Chapter 7d PARASITIC DISEASES OF THE GUT

PARASITIC DISEASES OF THE GUT

1. OVERVIEW

There is an awesome array of parasites that may infest humans. In the lesser developed parts of the world with poverty, malnutrition, poor sanitation and unclean water supplies they are legion. In addition they are also seen in the developed world, particularly where there is the potential for person to person transfer as in day care centers, among the rural poor and with those with impaired immunity. It is not intended to attempt to cover such an enormous field comprehensively but a brief overview emphasising the epidemiological aspects is in order. Although the organisms are numerous they may be considered under several categories which often share characteristics.

These, wide spread geographically may be considered under several headings. Extensive references are provided in standard texts (1). Gut parasites may be:

• Extra cellular

• Giardia lamblia

• Blastocystis hominis

• Entamobea histolytica

• Intra cellular

• Cryptosporidium parvum

• Cyclospora cayetanenis

• Isospora belli

The most frequent of these are Giardia lamblia and Blastocystis.

Cryptosporidia may be underestimated because special processes are needed

to identify the parasite. Blastocystis hominis is frequently found but its pathogenicity is controversial.

2. ENTAMOBEA HISTOLYTICA

This is an important pathogen in its morbidity and its mortality and only malaria and schistosomiasis produce more deaths. Its lethal nature is partly due to its capacity to invade the body, generally producing an abscess in the liver, although its major importance is in the colon where it may produce ulceration, even mimicking inflammatory bowel disease. The consequences of this may be catastrophic in patients thought to have ulcerative colitis and given high dose steroids. Suspected for decades, it is now clear that there are two microscopically indistinguishable species; E.histolytica and E.dispar, the latter being quite benign but much more frequent than E.histolytica in the asymptomatic. Both organisms are widespread throughout the tropical world being readily spread through faecal contamination of water supplies and food. The role of immunity development to them is unclear.

Epidemiology

E.histolytica is wide spread, as noted, in the tropics but it occurs occasionally in the developed world, largely in groups such as immigrants, travellers and inmates in mental institutions and in sexually active homosexual males. Although traditionally amebic infection is associated with dysentery, recent studies of Bangladeshi children show that nearly half were newly infected annually but only a tiny minority developed dysentery.

3. GIARDIA LAMBIA

This organism has an extraordinarily wide distribution, being reported to infect up to 5% of people in the developed world and around 1 in 4 persons in undeveloped countries. In the developed world it is seen predominantly in young children, especially in day care centers where the colonization rate may reach 90%, in travellers, those dependant upon infected water supplies and in homosexual males.

Epidemiology

Transmission is mainly through infected water subject to faecal

contamination and the relative resistance of the encysted form to

chlorination renders the process inadequate and the faecal-food-oral route

does occur. There is evidence that it is a zoonosis, carrier unknown. The

organism mainly infects the small bowel and may produce a rather characteristic low grade fatty diarrhea which may be prolonged.

4. BLASTOCYSTIS HOMINIS

The role of this very common organism is controversial. It may be that it acts in cooperation with known pathogens and its role as a sole pathogen in the immuno competent may be negligible.

5. DIENTAMOBA FRAGILIS

The role of this is unclear; it may produce mild abdominal symptoms and be associated with pin worm infestation.

6. CRYPTOSPORIDIA /ISOSPORA BELLI

May cause an acute diarrhoeal illness in the immuno competent but the AIDS epidemic has brought them into prominence; in the immuno suppressed they are capable of producing a chronic debilitating diarrhoea, virtually impossible to eradicate in cryptosporidiosis.

7. CRYPTOSPORIDUM PARVUM

Recognised for decades as a wide spread infection in all mammals, it is only since the AIDS epidemic that it has become recognised as a serious and frequent pathogen in humans, even the immuno competent.

Epidemiology

This organism is transmitted by cysts liberated in the intestine,

particularly in farm animals or humans, by means of food, water or person to

person contact with faecal-oral or sexual contact. Serological studies show

evidence of infection in up to one third of well people in the developed

world, particularly in those in contact with farm animals and up to 90% in

the lesser developed world. The organism is relatively resistant to

chlorination and so outbreaks, which may be massive, are associated with

breakdown in water treatment practice, swimming pools and, once again,

day-care centers. The infection may be life threatening in the

immunocompromised such as in untreated AIDS.

8. CYCLOSPORA CAYETANENIS

Only recent recognised, this organism is worldwide in distribution but seen especially in the lesser developed world. Transferred by food principally or water, it seems to be common in AIDS patients in the lesser developed world. In the immunocompetent, after some weeks of diarrhoea the infection spontaneously resolves.

9. HELMINTHS

Worms are not, in most situations, a major health burden in western type societies. In the undeveloped world, given their usual faecal-oral mode of transmission, they are frequent and a potent cause of morbidity and some mortality. They are sub divided into three different types:

- Round worms (Nematodes (( )

- Flat worms – Cestodes (tape worms) - Trematodes (flukes)

The clinical significance of this sub division is that the members of each group share major similarities in terms of life cycle, therapy and metabolism.

The morbidity of these worms depends in general on the worm load and it is now evident that there are major genetic determinants of susceptibility.

Details of their complex lifestyles and human impact are found in major texts (2, 3), however a brief overview is appropriate.

9.1 Nematodes (Round Worms)

These flourish in areas with primitive sanitation when environmental faecal contamination flourishes. Three general sorts of life cycle are recognised.

Ingestion of ova is followed by liberation of larvae into the intestine where the worms spend their lifetime. Trichuris trichiura and Enterobius vermicularis are examples.

Ingestion of ova followed by passage through the gut wall to the venous system to the lungs, alveoli, up the bronchi and then via the pharynx to the gut – Ascaris lumbricoides.

Penetration of the skin e.g. of bare feet and then passage through the

venous system to the lungs, up the bronchi and back into the intestine –

Strongyloides stercoralis.

9.1.1 Trichuris trichiura

These thrive in moist shaded temperate or tropical areas where there is faecal contamination of soil. Light infestations are asymptomatic. Heavy infestations of the colon are mostly seen in children leading to anaemia and growth retardation.

9.1.2 Enterobius vermicularis (thread worm)

This is very common in temperate as well as tropical areas and is mainly seen in school age children. The worm has a brief life for weeks and is characterised by the female worm emerging at night to lay eggs in the perianal area. Spread is facilitated by inadequate hygiene and is often intra familial.

9.1.3 Ascaris lumbricoides

Reaching up to 40cm in length, this worm is both important and prevalent throughout the world except in cold or arid climates. The female produces up to 200 000 eggs daily, hence the absolute need for adequate sanitation. Infection may be fatal but infestations are mainly seen in children. The numerous complications range from pulmonary symptoms from worm migration, abdominal symptoms, growth retardation and even intestinal obstruction from a worm bolus.

9.1.4 Strongyloides stercoralis

This is endemic throughout the tropical areas of Africa, Asia, Latin America and part of Europe and the US. It has several unusual characteristics. It is the only worm capable of completing the life cycle within the soil and the eggs hatch within the intestine so that auto infection takes place. Partly for this reason, in those infected e.g. in tropical war conditions, the infection may persist for years. This is important for the immuno suppressed or those on steroids, hyperinfection may occur with fatal results but strangely, HIV infection is not as dangerous as might be expected and cyclosporine, a potent immunosuppressant is benign in this context.

9.2 Cestodes (tape worms)

There are several genera of these infesting man; all have a most complex

lifestyle including sojourns in different hosts, vertebrate and invertebrate as

well as man. Infection takes place from eating inadequately cooked meat

(cattle or other herbivores) (T Saginata or T Solium). Biologically the necessity for human residence as part of the life cycle with contamination of animal food by human faeces attests to the long association of humans and these worms.

References

1. Li E, Stanley SL. Parasitic diseases : protozoa. In : Yamada T, Alpers DH, Kaplowicz N, Laine L, Owyang E, Powell DW eds. Textbook of Gastroenterology 4

Ed..(Lippincott Williams and Wilkins, Philadelphia, 2003), Vol. 2, pp. 2589-2607.

2. Pearson RD. Parasite diseases : helminthes. In : Yamada T, Alpers DH, Kaplowicz N, Laine L, Owyang E, Powell DW eds. Textbook of Gastroenterology 4

Ed (Lippincott Williams and Wilkins, Philadelphia, 2003) Vol. 2, pp. 2608-2625.

3. Guerrant RL, Walker DH, Weller P. Tropical Infectious Diseases : Principles, Pathogens

and Practice. (Churchill Livingstone, Philadelphia, 1999).