grade eosinophilia and occult blood in the stool are common.
Diagnosis can only be confirmed by endoscopy and the removal of the worms by biopsy forceps and microscopic identification.
Epidemiology and Prevention
Anisakis larvae are usually found in herring, mackerel, and North American salmon. Pseudoterranova are found in cod, halibut, rockfish, sardines, and squid.
Human infections result from eating raw fish, insufficiently cooked or smoked fish, marinated or salted fish. Freezing fish at -20°C (-4°F) for a minimum of five days kills the larvae and thus the fish is suitable for consumption in dishes like sushi. Smoking the fish at temperatures >65°C (149°F) renders it suitable for consumption. Marinating or salting the fish cannot be depended upon to kill the larvae, however adequately cleaning the fish can render it safe for consumption. Human anisakiasis has been virtually eliminated from the Netherlands due to the mandatory freezing of herring.
Several species of the genus Gnathostoma are responsible for zoonotic infections in man. Gnathostoma spinigerum is a nematode found in dogs, cats, and several other carnivores. Human infections of the disease have been reported from Japan, China, Thailand, the Far East, and the Philippines, with man acquiring the infection from eating various freshwater fish.
The life cycle of this parasite involves two intermediate hosts, cyclops and birds, snakes, fish and frogs where they mature before developing into adults in the definitive hosts.
Illustration 7-4. Diagram illustrating the life cycle of Gnathostoma spinigerum. In the natural definitive host (pigs, cats, dogs, wild animals) the adult worms reside in a tumor which they induce in the gastric wall. They deposit eggs that are unembryonated when passed in the feces . Eggs become embryonated in water, and eggs release first-stage larvae . If ingested by a small crustacean (Cyclops, first intermediate host), the first-stage larvae develop into second-stage larvae . Following ingestion of the Cyclops by a fish, frog, or snake (second intermediate host), the second-stage larvae migrate into the flesh and develop into third-stage larvae . When the second intermediate host is ingested by a definitive host, the third-stage larvae develop into adult parasites in the stomach wall . Alternatively, the second intermediate host may be ingested by the paratenic host (animals such as birds, snakes, and frogs) in which the third-stage larvae do not develop further but remain infective to the next predator . Humans become infected by eating undercooked fish or poultry containing third-stage larvae, or reportedly by drinking water containing infective second-stage larvae in Cyclops .
The adult female worms measure 25-54µm whereas the male measures 11-25µm. The anterior half of the worm is covered with leaf like spines. The male worms have a red tail while the larger females have a more curved tail. The larval worms are 4µm long.
Image 7-4. Adult worms of Gnathostoma spinigerum. The males are smaller than the females and possess a red tail. (x 1.3) (SOURCE: CDC/Medscape)
Image 7-5. Third stage larval Head of Gnathostoma spinigerum. (SOURCE: CDC/Medscape)
Humans are accidental hosts and after ingestion, the larvae do not mature but migrate throughout the body via the intestinal wall. Symptoms include epigastric pain, vomiting and anorexia. These symptoms subside as the larvae continue their migratory path through the cutaneous tissue. Evidence of migration appears as either lesions similar to cutaneous larvae migrans or migratory swellings accompanied by inflammation, redness or pain. The swelling is hard and non-pitted and lasts for several days. These migratory lesions may be accompanied by pruritis and pain. There is marked eosinophilia in patients with cutaneous involvement. Ocular involvement resulting in blindness may occur in serious disease. Eosinophilic myeloencephalitis may result from migration of the worms along the nerve tracks. Symptoms may include pain, paralyses, seizures, coma and death. The CSF may be xanthochromic (yellowish discoloration) or bloody.
Presumptive diagnosis may be made on the basis of clinical symptoms. Definitive diagnosis is the recovery and identification of the worm since the symptoms may be suggestive of Sparganosis, paragonimiasis and cutaneous larvae migrans and myiasis. A bloody spinal fluid or xanthochromia may resemble infection with Angiostrongylus cantonensis.
Trichinella spiralis was first seen by James Paget, but was named and described by his Professor, Richard Owen. The family Trichinellidae contains only one single genus; Trichinella, and was originally thought only to contain the one species; Trichinella spiralis, which causes the serious and often fatal disease in man known as trichinosis (Trichinellosis). It is a parasite of carnivorous animals and is especially common in rats and in swine fed on uncooked garbage and slaughter house scraps. Humans become infected by eating raw pork, with sausages being the most common cause of infection. It is a cosmopolitan parasite and is common in China, though prevalent in many European countries as well.
It is now thought that there are four varieties of this species that exists worldwide;
Trichinella spiralis is a ‘domestic’ parasitic nematode long recognized to cause a zoonosis transmitted to man by the ingestion of infected pork.
Infection in the definitive hosts is acquired by the hosts eating raw or undercooked flesh (e.g. pork, containing encapsulated larvae). Rats are probably the most highly infected ‘natural’ hosts and pigs become infected by eating infected pork scraps or occasionally rats which inhabit their stalls. For man, sausages are a dangerous source of the parasite as a small fragment of infected pork, (after mincing), may become widely distributed among a number of sausages.
Illustration 7-5. The life cycle of Trichinella spiralis. Trichinellosis is acquired by ingesting meat containing cysts (encysted larvae) of Trichinella. After exposure to gastric acid and pepsin, the larvae are released from the cysts and invade the small bowel mucosa where they develop into adult worms (female 2.2 mm in length, males 1.2 mm; life span in the small bowel: four weeks). After one week, the females release larvae that migrate to the striated muscles where they encyst . Trichinella pseudospiralis, however, does not encyst. Encystment is completed in 4 to 5 weeks and the encysted larvae may remain viable for several years. Ingestion of the encysted larvae perpetuates the cycle. Rats and rodents are primarily responsible for maintaining the endemicity of this infection. Carnivorous/omnivorous animals, such as pigs or bears, feed on infected rodents or meat from other animals. Different animal hosts are implicated in the life cycle of the different species of Trichinella. Humans are accidentally infected when eating improperly processed meat of these carnivorous animals (or eating food contaminated with such meat). (SOURCE: CDC/DPDx Trichinellosis)
The adult female worm is about 2-3µm long and 90µm in diameter. The male is smaller measuring 1.2µm long by 60µm in diameter.
The female adult worms are ovoviparous (born alive from the mother but held as an unshelled egg internally until birth, after having no umbilical connection to the mother) and up to 1500 larvae may be released by a single worm.
Image 7-6. A, B: Larvae of Trichinella, freed from their cysts, typically coiled; length: 0.8 to 1.0 µm. Alaskan bear. (SOURCE: CDC)
Symptoms during the intestinal phase may go unnoticed or may be severe. Epidemics can result in outbreaks of gastro-enteritis, two to seven days after the ingestion of raw pork. Diarrhea with or without abdominal pain may last for several weeks. Eosinophilia and fever occur in most cases. Leukocytosis is common and hyperglobulinaemia (an autoimmune, chronic blood disease) is characteristic. Myocytosis and circum orbital edema are classic signs. There can also be central nervous system involvement
The primary pathogenic effect of Trichinella comes from the destruction of the striated muscle fibers in which it encysts. There can be neurological manifestations of trichinosis and death may be ascribed to myocarditis, encephalitis or pneumonitis.
Diagnosis of trichinosis depends on the clinical signs, such as myalgia, periorbital edema, fever and eosinophilia in a patient with a history of eating pork or sausages.
Serological tests are available but may be negative if carried out within three to four weeks post infection. Circulating antibodies to T. spiralis appear from two to four weeks after infection. Redefined diagnostic antigens for their detection are currently being developed. A simple IFAT employing fragments of larvae as antigen is a useful diagnostic tool. Latex tests with extracted larval antigens have also proved valuable in the acute stage, during which high antibody titers develop.
Illustration 7-6. Diagrammatic representation of the larvae encapsulated in striated muscle. (SOURCE: Smyth, J.D, 1994)
Image 7-7. A, B: Encysted larvae of Trichinella in pressed muscle tissue. The coiled larvae can be seen inside the cysts. (SOURCE: CDC)
Muscle biopsy is available with the muscle being digested in pepsin, which frees the encapsulated larvae or by a simple device whereby the muscle sample is compressed between two glass plates to make it semi-transparent, allowing you to see any encapsulated larvae using a ‘trichinoscope’ (a simple magnifying system).