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  Air-borne nematode, Biology tutorial

Introduction:

The human pinworm Enterobius vermicularis is the ubiquitous parasite of man. It is anticipated that around 200 million people are infected yearly. It is more general in the temperate areas of the Western Europe and North America and is found specifically in children. Samples of Caucasian children in the U.S.A. and Canada have represented incidences of infection between 30 to 80 percent having identical levels in the Europe and however these areas are the parasites strongholds, it might be found all through the world, again often having high degrees of incidence. For instance in parts of South America the incidence in school children might be as high as 60 percent. Interestingly non-Caucasians come out to be relatively resistant to infection with this nematode. As a species and contrast to popular belief, E. vermicularis is wholly restricted to man, other animals harboring related however different species which are non-infective to humans, however their fur might be contaminated by eggs from the human species when stroked by someone having eggs on their hands. In man anywhere where there is large number of children gathered altogether, (like nurseries, play groups, orphanages and so on), particularly when conditions are unhygienic are ready sources of infection, as one child might rapidly convey the parasite to his or her fellows.

Enterobius vermicularis (the human pin-worm):

Morphology:

These creamy white colored nematodes are comparatively small having the female measuring only around 10 mm by 0.4 mm broad. The females encompass a cuticular expansion at their anterior ends, having a long pointed tail. The male parasites that are much less numerous than the females are much smaller, measuring only up to 5 mm long and encompass a curved tail having a small bursa similar to expansion and a single spicule. The head consists of a mouth having three small lips.

Life cycle:

The adult parasites live principally in the caecum. The male and females mate and the uteri of the females become filled by eggs. The gravid females (each having up to 15000 eggs) then migrate down the digestive tract to the anus.

From here they make normal nocturnal migrations out of the anus, to the perianal area, where air contact stimulates them to lay their eggs, before retreating back to the rectum. Finally the female die, their bodies disintegrating to discharge most of the remaining eggs. These eggs that are clear and measure around 55 by 30µm, then mature to the infectious phase (having L1 larvae) over 4 to 6 weeks. To infect the host, generally these eggs should then be ingested. The ingested eggs hatch in the duodenum.

The eggs themselves are sticky and encompass a feature shape, shared with all the members of the group Oxyuridea having an asymmetrical form, flattened on one side.

The larvae then experience a sequence of moults, as they migrate down the digestive tract. The adult worms then mature in the caecum, prior to copulating to complete the cycle (usually 6 weeks). Occasionally the eggs hatch in the perianal area itself, the resultant L1 larvae being completely infective, crawling back via the anus, then migrating up the intestine to the caecum (retro infection). 

Pathology of Infection:

The greater part of infections with this nematode is asymptomatic, however in some cases the emerging females and the sticky masses of eggs which they lay might causes irritation of the perianal area, which in some cases might be severe. As the females come out at night this might give mount to sleep disturbances and scratching of the influenced perianal area transfers eggs to the fingers and beneath the finger nails.

This in turn assists the transmission of the eggs, both back to the original host (that is, autoinfection) and to the other hosts. 

Diagnosis:

As eggs are seldom passed out with faeces, examination of faecal samples might not reveal them. This might account for negative outcomes of enterobiasis in most of the surveys for helminth infections comprising faecal samples in the tropical Africa. The most dependable diagnosis is by the cellophane tape swab. This comprises the attachment of a piece of cellophane to the perianal region overnight. This is then inspected for eggs beneath the microscope. Alternatively, the anus and perianal area can be inspected beneath bright light at night, at which time adult worms can be seen glittening in the light.

Epidemiology and Control:

The eggs of the parasite are air-borne, caught in household linen, clothing, curtain, carpets and so on. As such, infection is very common in dry season than rainy season in the tropics.  Maintenance of high standards of personal and domestic sanitation is thus imperative for the control and prevention.

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