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• Cestodes


  Parasitic flatworms, considered as primitive worms

  
• Cestodes


  • No digestive/nervous system; absorb nutrients and eliminate waste products through their outer surface called the tegument; commonly known as tapeworms

  
• Parasites


  • Protozoa
  • Amoeba
  • Flagellates
  • Ciliates
  • Sporozoa
  • Helminths

  
• Tegument


  Host-parasite interface, and metabolically active body covering performing all the vital activities such as protection, absorption and secretion

  
• Morphological features of cestodes


  • Parasites are flat and consists of three distinct regions – head, neck, body (proglottids)
  • Head contains an organ of attachment called the scolex, which may consists of either hooks, suckers, or sucking grooves
  • Neck is the region of growth and connects the head to the body of the worm; worm grows by adding new proglottids from the neck
  • Body is divided into multiple segments (hence, named tapeworm) called proglottids; series of proglottids is called strobila

  
• Reproductive organs of cestodes


  • All cestodes are hermaphroditic (self-fertilizing) with each proglottid containing both male and female reproductive organs
  • Each proglottid is capable of laying eggs = pregnant proglottid or gravid proglottid
• Life cycle of a typical cestode

  1. Egg

  2. Larva

  3. Adult worm

  
• Infection in humans is usually acquired through ingestion of the undercooked or raw flesh of the intermediate host containing the infective larvae

  
• Lifecycle of a typical cestode infection in humans


  1. Ingested larvae are transformed into adult worms in the intestines of the infected host

  2. The adult worm then undergoes self-impregnation with the gravid segment rupturing to release the eggs in the intestines

  3. The eggs are then passed out to the external environment during defecation

  
• Taenia saginata (Beef Tapeworm)


  • The intermediate host is cattle where the eggs enter the blood vessels within the cattle's intestines
  • 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, fish)

  
• Taenia saginata infection is common in the areas of the world where beef is routinely eaten, especially undercooked
• Taenia saginata infection is endemic in Eastern Europe, Russia, Eastern Africa, Latin America

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


  Disease produced by the adult worm

  
• 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 of taeniasis


  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 proglottids

  
• Treatment of taeniasis


  Drug of choice against the adult worm is praziquantel
• Prevention and control of taeniasis


  • Proper waste disposal and sanitation practices
  • Adequate cooking of beef
  • Freezing of beef meat for approx. 10 days may kill the encysted larvae
  • Prompt treatment of infected persons help prevent spread of the disease
• Taenia solium (Pork Tapeworm)


  • Infection is acquired through ingestion of improperly cooked or raw pork meat which contains the infective larvae called cysticercus cellulosae; has two infective stages: eggs and larvae
  • 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
  • Autoinfection may also occur; Pigs serve as the intermediate host while human serve as both intermediate and definitive hosts

  
• Two scenarios of Taenia solium infection in humans


  1. Ingestion of undercooked or raw pork meat - infective stage is the larval form which transforms into adult worm in the intestines of infected individuals. Humans serve as the definitive hosts.

  2. Ingestion of worm eggs - eggs hatch in the small intestines, burrow through the wall of the intestines into a blood vessel and disseminate to various organs. Humans serve as intermediate hosts.

  
• 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, Asia
• Adult worms produce little damage in the intestines
• Encysted larvae may produce damage in the tissues where they disseminate, such as in the brain where 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

  
• Taeniasis


  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.

  
• 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 - cystercosis of the brain, most feared and most severe involvement; symptoms associated with increased intracranial pressure such as seizures, headache, vomiting. Ocular cysticercosis may lead to visual disturbance due to development of inflammation of the uvea (uveitis) and retina (retinitis).

  
• Laboratory diagnosis of taeniasis


  Microscoping examination of stool specimen from infected persons is the diagnostic procedure of choice; 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.

  
• Laboratory diagnosis of cysticercosis


  Diagnostic procedure depends of demonstration of the cyst in tissue, through biopsy or CT scan

  
• Treatment of taeniasis and cysticercosis


  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 neurocystercosis; Surgical removal of the larvae may be necessary
• Prevention and control of pork tapeworm infection


  • Proper waste disposal and sanitary measures
  • Thorough cooking of pork meat
  • Prompt treatment of infected persons to prevent the spread of the parasite

  
• Diphyllobothrium latum (Broad Fish Tapeworm)


  • The longest of the tapeworms, can reach a length about 13 meters
  • Its eggs consist of ciliated larvae called corcodia (s. coracidium)
  • One end of the egg is occupied by a lid structure called an operculum
  • 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
  • Human infection is through ingestion of improperly cooked or raw fish containing the plerocercoid (infective stage), the precursor larval stage

  
• Life cycle of Diphyllobothrium latum


  1. After ingestion, the plerocercoid attaches to the intestinal mucosa and matures into the adult worm

  2. The adult worm self-fertilizes and the eggs are passed out with the stool

  3. If the eggs come to contact with fresh water, the coracidium hatches and is ingested by the first intermediate host, a tiny crustaceuan called a copepod (Cyclops sp.)

  4. After ingestion, the coracidium develops into the larval stage called the procercoid

  5. The copepod is then eaten by a freshwater fish (second intermediate host) where the procercoid develops intro the plerocercoid

  
• D. latum infections 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
• Diphyllobothriasis


  May manifest with symptoms of gastrointestinal involvement, which may include diarrhea and abdominal discomfort; when the adult worm attaches itself to the jejunum and ileum, the patient may develop deficiency of vit B12, leading to anemia similar to pernicious anemia and is characterized as megaloblastic anemia resulting from lack of malnutrition of red blood cells

  
• Laboratory diagnosis of diphyllobothriasis


  Based on the finding of the characteristic eggs and/or the proglottids (less frequent) in a stool specimen

  
• Treatment of diphyllobothriasis


  The drug of choice is praziquantel; an alternative drug is niclosamide