Cestodes

• Cestodes


  are classified under the sub-kingdom Metazoa, phylum Platyhelminthes.
• Cestodes


  ▪considered as primitive worms ;

  ▪no digestive system nor a nervous system;

  ▪they absorb nutrients and eliminate waste

  products through their outer surface called

  the tegument.

  ▪commonly known as tapeworms
• parasites are flat and consist of three distinct regions – the

  head, neck and body (proglottids).


  morphological features of cestodes
• parts of cestodes

  A) scolex

  B) neck

  C) strobila (plu), proglottids (sing,)

  
• the head contains an organ of attachment called the

  scolex, which may consist of either

  hooks, suckers, or sucking grooves.
• in some species, the scolex has a fleshy

  extension called a rostellum to which hooks

  may be attached.
• head part of cestodes

  
• ● the neck serves as the region of growth and connects the head to the body of the worm.

  ● the worm grows by adding new proglottids from the neck

  ● the oldest proglottids are found at the most distal part of the body of the parasite.
• ● the body is divided into multiple segments (hence, the

  name tapeworm) called proglottids.

  ○ A series of proglottids is called strobila

  (plural strobili).
• REPRODUCTIVE ORGANS

  ▪All cestodes are hermaphroditic (self-

  fertilizing) with each proglottid containing both male and female reproductive organs

  -each proglottid, therefore, is capable of

  laying eggs (now called a pregnant

  proglottid or gravid segment).
• REPRODUCTIVE ORGAN OF CESTODES

  A) VAGINA

  B) GENITAL PORE

  C) SPERM DUCT

  D) YOLK GLAND

  E) OVARY

  F) EXCRETORY DUCT

  G) TESTES

  
• LIFE CYCLE OF CESTODES

  A typical cestode life

  cycle is divided into three

  stages – egg, larva and

  adult worm.
• ● Infection in humans is usually acquired

  through ingestion of the undercooked or raw

  flesh of the intermediate host containing the

  infective larvae.

  ○ after ingestion, the ingested larvae are

  transformed into adult worms in the

  intestines of the infected host.

  ○ the adult worm then undergoes self

  impregnation with the gravid segment

  rupturing to release the eggs in the

  intestines.

  ○ the eggs are then passed out to the

  external environment during

  defecation.
• Intestinal Cestodes Taenia saginata (Beef Tapeworm) (Important Properties and Life Cycle)

  The intermediate host is cattle where the eggs enter

  the blood vessels within the cattle’s intestines.
• Taenia saginata (Beef Tapeworm)

  ● for the majority of cestodes, the

  egg contains an embryo called

  the oncosphere, which

  represents the first larval or

  motile stage.

  ● it is equipped with small hooks (hooklets) that eventually

  enable the parasite to pierce

  the wall of the intestines.

  ● the eggs are excreted in the

  feces of infected hosts and are

  transmitted to the intermediate

  hosts (cattle, pig, or fish).
• EPIDEMIOLOGY AND PATHOGENESIS

  Taenia saginata infection is common in the areas of the world

  where beef is routinely eaten, especially undercooked.

  Endemic in Eastern Europe, Russia, Eastern Africa, and Latin

  America (Centers for Disease Control and Prevention).

  • The adult worm do not produce significant damage in the small intestines.
• Disease : Taeniasis

  Majority of patients are asymptomatic.

  Those with high worm burden may complain of diarrhea,

  abdominal pain, loss of appetite with resultant weight loss, and

  body malaise.

  • The gravid proglottids may reach the anus where egg-laying may

  occur resulting in itchiness in the anal region (pruritus ani).
• Laboratory Diagnosis-Taenia saginata (Beef Tapeworm)

  Examination of fecal specimen from infected

  patients is the procedure of choice.

  • Eggs or gravid proglottids may be recovered from

  the stool although eggs are less often found than the

  proglottids.
• Treatment-Taenia saginata (Beef Tapeworm)

  The drug of choice against the adult worm is

  praziquantel.
• Prevention and Control-Taenia saginata (Beef Tapeworm)

  Proper waste disposal and sanitation practices as well as the

  adequate cooking of beef are the main preventive measures

  for taeniasis.

  Freezing of beef meat for approximately 10 days may kill the

  encysted larvae.

  Prompt treatment of infected persons help prevent spread

  of the disease
• Taenia solium


  Pork Tapeworm
• Intestinal Cestodes Taenia solium


  • Infection acquired through ingestion of improperly cooked or raw pork meat containing infective larvae called cysticercus cellulosae
  • Unlike beef tapeworm, Taenia solium infection can also occur through ingestion of food or water contaminated with human feces containing parasite eggs
  • Taenia solium has two infective stages - eggs and larvae
  • Autoinfection may occur
  • Pigs serve as intermediate host, humans serve as both intermediate and definitive hosts
• Unlike the beef tapeworm, Taenia solium infection can also occur following the ingestion of food or water contaminated with human feces that contain the eggs of the parasite
• Taenia solium has two infective stages – eggs and larvae
• Autoinfection may also occur
• Pigs serve as the intermediate host while humans serve as both intermediate and definitive hosts
• ❏ There are two scenarios that can occur depending on which infective stage

  entered the human host.

  ❏ In cases where infection is acquired through ingestion of undercooked or

  raw pork meat, the infective stage is the larval form which transforms into

  adult worm in the intestines of infected individuals. In this instance,

  humans serve as the definitive hosts.

  ❏ On the other hand, ingested worm eggs hatch in the small intestines,

  burrow through the wall of the intestines into a blood vessel, and

  disseminate to various organs. In this instance, humans serve as

  intermediate hosts.
• Epidemiology and Pathogenesis

  ➔ T. solium infection is more prevalent is

  underdeveloped communities with poor sanitation

  and where people eat raw or undercooked pork.

  ➔ Higher rates of illness : Latin America, Eastern

  Europe, sub-Saharan Africa, India, and Asia

  (Centers for Disease Control and Prevention).
• Epidemiology and Pathogenesis-Taenia solium (Pork Tapeworm)

  ❏ Adult worms produce little damage in the intestines.

  ❏ Encysted larvae may produce damage in the tissues where they disseminate.

  ❏ for instance, in the brain, they may manifest as space-occupying lesions.

  ❏ although the larvae may encyst in various tissues of the body, they evoke

  little inflammatory response.

  ❏ however, when the encysted larvae die, they may release substances that

  may induce an allergic reaction in the host which may potentially be fatal

  due to the development of anaphylactic shock.
• Disease : Taeniasis

  Taeniasis – the disease produced by the adult

  worm. Most cases are asymptomatic but in the

  presence of high worm burden, manifestations may

  be similar to beef tapeworm infection.
• 2. Cysticercosis – the result of larval encystation in various tissues of the

  body. The most common involvement is that of the skeletal muscles where

  patients may complain of muscle pain.

  ● Neurocysticercosis : or cysticercosis of the brain, is the most feared

  and most severe involvement.

  ○ symptoms associated with increased intracranial pressure such as

  seizures, headache, and vomiting.

  ○ Ocular cysticercosis may lead to visual disturbance due to

  development of inflammation of the uvea (uveitis) and retina

  (retinitis).
• Laboratory Diagnosis-Taenia solium

  ● Microscopic examination of stool specimen from infected

  persons is the diagnostic procedure of choice in patients with

  taeniasis.

  ● Demonstration of ova or proglottids may help establish the

  diagnosis. The demonstration of the typical morphology of the

  scolex can differentiate pork tapeworm from beef tapeworm.

  ● For cysticercosis, diagnostic procedure depends on demonstration

  of the cyst in tissue, through biopsy or CT scan.
• Treatment-Taeniasis

  Drug of choice is praziquantel.

  For cysticercosis, praziquantel may also be effective but it is usually not

  recommended for ocular and CNS involvement.

  Alternative drugs include albendazole, paromomycin, and quinacrine

  hydrochloride.

  Anticonvulsants may be given in cases of neurocysticercosis.

  Surgical removal of the larvae may be necessary.
• Prevention and Control - Taenia solium

  ● Important preventive measures for pork tapeworm

  infection are the same as of that for beef tapeworm

  and include :

  ○ proper waste disposal and sanitary measures

  ○ thorough cooking of pork meat ; and

  ○ the prompt treatment of infected persons to

  prevent the spread of the parasite.
• Comparison between T.saginata and T.solium

  ▪The eggs of Taenia saginata are usually indistinguishable from the

  eggs of the pork tapeworm Taenia solium.

  ▪Both species may be differentiated by the appearance of their

  scolices and the structures of their proglottids.

  ▪the scolex of Taenia solium contains a rostellum while that of Taenia

  saginata do not.

  ▪Both species may be differentiated by the appearance of their

  scolices and the structures of their proglottids.
• Intestinal Cestodes Diphyllobothrium latum (Broad Fish Tapeworm)

  ● the longest of the tapeworms, the fish

  tapeworm can reach a length of

  about 13 meters.

  ● its eggs consist of ciliated larvae

  called coracidia (s. coracidium).

  ● One end of the egg is occupied

  by a lid structure called an

  operculum.
• Important Properties and Life Cycle

  ● its scolex contains a pair of long sucking

  grooves.

  ● the gravid segments contain a uterine

  structure that is centrally located and

  assumes a rosette formation.
• Important Properties and Life Cycle

  ❏ Human infection with D. latum is through ingestion of improperly

  cooked or raw fish containing the plerocercoid (infective stage), the

  precursor larval stage.

  ❏ After ingestion, the plerocercoid attaches to the intestinal mucosa and

  matures into the adult worm.

  ❏ The adult worm self-fertilizes and the eggs are passed out with the

  stool.

  ❏ If the eggs come to contact with fresh water, the coracidium hatches

  and is ingested by the first intermediate host, a tiny crustacean called

  a copepod (Cyclops sp).
• Important Properties and Life Cycle

  After ingestion, the coracidium develops into the larval stage called the

  procercoid.

  The copepod is then eaten by a freshwater fish (second intermediate host)

  where the procercoid develops into the plerocercoid.

  • Definitive hosts for the parasites are humans and other fish-eating

  mammals such as dogs, cats, bears and seals.
• Epidemiology and Pathogenesis-D. LAtum

  ● D. latum infection occurs in countries where raw freshwater fish is

  consumed.

  ● Little damage is produced in the small intestines of the human

  hosts.

  ● In some individuals, the parasite may compete with the host for

  vitamin B12, leading to a deficiency of this vitamin.