Microbiology Education Series - Parasitology No.  1

 

 Ascaris lumbricoides

 

Nematodes: Ascaris lumbricoides

Fig.1: A. lumbricoides in faecal preparation       Fig.2: A. lumbricoides egg in faecal preparation

 

1

 Background / History

  • Intestinal Nematodes are more common than Nematode infections of the blood and tissue.
  • Nematodes are the most easily identified form of intestinal parasite, owing to their relatively large size, non-segmented bodies and cylindrical shape. There morphology gives rise to their common name - Roundworms.
  • Nematode infection is mainly diagnosed by the detection and identification of eggs in faeces.
  • Eggs of A. lumbricoides are identified by their characteristic size and shape and structure of larvae in the egg.

 

2

 Classification

Table 1: Soil-transmitted Nematode classification

Classification

A. lumbricoides

Phylum

Nematoda

Class

Secernentea

Order

Ascaridida

Family

Ascarididae

Genus

Ascaris

Species

lumbricoides

Trival Name

Roundworm

No of species/sub species

16

No of species infecting humans

12

 

 

3

 Morphology

Adult A. lumbricoides are 20-35cm long (females), 10-30cm long (males).

  • Pinkish white, non-segmented cylindrical roundworms marginally narrower at the head.
  • Male worms more slender than females.
  • Females have a vulva about one third of the way down the body from the head, with a blunt tail.
  • Males and females have a mouth and finely striated cuticle (characteristic of all ascarids).
  • Internally A. lumbricoides has a similar structure to all Nematodes with a cylindrical oesophagus opening into a ribbon-like flattened intestine.
  • A. lumbricoides eggs are easily distinguishable, having thick shells comprising of thick, transparent inner shell covered in a course, bumpy outer coat.

 

4

 Replication / Life Cycle

Figure 3: Life cycle of A. lumbricoides

 

5

 Clinical Disease

  • Larval migration causes tissue damage, for example A. lumbricoides can perforate the intestine causing peritonitis and secondary bacterial infections. Migration of worms to the bile duct can cause blockage of bile flow resulting in jaundice. Blockage of the trachea can lead to suffocation.
  • Adult worms do not attach themselves to the mucosa of the intestine and are in constant motion to stay within the intestine.
  • Pneumonitis is caused by multiple larval migration to the lungs, emulating asthma.
  • Bowel obstruction is caused by a tangled bolus of adult worms. This can lead to malnutrition. Large number of adult worms can also lead to abdominal tenderness, distension, fever and vomiting.
  • During mixed worm infections, A. lumbricoides should be treated first as other non-Ascaris drugs can cause A. lumbricoides to migrate to and perforate the bowel.

 

6

 Laboratory Diagnosis

  • Sample - fresh faeces, preferably those produced by laxative as they are reported to yield higher numbers of parasites.
  • Faeces can be examined macroscopically for traces of blood and mucous.
  • Microscopic examination involves wet preparation of a small amount of faeces in saline with a loopful of Lugol's iodine. Examine under 10x objective and 40x for structural detail.
  • Faeces can be subjected to parasite concentration methods that reveal knobbly-coated, bile stained eggs. Typically 55-75 microns long and 50 microns wide. The thick outer wall can be partially removed or absent.
  • Adult worms may occasionally pass out in the faeces. In situ A. lumbricoides adults can be identified by radiological examination. Cholangiograms show their presence in the biliary tract.
  • Eosinophils and larvae in sputum is characteristic of the pulmonary stage of disease.

 

Fig.4: Parasitology products available from Cosmos Biomedical Ltd

 

Product

Kit Details
 

Kit Copro-Duo (2x12 tests): A complete box in Parasitic Coprology. Two concentration techniques: MIF and Bailenger.

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Kit Copro-Duo (2x24 tests): A complete box in Parasitic Coprology. Two concentration techniques: MIF and Bailenger.

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Kit MIF Color: A single technique for concentrating, staining and preserving parasitic elements . Between 50 and 100 tests per kit.

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Kit 555: Fast-acting variation of May-Grunwald Giemsa stain for Plasmodium, Trichomonas, Microfilaria, Tissular Protozoa, Cryptosporidium, Pn.carinii, Fungi contributing to deep Mycosis and for Veterinary Parasitology

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Digital Microscope Camera: Capture microscopy images directly to your PC via USB connection. Supplied with operating software  and eyepiece adaptor

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7 Treatment & Prevention / Control

  • First choice drug is mebendazole. Alternatives are pyrantel pamoate and piperazine.
  • Mixed parasitic infections, for example, A. lumbricoides and Giardia or Entamoeba, should have ascariasis treated first to avoid migration of A. lumbricoides to the intestine and intestinal perforation.
  • Effective preventative methods include hygiene education, improved water supplies and basic sanitary conditions. Food handlers in endemic areas should adhere to strict hygiene regulations.
  • Limiting use of human faeces as fertiliser. A. lumbricoides can remain viable in soil for in excess of three years.
  • Control measures include the mass treatment of at-risk populations, although ecomonic issues can be inhibitory to effective mass treatment initiatives.

 

Appropriate resources may not be available to implement effective treatment and control strategies in many developing countries.

 

8

 References

  • Basic Medical Laboratory Technology, 2nd Ed. Kirk, Peel, James, Lewis & Waft. pp.114-117
  • Topley & Wilson's Microbiology and Microbial Infections: Vol.5 Parasitology, 9th Ed. Cox, Kreier & Wakelin. pp.561-584
  • Medical Microbiology, 4th Ed. Murray, Rosenthal, Kobayashi & Pfaller. pp.728-731

 

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