Volume 7.  Tissue Dwelling Nematodes



Co- Authors:  M. Arcari 1, A. Baxendine 1 and C. E. Bennett2


1. Intersep Ltd    2. University of Southampton




          Helminth Parasites                                                            


7.       Unusual tissues dwelling nematodes                                           1

Angiostrongylus cantonensis                                                          1

Angiostongylus costaricensis                                                         4

Anisakiasis                                                                                    6

Gnathostoma spinigerum                                                               8

Trichinella spiralis                                                                        10



7. 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, whilst man is normally an accidental definitive host.



Angiostrongylus cantonensis



Angiostrongylus cantonensis was first described in 1945 in Taiwan from a boy with suspected meningitis. Since then it has been recognised 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 (Peters & Gilles, 1995)


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 molluscs of the giant land snail (intermediate hosts) or food which is contaminated by the snails’ bodies.


Life cycle


The definitive host of A. cantonensis is the rat. Adult worms reside in the pulmonary arteries. (Diag. 1) The eggs are released, carried to the lungs in the bloodstream, they break through the pulmonary tract and are then swallowed.  The eggs hatch in the intestine and the 1st stage larvae are passed in the stools. The 1st stage larvae in the rats faeces are eaten by the intermediate hosts, from the various genera, Achatina, Cipangopaludina and Bradybaena and also various slugs. (Fig. 1)The larvae develop within these intemediate hosts to the 3rd stage larvae. These, infected molluscs are then eaten by rats where the larvae can complete the cycle. 


Human infection occurs by the ingestion of infective larvae in the slugs, snails or on vegetation.  The larvae migrate to the brain and cannot continue their growth cycle. It is the death of the larvae which invokes the inflammatory response in the meninges resulting in eosinophilic meningitis.













Diagram 1. Life cycle of the rat nematode Angiostrongylus cantonensis, which is known to infect humans and is the main cause of eosinophilic meningitis.



Angiostrongylus cantonensis is a long slender worm measuring between 17 and 25 mm long by 0.26 to 0.34 mm wide. (Fig. 2)



Figure 1. The 1st stage larvae of Angiostrongylus cantonensis. The larvae are found in the faeces of rats. (x 160)  (Peters & Gilles, 1995)



Figure 2. The adult worms of Angiostrongylus cantonensis. They measure between 17 – 25mm long and reside in the pulmonary arteries and arterioles of the definitive hosts. (x 3.5) (Peters & Gilles, 1995)



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, parasthesia, neck stiffness and fever.  It is accompanied by moderate eosinophilia in the CSF i.e. the cell count is 100 – 2000/mm3.  Blood eosinophilia is also common. Eye involvement is accompanied by visual impairment, ocular pain, keratitis and retinal oedema.  Living worms have been reported and are removed surgically.


Laboratory diagnosis

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 can also provide confirmation.


Angiostrongylus 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.


Life Cycle

The life cycle is similar to A. cantonensis, (Diag. 1) 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. costaricensus 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 300mm and the males measure 22 x 140mm. (Fig. 3 & 4)



Figure 3. Picture showing the caudal end of the male adult worm, Angiostrongylus costaricensis, showing clearly the copulatory bursa. (www.cdfound.to.it)



Figure 4.  Picture showing the adult Angiostrongylus costaricensis worms in the natural host of a rat. The worms normally localise within the mesenteric arteries, especially those of the ileocaecal region of the natural definitive host. (www.cdfound.to.it)



Clinical Disease

The most common symptoms are pain and tenderness with fever, vomiting and diarrhoea.  A tumour-like mass is often palpable and can mimic malignancy.  The symptoms of abdominal pain, vomiting and diarrhoea, 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, Tarranova 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 caecum.


Life cycle

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 faeces 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 moult 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 and 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.



The larvae reach a length of 50mm with a diameter of 1-2 mm. (Diag. 2) Classification of the Anisakids is made by the structure of the digestive tract.


Clinical Disease

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 diarrhoea, 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 characterised 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.




Diagram 2. Line diagram respresenting the morphology of the third stage larvae of Anisakis simplex  (from fish), it is the causative organism of anisakiasis in man. (Smyth, J.D, 1994)


Laboratory diagnosis

Diagnosis can only be confirmed by endoscopy and the removal of the worms by biopsy forceps and microscopical identification.


Epidemiology and prevention

Anisakis larvae are usually found in herring, mackerel and North American salmon. Pseudoterranova are found in cod, halibut, rockfish, sardine and squid.


Human infections result from eating raw fish, insufficiently cooked or smoked fish, marinated or salted fish. Freezing fish at -20°C for a minimum of 5 days kills the larvae and thus the fish is suitable for consumption in dishes like sushi. Smoking the fish at temperatures >65°C renders it suitable for consumption.  Marinating or salting the fish cannot be depended on 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.


Gnathostoma spinigerum



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, Far East and Philippines, with man acquiring the infection from eating various freshwater fish.


Life cycle

The life cycle of this parasite involves 2 intermediate hosts, cyclops and birds, snakes, fish and frogs where they mature before developing into adults in the definititve hosts. (Diag. 3)





 Diagram 3.  Diagram illustrating the life cycle of Gnathostoma spinigerum.


In the definitive hosts, which normally are carnivores, the adult nematodes live in nodules in the stomach wall.  The larvae develop inside the eggs, which are passed in the faeces and then into water where they are ingested by species of Cyclops.  They, in turn, are ingested by a number of fresh water fish, frogs, snakes and birds. The larvae pierce their gastric wall and develop into 3rd stage larvae and become encysted in the muscle.  Cats and dogs ingest the raw flesh of these animals and the larvae excyst and mature into adults in the stomach of cats and dogs.


If the infected fish or frogs are eaten by other hosts, apart from the definitive hosts, (paratenic host), such as herons, pigs and man, they do not mature but migrate through the subcutaneous tissues causing visceral and cutaneous larva migrans.



The adult female worms measure 25 - 54 mm whereas the male measures 11 - 25 mm. (Fig. 5) The anterior half of the worm is covered with leaf like spines. The male worms have a red tail whilst the larger females have a more curved tail. The larval worms are 4mm long.



Figure 5. Adult worms of Gnathostoma spinigerum. The males are smaller than the females and possess a red tail. The males measure between 11- 25mm whilst the females are larger, measuring between 25 – 54mm long. (x 1.3) (Peters & Gilles, 1995)


Clinical Disease

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. Eosinophillic 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 or bloody.


Laboratory diagnosis

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 Angiostrongylius cantonensis.

Trichinella spiralis



Trichinella spiralis was first seen by James Paget but was named and described by his Pofessor, 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 (trichinelosis). 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 prevalent in many European countries with the highest interest being in China.


It is now thought that there are four varieties of this species that exists worldwide;

Trichinella spiralis spiralis - temperate zone – high infectivity for pigs, rats and man.

Trichinella spiralis nelsoni - Tropics - low infectivity for pigs and rats and high infectivity for lions, hyenas.

Trichinella spiralis nativa - Arctic - low infectivity for pigs, found in polar bears, resistant to freezing.

Trichinella spiralis pseudospiralis- New Zealand – low infectivity for pigs, rats and mice.


Trichinella spiralis is a ‘domestic’ parasitic nematode long recognised to cause a zoonosis transmitted to man by the ingestion of infected pork.


Life cycle

Infection in the definitive hosts is acquired by the hosts eating raw or undercooked flesh e.g. pork, containing encapsulated larvae. (Diag. 4 & 6 & Fig. 7) 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.

 Humans become infected by eating raw meat containing encysted larvae.  The cyst becomes digested and releases the larvae which invade the intestinal mucosa. They develop and mate in the second day.  By the 6th day of infection, the female adults deposit motile larvae which are carried by the intestinal lymphatics or mesenteric venules to other tissues in the body.  The very active muscles, such as the diaphragm, jaws, tongue, larynx and eyes, are invaded and the larvae become encapsulated by the 21st day following infection. Calcification of the cysts occurring as early as five months, but usually begins after 6 –18 months. The cyst wall is derived from the host's muscle fibre and the larvae remain viable for many years with no further development occurring. When muscles are eaten by the definitive host, sexual maturation in the intestinal phase, as explained earlier, rapidly occurs.



Diagram 4. Diagram illustrating the life cycle of Trichinella spiralis which is the causative agent of the disease trichinosis which is often terminally fatal in man.




The adult female worm is about 2-3mm long and 90mm in diameter. The male is smaller measuring 1.2mm long by 60mm in diameter. (Diag. 5 & Fig. 6)


The female adult worms are ovoviparous and up to 1500 larvae may be released by a single worm.



Diagram 5.  Diagram illustrating the morphology of the adult male and female nematodes, Trichinella spiralis. The adult female measures between 2 – 3cm long whilst the male worms are smaller in size measuring approximately 1.2mm long.





Figure 6. Parasitic female of Trichinella spiralis. The adult female worms penetrate the mucosa of the host where they liberate the larvae, which are then carried in the bloodstream to the muscle where they encyst.
Clinical Disease

Symptoms during the intestinal phase may go unnoticed or may be severe.  Epidemics can result in outbreaks of gastro-enteritis, 2 to 7 days after the ingestion of raw pork.   Diarrhoea with or without abdominal pain may last for several weeks.  Eosinophilia and fever occur in most cases. Leucocytosis is common and hyperglobulinaemia is characteristic.  Myocytosis and circum orbital oedema 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 fibres in which it encysts. (Diag. 6 & Fig. 7) There can be neurological manifestations of trichinosis and death may be ascribed to myocarditis, encephalitis or pneumonitis.

Laboratory diagnosis

Diagnosis of trichinosis depends on the clinical signs, such as myalgia, periorbital oedema, 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 3 - 4 weeks post infection. Circulating antibodies to T. spiralis appear from 2 – 4 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 titres develop.



Figure 6.  Diagrammatic representation of the larvae encapsulated in striated muscle. (Smyth, J.D, 1994)


Figure 7 . Encapsulated Trichinella spiralis  larvae in the muscle of the definitive hosts. Man becomes infected by eating infected raw meat.


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 2 glass plates to make it semi-transparent, allowing you to see any encapsulated larvae using a ‘trichinoscope’ (a simple magnifying system). (Fig. 7)



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