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Chapter  8.
Larval Cestodes and Nematodes


8.1 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.

Echinococcus granulosus causes cystic echinococcosis, and is the form most frequently encountered; E. multilocularis causes alveolar echinococcosis; E. vogeli causes polycystic echinococcosis; and E. oligarthrus is an extremely rare cause of human echinococcosis.

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.  

Life Cycle

Illustration 8-1. Life Cyle of Echinococcus granulosus. The adult Echinococcus granulosus (3 to 6 mm long)  resides in the small bowel of the definitive hosts, dogs or other canids.  Gravid proglottids release eggs  that are passed in the feces.  After ingestion by a suitable intermediate host (under natural conditions: sheep, goat, swine, cattle, horses, camel), the egg hatches in the small bowel and releases an oncosphere  that penetrates the intestinal wall and migrates through the circulatory system into various organs, especially the liver and lungs.  In these organs, the oncosphere develops into a cyst  that enlarges gradually, producing protoscolices and daughter cysts that fill the cyst interior.  The definitive host becomes infected by ingesting the cyst-containing organs of the infected intermediate host.  After ingestion, the protoscolices  evaginate, attach to the intestinal mucosa  , and develop into adult stages  in 32 to 80 days.  The same life cycle occurs with E. multilocularis (1.2 to 3.7 mm), with the following differences: the definitive hosts are foxes, and to a lesser extent dogs, cats, coyotes and wolves; the intermediate host are small rodents; and larval growth (in the liver) remains indefinitely in the proliferative stage, resulting in invasion of the surrounding tissues.  With E. vogeli (up to 5.6 mm long), the definitive hosts are bush dogs and dogs; the intermediate hosts are rodents; and the larval stage (in the liver, lungs and other organs) develops both externally and internally, resulting in multiple vesicles.  E. oligarthrus (up to 2.9 mm long) has a life cycle that involves wild felids as definitive hosts and rodents as intermediate hosts.  Humans become infected by ingesting eggs  , with resulting release of oncospheres  in the intestine and the development of cysts  ,  ,   ,  ,    in various organs. (SOURCE: CDC/DPDx Echinococcus)

 

Morphology

The adult worm measures approximately 3–8.5µm long. The scolex has four 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 feces.

Image 8-1.  Echinococcus granulosus scolex. (SOURCE: CDC)


Image 8-2.  Echinococcus granulosus immature proglottid. (SOURCE: CDC)

Image 8-3.  Adult female worms of Echinococcus granulosus with gravid proglottids. (SOURCE: CDC)

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 characterized 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.

 

Image 8-4. Unilocular cyst of Echinococcus granulosus. This is characterized 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 five to 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 hemagglutination 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. One percent 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.

Visualization 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 visualized 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

·       Safe disposal of dog feces.

·       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 (1) are the primary definitive hosts although in domestic circumstances dogs (1a) and cats (1b) can act as the definitive host.  Rodents (4) are the intermediate hosts.  Man (4a) is an accidental host by the ingestion of eggs (2) where multilocular cysts are formed (3). In these cysts, the limiting membrane is thin and the germinal epithelium may bud off externally resulting in proliferation in any direction (5).  Metastases may occur.  Unlike E. granulosus, there is little fluid in the cysts of E. multilocularis.

Illustration 8-3: Life cycle of Echinococcus multilocularis. (SOURCE: Unknown)


Morphology

Larvae - The larval E. multilocularis is very different from that of E. granulosus. In this case the 'cyst' grows invasively by external budding, forming a diffuse growth through the infected organ, replacing that organ’s tissues. The growth itself, (it cannot truly be called a cyst as there is no real cyst wall), is composed of numerous cavities containing a gelatinous matrix within which protoscolices and numerous brood capsules are produced, and which in its behavior, most closely resembles a malignant neoplasm. In contrast to E. granulosus this growth is also very rapid.  Infective protosocialises being present after only wo to three months, as compared to the one to two years in the related metacestode.

Adults - The adult parasite is very similar to E. granulosus, being slightly smaller, with a maximum length of approximately 4µm, and consisting of four to five proglottids.

 

 
Illustration 8-4.
Diagrammatic representation of the adult worm if Echinococcus multilocularis. (SOURCE: Unknown)


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. 

Laboratory 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 8-1. Differences between the hydatid cysts of E. granulosus and E. multilocularis. (SOURCE: White)


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.  Multiple scolices burst from the inner surface of the cyst wall.  The cyst resembles that of a cysticercus (Cysticercus cerebralis).  It is filled with fluid, 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.

 


Illustration 8-5.
Diagrammatic representation of a Multiceps (Taenia) multiceps coenurus cyst. (SOURCE: Unknown)


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.

Laboratory Diagnosis

There are no serological tests available. Diagnosis is achieved by the finding of the coenurus cyst in the brain and spinal cord, or in the subcutaneous tissue.


8.2  Larval Nematodes which Infect Man

Intestinal capillariasis caused by Capillaria Philippinensis appeared first in the Philippines and subsequently in Thailand, Japan, Iran, Egypt, and Taiwan, but most infections occur in the Philippines and Thailand. As established experimentally, the life cycle involves freshwater fish as intermediate hosts and fish-eating birds as definitive hosts. Embryonated eggs from feces fed to fish hatch and grow as larvae in the fish intestines. Infective larvae fed to monkeys, Mongolian gerbils, and fish-eating birds develop into adults. Larvae become adults in 10 to 11 days, and the first-generation females produce larvae. These larvae develop into males and egg-producing female worms. Eggs pass with the feces, reach water, embryonate, and infect fish. Autoinfection is part of the life cycle and leads to hyperinfection. Humans acquire the infection by eating small freshwater fish raw. The parasite multiplies, and symptoms of diarrhea, borborygmus, abdominal pain, and edema develop. Chronic infections lead to malabsorption and hence to protein and electrolyte loss, and death results from irreversible effects of the infection. Treatment consists of electrolyte replacement and administration of an antidiarrheal agent and mebendazole or albendazole.

Capillariasis Philippinensis is considered a zoonotic disease of migratory fish-eating birds. The eggs are disseminated along flyways and infect the fish, and when fish are eaten raw, the disease develops.


Capillaria Philippinensis

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 Philippinensis is a small nematode which burrows into the intestinal mucosa of its human host. 

Humans and birds become infected when they eat uncooked or poorly cooked fish. Therefore, infections are seen in endemic proportions where uncooked fish is considered 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 feces 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.

 

 

 

Illustration 8-6. Diagram of the life cycle of Capillaria Philippinensis. Humans become infected from eating infected fish.  The disease in man is known as Capillariasis.
Typically, unembryonated eggs are passed in the human stool  and become embryonated in the external environment  ; after ingestion by freshwater fish, larvae hatch, penetrate the intestine, and migrate to the tissues  .  Ingestion of raw or undercooked fish results in infection of the human host  .  The adults of Capillaria Philippinensis (males: 2.3 to 3.2 mm; females: 2.5 to 4.3 mm) reside in the human small intestine, where they burrow in the mucosa  .  The females deposit unembryonated eggs.  Some of these become embryonated in the intestine, and release larvae that can cause autoinfection.  This leads to hyperinfection (a massive number of adult worms)  Capillaria Philippinensis is currently considered a parasite of fish eating birds, which seem to be the natural definitive host  (CDC 3382 - CDC/Alexander J. da Silva, PhD/Melanie Moser)

Morphology

The adult female worm measures 2.5-4.3µm, while the males are marginally smaller measuring 2.3-3.2µm.  Adult females can produce immature and mature ova as well as free larvae that can auto-infect.

The eggs measure 45µm x 21µm, and resemble those of Trichuris but have less prominent polar plugs.  They have a thick striated shell.

Figure 8-5. Eggs of Capillaria Philippinesis. They are oval in shape and closely resemble the eggs of Trichuris but have less prominent plugs. (SOURCE:  PHIL 1376/653 - CDC/Dr. Mae Melvin)


Clinical Disease

Many cases may be asymptomatic, but any manifested symptoms will be related to the worm burden. The most common symptoms are watery stools with large fluid loss, anorexia, nausea, vomiting, and hypotension.  Abdominal distension and edema may also develop.  Death can ensue from pneumonia, cerebral edema, 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.  

Laboratory Diagnosis

Diagnosis depends on finding the characteristic eggs or larvae in the feces. Pulmonary capillariasis diagnosis relies on finding adult worms or eggs in lung biopsies.

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Ch 1. The Ameba
Ch 2. The Ciliates, Coccidia, and Microsporidia
Ch 3. The Flagellates
Ch 4. The Cestodes
Ch 5. The Nematodes
Ch 6. The Trematodes
Ch 7. Tissue Dwelling Nematodes
Ch 8. Larval Cestodes and Nematodes
Ch 9. Malaria
Ch 10. The Blood Nematodes
Ch 11. Babesia, Trypanosomes, and Leishmania
Ch 12. Arthropod Vectors
Ch 13. Artifacts and Confounders