Larval Cestodes and Nematodes which Infect Man

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http://www.soton.ac.uk/~ceb/Diagnosis/Vol8.htm

 

 

Volume 8. Larval Cestodes and Nematodes which Infect Man

 

 

 

 

 

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

 

 

 

1. Intersep Ltd 2. University of Southampton

 

 

 

More information can be obtained on www.intersep.com and

www.soton.ac.uk/~ceb/, Ectoparasites and Endoparasites.

 

 

 

 

 

 

Contents

 

 

 

 

 

 

 

8. Larval Cestodes which Infect Man

 

Echinococcus granulosus

 

Echinococcus multilocularis

 

Multiceps multiceps

 

Capillaria philippensis

 

 

 

 

 

References

 

 

 

 

 

Larval Cestodes which Infect Man

 

 

 

Infections in man with Echinococcus granulosus, Echinococcus

multilocularis and Multiceps multiceps are caused by the accidental

ingestion of eggs which are excreted by the definitive animal host.

The disease that is produced due to the invasion of these parasites is

caused by the larval stages or hydatid cyst, is known as hydatid

disease or hydatidosis.

 

 

 

Each cestode possesses an elongated tape-like body which lacks an

alimentary canal. The adult tapeworms are strings of individuals

having a complete set of reproductive organs (proglottids) in

progressive degrees of sexual maturity and budding off from a body

attached to the host tissue by a head or scolex.

 

 

 

The larval stage, show a wide variation being found in almost any

organ of both vertebrate and invertebrate hosts.

 

 

 

Echinococcus granulosus

 

 

 

Introduction

 

Echinococcosis or Hydatid disease in man is caused by the larval stage

of the dog tapeworm, Echinococcus granulosus. Hydatid disease is most

extensively found in East Africa, North Africa, South Africa, the

Middle East and parts of South America and Australia. The intermediate

hosts are cattle, sheep, pigs, goats or camels and the definitive host

for this disease is the dog or other canids.

 

 

 

Liife cycle of the cestode, Echinococcus granulosus. Larval infection

in man causes hydatid disease.

 

 

 

Adult worms are only seen in the definitive hosts, dogs, they cannot

develop in man. Man is an accidental intermediate host of hydatid

disease. When the ova are ingested by a suitable intermediate host,

they hatch in the duodenum and the oncosphere migrates to the blood

stream where it is carried to the liver, lungs and other organs of the

body. Here it develops into a hydatid cyst which consists of an outer

thick laminated cyst wall and an inner, thin nucleated germinal layer.

From the inner layer brood capsules are produced which contain

protoscoleces. The brood capsules detach from the germinal layer,

releasing free protoscoleces. Hydatid sand is the name given to the

fluid in the cysts which consists of protoscoleces, tissue debris and

sometimes free hooklets. Here, the life cycle stops in humans, but is

continued when a hydatid cyst containing protoscoleces eg. in sheep

liver, is ingested by a suitable canine host where the protoscoleces

develop into adult worms. (Fig. 1)

 

 

 

Morphology

 

The adult worm measures approximately 3 – 8.5mm long. The scolex has 4

suckers and a rostellum with hooks, the latter becoming tightly

inserted into the crypts of Lieberkühn. The mature strobila has only 3

– 4 proglottids, one is immature, one is mature and the final one is

gravid; when gravid the eggs are expelled in the faeces. (Fig. 2 & 3)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2. Diagrammatic representation of the mature proglottid of

Echinococcus granulosus

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3. Adult female worm of Echinococcus granulosus. The mature

strobila has only 3 – 4 proglottids; one is immature, one is mature

and the final one is gravid. The eggs are released from the gravid

proglottid in the faeces.

 

 

 

 

 

Due to the close similarity of the eggs to other Taenia species found

in dogs they were until recently thought to be morphologically

indistinguishable.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The larvae in man develop into a unilocular cyst which gives rise to

unilocular hydatid disease. This is characterised as having only one

bladder or many completely isolated bladders, each enclosed in its own

well-developed envelope. The latter consists of several layers, the

most prominent being the laminated layer. Within this again is the

germinal membrane from which the brood capsules arise inside which

develop thousands of larvae or protoscoleces, the whole being

suspended in a hydatid fluid. (Figure 4, 5 & Table 1)

 

 

 

 

 

 

 

 

 

Figure 4. A diagrammatic representation of the tranverse section of a

hydatid cyst of Echinococcus granulosus.

 

 

 

 

 

Figure 5. Unilocular cyst of Echinococcus granulosus. This is

characterised as having only one bladder or many completely isolated

bladders, each enclosed in its own well-developed envelope. Inside

which develop thousands of larvae or protoscoleces, the whole being

suspended in a hydatid fluid. These cysts in man give rise to

unilocular hydatid disease.

 

 

 

 

 

 

Clinical disease

 

Hydatid disease in humans is potentially dangerous depending on the

location of the cyst. Some cysts may remain undetected for many years

until they become large enough to affect other organs. Symptoms are

then of a space occupying lesion. Lung cysts are usually asymptomatic

until there is a cough, shortness of breath or chest pain. Hepatic

cysts result in pressure on the major bile ducts or blood vessels.

Expanding hydatid cysts cause necrosis of the surrounding tissue.

 

 

 

Slow leakage of the hydatid fluid results in eosinophilia and rupture

of an abdominal hydatid cyst results in severe allergic symptoms.

 

 

 

Symptoms may not manifest themselves for 5 – 20 years after the infection.

 

 

 

Laboratory Diagnosis

 

1. Imaging and serodiagnosis are the mainstay of diagnosis.

Serological tests include Enzyme linked immunosorbent assay (ELISA),

an indirect haemagglutination test a complement fixation test and a

Western Blot system.

 

2. Microscopic examination of the cyst fluid to look for the

characteristic protoscoleces which can be either invaginated or

evaginated. The cyst fluid will also reveal free hooklets and tissue

debris. 1% eosin may be added to the fluid to determine the viability

of the protoscoleces. Viable protoscoleces exclude eosin whereas

nonviable protoscoleces take up the eosin.

 

3. Histological examination of the cyst wall after surgical removal.

 

 

 

 

 

Western Blots

 

One serological test which has proved to be of value to diagnosing

Hydatid disease is the Western Blot. The test presents a definitive

means for detection of human antibodies to the cestode E. granulosus.

 

 

 

Diagnosis can be achieved using the Western Blot assay for the

detection of IgG antibodies in serum reactive with E. granulosus

antigens present on a membrane. Field studies support a sensitivity of

80% and specificity of 100% in patients with hepatic cysts.

 

 

 

This assay is known as the QualicodeÔ Hydatid Disease Kit, the

principle behind the test is that it is a qualitative membrane-based

immunoassay manufactured from E. granulosus proteins. The E.

granulosus proteins are fractionated according to molecular weight by

electrophoresis on a ployacrylamide slab gel (PAGE) in the presence of

sodium dodecyl sulfate (SDS). The separated E. granulosus proteins are

then transferred via electrophoretic blotting from the gel into strips

for testing of individual samples.

 

 

 

During the procedure, the strips containing the E. granulosus proteins

are incubated with serum specimens and washed to remove unbound

antibodies.

 

 

 

Visualisation of human immunoglobulins specifically bound to E.

granulosus proteins is performed by sequential reaction with goat

anti-human immunoglobulin-alkaline phosphatase conjugate and BCIP/NBT

substrate. Bands corresponding to the positions of the resoled E.

granulosus proteins will be visualised on the strip, indications the

presence in the serum sample of IgG antibodies direct against E.

granulosus antigens. Band positions are compared to those on a

reference strip developed using the Hydatid disease positive control.

 

 

 

Prevention

 

1. Safe disposal of dog faeces.

 

2. Education to prevent feeding uncooked offal to dogs.

 

 

Echinococcus multilocularis

 

 

Introduction

 

The larvae of Echinococcus multilocularis is a particularly dangerous

species causing multilocular (alveolar) hydatid disease in man and

animals and is common in the highlands of Europe i.e. Switzerland and

Germany, in Canada, Alaska and Northern Russia. The most common

definitive hosts are foxes and wolves in addition to domestic cats and

dogs when they have access to infected rodents.

 

 

Life cycle

 

Foxes are the primary definitive hosts although in domestic

circumstances dogs can act as the definitive host. Rodents are the

intermediate hosts. Man is an accidental host by the ingestion of

eggs where multilocular cysts are formed. In these cysts, the

limiting membrane is thin and the germinal epithelium may bud off

externally resulting in proliferation in any direction. Metastases

may occur. Unlike E. granulosus, there is little fluid in the cysts

of E. multilocularis.

 

 

 

Morphology

 

The morphology is in general very similar to that of E. granulosus,

but the adults are much smaller. (Fig. 6 & Table 1)

 

 

 

Unlike E. granulosus, cysts of E. multilocularis in man do not contain

daughter cysts with scolices. Instead the larval cyst, or as it is

referred to as an alveolar or multilocular hydatid cyst forms a

multicystic structure made up of proliferating vesicles embedded in a

dense fibrous stroma, which is often mistaken for a hepatic sarcoma.

In older cysts the hydatid fluid is replaced by a jelly-like mass.

 

 

 

 

 

 

 

 

 

 

Figure 6. Diagrammatic representation of the adult worm if

Echinococcus multilocularis.

 

 

 

 

 

 

Clinical Disease

 

Cysts form primarily in the liver and growth in the vena cava or

portal vein results in metastases in the lung or brain. Clinical

disease is similar to that of E. granulosus.

 

 

Diagnosis

 

1. Laboratory diagnosis is can be made by ELISA.

 

2. Clinical diagnosis is made by ultrasound.

 

 

 

 

 

 

 

Echinococcus granulosus

 

 

Echinococcus multilocularis

 

Slow development of cyst

 

 

Rapid development of cyst

 

Cysts have thick-walled chambers

 

 

Cyst has thin-walled chambers

 

Separated by connective tissue

 

 

Not separated by connective tissue

 

Cyst is fluid filled

 

 

Cyst is gelatinous filled

 

Cyst is free of host material

 

 

Cyst is contaminated by host material

 

 

 

Table 1. Differences between the hydatid cysts of E. granulosus and E.

multilocularis.

 

 

Multiceps multiceps

 

 

Introduction

 

Multiceps multiceps, is a cestode of cosmopolitan distribution and

causes coenuriasis in man. The dog is the common definitive host in

the USA and UK. With the intermediate stages developing in many

ungulates, especially sheep.

 

 

Life cycle

 

The adult worm is found in dogs and other canidae. The intermediate

hosts comprise of a number of herbivorous animals, where the cysts

develop in the brain and spinal cord causing a disease called

`staggers', which affects the balancing powers of the animals. Man

becomes infected by the accidental ingestion of eggs. The oncosphere

hatches and penetrates the intestinal wall and the embryo is carried

by the blood stream to various parts of the body including the central

nervous system where it lodges and the cyst or coenurus develops.

(Fig. 7) Multiple scolices burst from the inner surface of the cyst

wall. The cyst resembles that of a cysticercus (Cysticercus

cerebralis). It is filled with fliud, semi-transparent and glistening

white. The cysticercus possesses unusual asexual multiplication,

forming a bladder (or coenurus) which gives rise to hundreds of

daughter protoscoleces directly from its inner wall. It therefore

differs from a hydatid cyst.

 

 

 

 

 

 

 

Figure 7. Diagrammatic representation of a Multiceps multiceps

coenurus cyst. Hundred of daughter protoscoleces are released from the

cyst, therefore differing from a hydtatid cyst.

 

 

 

Clinical Disease

 

In humans, coenuri are most frequently found in the brain and spinal

cord but also in the subcutaneous tissue. Symptoms include headache,

vomiting, paraplegia seizures and eye problems. The coenurus may cause

serious damage or even death, but only a few have ever been reported.

 

 

Diagnosis

 

There are no serological tests available.

 

 

 

 

 

Capillaria philippensis

 

 

 

Introduction

 

Intestinal capillariasis was first observed in the Philippines in 1962

and since then it has been noted, although less frequently in

Thailand, with scattered reports from Taiwan, Japan, Egypt, and Iran.

Capillaria philippensis is a small nematode which when in its human

host burrows into the intestinal mucosa.

 

 

 

Man and fish eating birds become infected when eaten uncooked or

poorly cooked fish. Therefore, infections are seen in endemic

proportions where uncooked fish is seen as a delicacy.

 

 

 

Life cycle

 

The complete life cycle is not known however human infection is

initiated by the consumption of raw fish. The infective larvae are

found in the intestine of the fish. When infective fish are eaten by

humans, the larvae mature and the adult worms live in the intestinal

mucosa, mainly in the jejunum, where they are usually present in large

numbers. Larval stages, oviparous and larviparous females are also

found in the host's intestine, which suggests that the nematode

multiplies in the intestine resulting in auto-infection as in

Strongyloides infection. The eggs are passed out in the faeces and

embryonate in the soil, a process which takes about 12 days. The eggs

are ingested by fresh-water fish, they hatch and develop into the

infective form in the intestine of the fish. (Fig.8)

 

 

 

 

 

 

 

 

 

Figure 8. Diagram of the life cycle of Capillaria philippensis. Humans

become infected from eating infected fish. The disease in man is known

as Capillariasis.

 

 

 

 

 

 

 

If man ingests the eggs, the resultant larvae migrate from the

intestine to the liver where they form adults and cause hepatic

capillariasis.

 

 

 

 

 

Morphology

 

The adult female worm measures 2.5 - 4.3mm, whilst the males are

marginally smaller measuring 2.3 -3.2mm. Adult females can produce

immature and mature ova as well as free larvae that can auto-infect.

(Fig. 9)

 

 

 

 

 

 

 

 

 

Figure 9. Adult female worm of Capillaria philippensis. They measure

approximately 2.5-4.3 mm in length. Anterior part contains the

esophagus and " stichosome " . Posterior part contains the intestine and

reproductive system. (www.medicine.cmu.ac.th)

 

 

 

 

 

The eggs measure 45mm x 21mm, and resemble those of Trichuris but have

less prominent polar plugs. They have a thick striated shell. (Fig. 10)

 

 

 

 

 

Figure 10. Egg of Capillaria philippensis. They are oval in shape

measuring 45mm by 21mm. They closley resemble the eggs of Trichuris

but have less prominent plugs. (www.medicine.cmu.ac.th)

Clinical Disease

 

Many cases may be asymptomatic but by in large, the symptoms are

related to the worm burden. The most common symptoms are watery stools

with large fluid loss, anorexia, nausea, vomiting, and hypotension.

Abdominal distension and oedema may also develop. Death can ensue

from pneumonia, cerebral oedema, hypokalemia and heart failure. The

large worm burden can cause electrolyte imbalance, plasma protein

imbalance, and fluid loss, proving fatal within 4 – 6 months if

untreated. Villi blunting leads to malabsorption but eosinophilia is

not a feature of this disease.

 

 

Diagnosis

 

Diagnosis depends on finding the characteristic eggs or larvae in the

faeces. Pulmonary capillariasis diagnosis relies on finding adult

worms or eggs in lung biopsies.

 

References

 

 

 

Murray, PR, Drew, WL, Koyayashi, GS & Thomson, JH: Medical

Microbiology. Mosby Books Inc., New York (1990)

 

 

 

Peters, W & Gilles, HM: Tropical Medicine & Parasitology. Wolfe

Medical Publications Ltd.

 

 

 

Jeffrey & Leach: Atlas of Medical Helminthology and Protozoology. E &

S Livingstone Ltd.

 

 

 

Ash, LR & Orihel, TC: Atlas of Human Parasitology. ASCP Press, Chicago.

 

 

 

Garcia, LS & Bruckner, DA: Diagnostic Medical Parasitology. Elsevior

Science Publishing Co. Inc.

 

 

 

Muller, R & Baker, JR: Medical Parasitology. Gower Medical Publishing.

 

 

 

Smyth, J.D: Introduction to Animal Parasitology. Cambridge University

Press (1994)

 

 

 

Snell, JJS, Farrell, ID & Roberts, C: Quality Control, Principles and

Practice in the Microbiology Laboratory. Public Health Laboratory

Service. ISBN 0 901 144 312.

 

 

 

I would like to thank the authors of the following web pages:

 

www.medicine.cmu.ac.th