Medical Parasitology

Examination of faeces for parasites Medical ParasitologyCollection of faecal specimenThe administration of drugs or anti-diarrheal substances such as mineral oil, barium, bismuth, magnesium and antibiotics should be terminated at least one week before stool collection. These substances may interfere with the ova and parasite examination by reducing the visibility of organisms.Stool sample should be collected in a clean dry container.Stool sample should not be contaminated with urine, water or any other fluids.

A good stool specimen is one that has been sampled from areas containing blood, mucous and areas of loose or watery consistency. One of the most important steps is the mixing of the stool with fixative. The purpose of fixative is to preserve the morphology of the parasites. The trophozoite stage is a very fragile stage and chances for recovery are increased when the stool is fixed completely and as quickly as possible.

Three stool specimens collected on alternate days is recommended as the standard for the ova and parasite examination. Recently, one specimen properly collected, fixed and examined for the proper length of time will yield 90% of the intestinal parasites present. A second and third specimen will yield 8% and 2% respectively.

1st stool examination

Negative for parasites +Asymptomatic patient

No need for the 2nd & 3rd stool examination

Negative for parasites +Symptomatic patient

2nd & 3rd stool examination is recommendedCollection of stool sample in children:For children in diapers, line the inside of the diaper with plastic wrap, to prevent the diaper from absorbing the stool. For older children, put plastic wrap, newspaper, or a special collection container under the toilet seat to catch the stool. Never take the sample out of the water in the toilet bowl and do not allow urine to touch it. If the child urinated at the same time, do not collect this stool. Toilet water and urine will destroy the germs or parasites.

Stool analysisMacroscopic examination:Colour: normally brownConsistancy: Formed, semi formed or wateryOdour: offensiveReaction: normal stool PH is 7.0-7.5Presence of :Blood, mucus, pus, fat.Parasites: adult worms or worm segments.

Microscopic for:RBCsPus cellsBacteriaParasites: helminthic ova or protozoal infecction.Parasites that can be found in stool specimen

HelminthesI. Adult wormsEntrobius vermicularis.Ascaris lumbericiodes.Mature and gravid segments of Taenia saginata.

II. Helminthic eggs Fascciola eggsSchistosoma mansoni eggs.Heterophyes eggs.Taenia eggs.Diphylobothrium eggs.Hymenolepis nana eggs.Trichuiris eggs.Ascaris eggs.Ancylostoma eggs.

III. Intestinal protozoa:Entamoeba histolytica trophoziotes and cysts.Gardia lamblia trophoziotes and cysts.Balantidium coli cysts.Cryptosporidium oocysts.Isospora oocysts

Examination of faeces for helminthes eggs and intestinal protozoaDirect smear method.Iodine smear method.Concentration methods.Direct smear methodSmall amount of stool is emulsified in one drop of saline on a microscopic slide and covered with a cover glass, then examined microscopically.Using a piece of stick or wire loop, mix a small amount of fresh specimen (especially mucus and blood) with each drop.Cover each preparation with a cover glass.

Too much stoolone drop of saline on a microscopic slideSmall amount of fresh specimen by wooden stickEmulsify

Avoid air bubblesPress cover slip slightly,remove excess liquidwith paper towel

STOOL EXAMINATION

Saline smearIodine smear

salineIodine 1%Eggs Protozoal troph. Motility (Amoeb, flagellates)Cyst morphological detailsStart with the scanning low power x4 objective lens and then higher powers for more details. use X100 lens for protozoa.Examine the preparations using the 10 and 40 objectives with the condenser iris closed sufficiently to give good contrast.Look especially for motile E. histolytica trophozoites containing red cells, motile G. lamblia trophozoites, motile Strongyloides larvae,. theLook for eggs and cysts of parasitic pathogens.

It should be noted that:In cases of parasites of large intestine like Schistosoma mansoni, eggs are deposited on the surface of the formed faeces and not mixed with the faecal mass as in eggs of parasites living in small intestine.In case of protozoa:Living trophoziotes can be detected by their movement.The addition of iodine solution stains the nuclei and glycogen vacuoles making identification easier.