Scientists of the 19^th century^1,2 and the early 20^th centruy^3,4 established the etiological correlation between Entamoeba Histolytica and the liver abscess. As a natural sequence, those interested in this problem next directed their energies towards understanding the process of abscess formation by E. Histolytica. Unfortunately, in spite of a lot of laudable work done by various workers, we are still in the dark regarding the pathogenesis of the amoebic liver abscess. The long latent period after the attack of colonic infection is difficult to explain. Possibly infection persists without symptoms until some stimulus provokes multiplication of Entamoeba Histolytica or the original infection incites a hypersensitive reaction.

Enigmatic journey of the Entamoeba Histolytica to the liver
The title itself has laid bare the contents. There is no direct proof to pinpoint the route taken by the E. Histolytica to reach the liver.
There are three theoretical possibilities:

1. Direct extension from the gut to the liver
2. Via the lymphatics
3. Along the portal blood stream

Rogers³ had put forth the first hypothesis of direct extension. He had noticed that the abscesses were more commonly found in the right lobe and nearer the surface. He thought that the proximity of the hepatic flexure of the liver may allow direct extension of the infection from one organ to the other. However later, in his Lettsomian Lectures⁵ delivered before the Medical Society of London in 1922, he himself acknowledged, that he had encountered no proof to support his hypothesis as he had found only one case in which this type of extension could have occurred.
The second alternative route could be along the lymphatics. On postmortem examination of patients with colonic lesions, the amoebae are sometimes seen in large numbers in the lymph sinuses but not in the lymph nodes.^6,7 Hence this also does not appear to be the likely path.
The third possibility of the E. Histolytica reaching the liver via the portal blood stream has some evidences in its favour. On microscopic examination of the abscesses, many workers have found multiple amoebae in the small interlobular branches of the portal vein.^5,8-10 It was also noted that the bile canaliculi and the hepatic artery radicles were normal and did not show any amoebae.^5,8 Successful production of liver abscess in experimental animals by injecting E. Histolytica in the portal vein further supports this hypothesis.^11,12
Amoebic abscess occurs in organs like the brain^13,14 and the lungs^6,15 (without any abscess in the liver). The blood stream could be the only portal of entry in them. Thus, one may conclude that the spread of amoebic infection to the liver, must in all probability be blood-borne.

The process of abscess formation
How these unicellular organisms produce such large sized abscesses is another unsolved puzzle. Various workers have put forth direct and indirect evidences in support of the various possibilities. Infarction, enzymatic hydrolysis and immunological reaction either independently or in combination may bring about the formation of an abscess.

Infarction
Some workers believe that the trophozoites reach small portal radicles in large numbers and set up a thrombus formation which gives rise to an infarct.^5,8,9,12,16 The wall of the vessel is destroyed^5,8 and the amoebae come out and probably feed on the products of cytolysis. They may then enter other open portal radicles and the process may be repeated. Many workers have found thrombosed radicles of the portal vein in the walls of the abscess. Some have even found amoebae entangled in the thrombus.^5,9,10 It is not surprising then that some scientists have been able to demonstrate an actual infarct because this would only be a very early pre-clinical phenomenon. Lesions resembling an infarct have been found in the wall of a liver abscess by Palmer.⁸ He has described them as dark, reddish brown, wedge shaped, with sharply demarcated borders. On microscopic examination they had a moth eaten appearance due to multiple small abscesses 1-3mm in diameter, extending from the portal vein radicles had undergone dissolution but the hepatic artery and the bile duct branches of the triad were normal.
While working on the experimental model of amoebic colitis, 2 of the 183 kittens of Carrera¹² died. On post-mortem examination he found in them lesions similar to the ones described by Palmer.⁸ Hence infarction may be said to have some role to play in the formation of a liver abscess.

Enzymatic Dissolution
The E. Histolytica produces enzymes and thus theoretically it can produce hydrolytic dissolution of the liver tissue. The first clue to the possibility of chemical action on the tissues by the E. Histolytica came from the fact that in tissue sections, clear ‘halo-like’ areas were seen around the amoebae^17,18 (Fig.1). Some authorities feel that the clear spaces could be artifacts produced during fixing and staining of the tissues. The tissue destruction is out of proportion to the number of amoebae present⁶ (Fig.2). Histological examination shows a pattern of diffuse lytic-necrosis and toxic oedema.^7,19,20.
As early as 1927, Craig²¹ discovered haemolytic and cytolytic activities in the extracts of E. Histolytica.
Harinasuta et al²² found that live amoebic trophozoites or even their saline extracts could produce digestion of thin gelatin films. Attempts to identify the enzymes have been made by many workers. Jarumilinta et al²³ studied the chromatographic pattern of amino acids released from the gut epithelium by the action of trypsin and those released by the action of trophozoites of E. Histolytica or their extracts. They found a similarity in them. Hence, they concluded that the trophozoites could produce an enzyme with trypsinlike activity. This could bring about dissolution of gelatin, casein, fibrin, haemoglobin and suspensions of epithelial cells from the guinea pig caecum. As this enzyme is effective over a wide range of pH. i.e. 5.0-8.0,²² it can also be effective in the liver. Pepsinlike activity of the E. Histolytica trophozoite was also discovered. The trophozoites do not seem to produce any enzyme with chymotrypsin-like activity.
However, the amount of enzyme produced by an individual trophozoite is minute.²³ Thus, for producing very large lesions like the ones seen in clinical practice, either the amoebae have to be present in large numbers or there has to be some spreading factor like hyaluronidase to assist the process. De Lamater et al²⁴ concluded from their experimental work that the E. Histolytica does not produce any extra or intracellular hyaluronidase. However, Jarumilinta and Maegraith²⁵ have discovered the presence of hyaluronidase by using a different technique. Other enzymes produced by the E. Histolytica are glutaminase,²⁶ amylase, maltase, esterase, succinyl dehydrogenase and gelatinase.²⁷.
On electron microscopic examination, the E. Histolytica trophozoite does not show any separate vacuoles containing the enzymes.²⁸ The entire trophozoite is enveloped by a fuzzy coat^28,29 and some workers wonder if that contains the enzymes. Food particles engulfed and held in food vacuoles undergo digestion.³⁰ Hence it is possible that the enzymes are in an inert state in the cytoplasm and get activated in or on the surface of the food vacuole.^30,31. No separate mechanism for extracellular release or these enzymes could be found.²⁸ Some scientists believe that the enzymes are liberated only after the death of amoebae.^32,33 They may as well be released as lysosomal vesicles³³ as the cytotoxic effects are seen both after direct contact with the cells or from a distance.^34-36.
Some workers^37,38 have shown that the e. Histolytica trophozoite can bring about complete destruction of the polymorphs. The actual death of these cells is preceded by a stage of degranulation. It is a farfetched possibility that the destruction of the hepatic tissue may be a combined effect of the enzymes of the E. Histolytica as well as the hydrolytic enzymes released by the inflammatory cells due to the action of E. Histolytica on them.

Phagocytosis
Till very recently phagocytosis was not considered to be an important pathogenic mechanism of the amoebae. However, recently, the amoebae have been shown to use phagocytosis^39,40 and pinocytosis as methods for tissue damage.

References

1. Councilman, W T, and LaFleur, H A, Bull. John Hopkins Hos. Rep, 1891, 2, 345
2. Walters, J. J. Trop. Med. Hyg., 1964, 67, 133
3. Rogers L. Brit. Med. J., 1902, 2, 844
4. Charles, R H, Brit. Med. J., 1908, 2, 1235
5. Rogers, L, Lancet, 1922, 1, 463
6. James, W M, Ann. Trop. Med. Parasitol., 1928, 22, 201.
7. Meleney, H E, J. Am. Med. Ass., 1934, 103, 1213.
8. Palmer, R B, Arch. Path., 1938, 25, 327
9. Rogers, L. Brit. Med., J., 1903, 1, 1315.
10. Reddy, D G, and Subramaniam, K, Ind. J. Med. Sci., 1962, 16, 203
11. Maegraith, B G, and Harinasuta, C, Ann. Trop. Med. Parasitol., 1954, 48, 428.
12. Carrera, G M, Arch. Path., 1950, 50, 440
13. Kean, B H Gilmore, H R, et al, Ann. Int. Med., 1956, 44, 831
14. Clark, H C, Am. J. Trop. Med. Hyg., 1925, 5, 157
15. Ramamoorthy, K. Kataria, M K, et al, J. Ind. Med. Ass., 1962, 38, 232
16. Lamont, N M, and Pooler, N R, Quart. J. Med., 1958, 27, 389.
17. WHO Tech. Rep. Series. No.421, Amoebiasis, 1969
18. Malhotra, K C, Singh Amarjit, et al, J. Ass. Phys. Ind. 1966, 14, 603
19. Faust, E C, Amoebiasis, Charles C. Thomas Publishers, Springfield, Illinois. 1954
20. Faust, E C, and Kagy, E S, Am. J. Trop. Med., 1934, 14, 221.
21. Craig C F, Am. J. Trop. Med., 1927, 7, 225
22. Harinasuta, C, and Maegraith, B G, Ann. Trop. Med. Parasitol;, 1958, 52, 508.
23. Jarumilinta, R, and Maegraith, B G, Bull, WHO, 1969, 41, 269
24. De Lamager, J N, Michaelson, J B, et al, Am. J. Trop. Med. Hyg., 1954, 3, 1.
25. Jarumilinta, R, and Maegraith, B G, Ann. Trop. Med. Parasitol., 1960, 54, 118
26. Nakamura, M, and Goldstein, L. Nature, 1957, 179, 1134
27. Rees, C W Baernstein, H D, et al, Am. J. Trop. Med. Hyg., 1953, 2, 1002
28. El Hashmi, W, and Pittman, F, Am. J. Trop. Med. Hyg., 1970, 19, 215.
29. Proctor, E M, Proc. Internat. Conf. on Amoebiasis 1975, 311, Ed. By Sepulveda, B, and Diamond, L. S, Instiuto Mexicano Del Seguro Social, Mexico, 1976
30. Fletcher, K A, Maegraith, B G, et al, Ann. Trop. Med. Paristol., 1962, 56, 496
31. Fastag de Shor, A, Villegas Silva R, et al, Proc. Internat. Conf. On Amoebiasis, 1975, 142, Ed. By Sepulveda, B and Diamond, L, S, Instituto Mexicano Del Seguro Social, Mexico, 1976.
32. Villaregos, V, Trop. Dis. Bull., 1962, 59, 780.
33. Pittman, F E Pittman, J C, et al, Proc. Internat. Conf. on Amoebiasis, 1975, 398, Ed. By Sepulveda, B, and Diamond, L S, Instituto Mexicano Del Seguro Social, Mexico, 1976
34. Chevez, A, Sepulveda, B, Proc. Internat. Conf. on Amoebiasis, 1975, 500, Ed., by Sepulveda, B, and Diamond, L S, Instituto Mexicano Del Seguro Social, Mexico, 1976.
35. Chevez, A, Sepulveda, B, Proc. Internat. Conf. on Amoebiasis, 1975, 477, Ed. by Sepulveda, B, and Diamond, L S, Instituto Mexicano Del Seguro Social, Mexico, 1976.
36. Chevez, A, Sepulveda, B, et al, Proc. Internat. Conf. on Amoebiasis, 1975, 428, Ed. by Sepulveda, B, and Diamond, L S, Instituto Mexicano Del Seguro Social, Mexico, 1976.
37. Jarumilinta, R, and Kradolfer, F, Ann. Trop. Med. Parasitol., 1964, 58, 325
38. Takenchi, A, and Philips, B P, Proc. Internat. Conf. On Amoebiasis, 1975, 382, Ed. by Sepulveda, B, and Diamond, L S, Instituto Mexicano Del Seguro Social, Mexico, 1976.
39. Chevez, A, Sepulveda, B, et al, Proc. Internat. Conf. on Amoebiasis, 1975, 441, Ed. by Sepulveda, B, and Diamond, L S, Instituto Mexicano Del Seguro Social, Mexico, 1976.
40. Kairalla, A B, and Hofbauer, A F, Proc. Internat. Conf. On Amoebiasis, 1975, 238, Ed. by Sepulveda, B, and Diamond, L S, Instituto Mexicano Del Seguro Social, Mexico, 1976.