L2: Medical Helminthology

Question 1

What does helminth mean?

Answer 1

Helminth is a general term meaning worm.

Question 2

What are helminths classified into?

Answer 2

Helminths are classified into flatworms or platyhelminths (trematodes and cestodes) and nemathelminths (nematodes).

Question 3

What are the general characters of trematodes?

Answer 3

1) Dorsoventrally flattened (Schistosoma females are cylindrical), leaf-shaped, bilaterally symmetrical.
2) Possess an oral sucker around the mouth and a ventral sucker (for fixation).
3) A well-developed alimentary canal with a muscular pharynx, esophagus, and a branched intestine.
4) Body cavity is lacking.
5) Hermaphroditic; having both male and female reproductive organs in the same adult (except schistosomes).

Question 4

What are the general characters of schistosomes?

Answer 4

 Belong to trematodes.
 Have separate sex.
 Females are cylindrical.

Question 5

What are the general characters for cestodes?

Answer 5

• Flattened, elongated, segmented.
• Body is divided into:
   A scolex or head bears the organs of attachment (acetabula, rostellum, hooks).

 A neck: the region of segment proliferation.

 A chain of proglottids called the strobila:
• The strobila elongates as new proglottids (segments) form in the neck region.
• The segments nearest the neck are immature (sex organs not fully developed)
and those more posterior are mature.
• The terminal segments are gravid, with the egg-filled uterus as the most prominent feature.

 Absent alimentary canal.

 Hermaphroditic.

Question 6

What are the general characters for nematodes?

Answer 6

 Cylindrical rather than flattened.

 Complete alimentary canal

 Mouth: surrounded by lips bearing sensory papillae, in some species is provided with teeth. It leads into a buccal cavity

 Esophagus: a muscular structure that pumps food into the intestine.

 Intestine: a tubular structure that ends with the anus.  Have separate sex, males are usually smaller than females.

Question 7

What are the modes of transmission of helminth infections?

Answer 7

1) Ingestion of contaminated water or food.
2) Ingestion of infected fish or infected meat.
3) Inhalation to the nasopharynx.
4) Skin penetration.
5) Arthropod bite.
6) Contact with domestic animals, such as dogs.
7) Infected persons may be direct sources of infection.

Question 8

What are the clinical presentations of helminth infections?

Answer 8

1) Gastrointestinal affection: abdominal pain, nausea, vomiting, diarrhoea, dysentery, or constipation.
2) Lung affection: chest pain, fever, cough, pneumonia, or haemoptysis.
3) Liver affection: biliary colic, jaundice, fever, or right hypochondrial pain.
4) Loss of weight, anaemia, vitamin deficiency.
5) Complications: intestinal obstruction, haematemesis, melena, or cancer.

Question 9

How are helminths diagnosed?

Answer 9

 Clinical.

 Laboratory:

 Macroscopic and Microscopic examination of stool for ova, gravid segments, adults, or larvae.
 Other samples that can be used to diagnose helminth infections include duodenal aspirate, urine, sputum, or blood.
 Immunological: ELISA, IHAT.
 Molecular: PCR.

 Radiological: X-ray, US, CT, MRI.

 Endoscopy: e.g. cystoscopy or sigmoidoscopy.

 Histopathological diagnosis: e.g. muscle biopsy.

Question 10

What are the general preventive and control measures for helminth infection?

Answer 10

1) Thorough washing of vegetables and fruits.
2) Provision of safe water supply.
3) Proper cooking of meat (beef or pork).
4) Proper cooking of fish.
5) Snail control.
6) Anti-fly and anti-mosquito measures.
7) Destruction of reservoir hosts (e.g. rodents).
8) Periodic examination and deworming of pets.
9) Treatment of infected cases or animals.
10) Health education: mode(s) of transmission, and prevention and control measures.
11) Environmental sanitation: sanitary disposal of feces.

Question 11

What are the diagnostic samples for helminths?

Answer 11

 Stool.
 Duodenal aspirate.  
 Urine. 
 Sputum.
 Blood.

Question 12

What is the habitat of trematodes?

Answer 12

Small intestine, biliary passages, blood vessels

Question 13

What is the habitat of cestodes?

Answer 13

Small intestine, tissues

Question 14

What is the habitat of nematodes?

Answer 14

Small or large intestine, tissue

Question 15

Eggs of trematodes

Answer 15

• Operculated; has a lid (Schistosoma eggs are non- operculated)
• Mature (contains miracidium) or immature

Question 16

Eggs of cestodes

Answer 16

• Most of them have non- operculated eggs

- Mature (contains hexacanth embryo or oncosphere)

Question 17

Eggs of nematodes

Answer 17

 Non-operculated

 Mature (contains completely formed larva) or immature

Question 18

Eggs of trematodes pass to?

Answer 18

Fresh or brackish water

Question 19

Eggs of cestodes pass to?

Answer 19

• Most of them need soil to complete their life cycle
• Tapeworms shed gravid proglottids into the intestine.
• Some proglottids disintegrate, releasing eggs that are voided in the feces, other proglottids are passed intact

Question 20

Eggs of nematodes pass to?

Answer 20

Soil

Question 21

What are the stages of the life cycle of helminths?

Answer 21

• Egg → larva(e) (in intermediate host) → adult (in the definitive host)
• Some nematodes produce larvae instead of eggs

Question 22

What are the larval stages of trematodes?

Answer 22

MSR CE

 Miracidium 
 Sporocyst
 Redia 
 Cercaria
 Encysted metacercaria
 (Schistosomes have no redia)

Question 23

What are the larval stages of cestodes?

Answer 23

 Cysticercus larva  Cysticercoid larva  Hydatid larva

Question 24

What are the stages of larvae of nematodes?

Answer 24

 Rhabditiform larva

 Filariform larva (Each larval stage is followed by a molt)

Question 25

Which type of helminth needs a snail as an intermediate host?

Answer 25

Trematodes

Question 26

What are the hosts of trematodes?

Answer 26

Two

Question 27

What is the number of hosts of cestodes?

Answer 27

Two hosts (except Hymenolepis nana)

Question 28

What is the number of hosts of nematodes?

Answer 28

Some need an arthropod vector or animal host, while others develop with no IH.