No disease entity can be studies in detail unless there is an experimental model available for the same. There is as yet no experimental model for amoebic liver abscess which can mimic the human disease in all its details and which is easily reproducible with out loss of many animals.
The three aspects to be taken into consideration are the host (animal), the agent (amoebae) and the various methods and modifications for producing the lesion.

The ‘Host’ or the susceptible animal
Experiments were carried out with various animals, eg. dogs, kittens, rats, guinea pigs and golden hamsters.
Dogs are found to be unsuitable because the picture was not always unform.¹ Kittens were useful only for the initial screening. In rats and guinea pigs, an abscess does form but even as early as on the fourth day, it is surrounded by a fibrous band^1,2 and the infection is self-limiting. Hence these animals were not considered ideal to work with.
In golden hamsters the abscess which is formed in the liver resembles the lesion found in the human beings. The amoebae can be easily recovered from the abscess upto a period of four days. There is no limiting wall to contain the infection and therefore it spreads, sometimes to occupy the whole lobe. Hence golden hamsters were considered ideal animals for creating an experimental model of amoebic liver abscess.^3,4
An experiment carried out by Jarumilinta et al⁴ showed that these animals (golden hamsters) could develop an abscess in the liver even after injecting amoebae in the caecum. This method has the closest resemblance of the pathogenesis of human disease.
Feeding of steroids, specially progesterone was found to increase the susceptibility of the animals to amoebic infection.⁵ Others have found hyper-cholesterolaemic animals to be more susceptible to amoebic infection.^6,7

The ‘Agent’ (Amoeba)
All the strains of amoebae are not capable of producing an abscess in experimental animals.⁸ The organisms which are received from patients suffering from amoebic liver abscess are the most virulent.⁸ Those amoebae which are isolated from patients with intestinal lesions are also virulent^.9,10 However, if organisms are collected from carriers, no abscess or intestinal lesion can be produced in experimental animals.^9,10
Even an originally virulent strain becomes avirulent after multiple subcultures.^8,11 Still if these are passed through guinea pig or hamster liver, the property of virulence is restored.^8,12 Nevertheless, an originally avirulent strain cannot be changed to a virulent one even by this method.¹²
The intensity of amoebic infection differs from patient to patient. Surprisingly the amoebae from these different groups have varying intensity of virulence. Neat et al¹² found that amoebae from patients of amoebiasis who were asymptomatic and whose serology tests were weakly positive were less virulent as compared to those from symptomatic patients with serological tests positive in high dilutions.
Some workers believe that the presence of certain bacteria increase the virulence of amoebae. Others feel that the presence or absence of bacteria does not change the virulence of the amoebae.⁸ However, the invading capacity as well as the capacity of particle phagocytosis is found to be increased in the amoebae incubated with viruses.¹⁴
The virulent strains of amoebae have different surface properties as compared to avirulent ones.¹⁵ However, exact details of explanation for differences in virulence are not known. Future work on the latest discovery of the position of the enzyme bands would be of help.¹⁶

The methods used
Different routes have been tried for introducing amoebae in the experimental animals.
Simple production of colitis is not followed by spontaneous appearance of amoebic liver abscess. However, a small number of golden hamsters do develop an amoebic liver abscess spontaneously secondary to production of an intestinal infection.⁴
Amoebic liver abscesses have been produced in rates and guinea pigs by injecting amoebae either in the mesenteric¹⁷ or portal venin.^18,19 The portal injection gave better results.¹⁸ The success rate was higher in those animals who had chronic intestinal infection.²⁰
Direct inoculation of the liver²¹ is the most preferred method. The amoebae are either injected into the substance of liver or left on the surface (Fig.10).

The process of abscess formation
When the amoebae are injected in the veins, they form infarcts by probably obstructing the lumen of the vessels. Scattered lesions appear within a few hours. The major changes are seen at the portal triad. After ten hours the lesions can be seen with the naked eye as well. Within twenty-four hours greyish yellow abscesses appear. The amoebae can be seen and even cultured from these lesions.¹⁷

Some special problems
Axenic culture of E. Histolytica. Till very recently it was not possible to culture amoebae axenically. Hence, while introducing amoebae in the experimental animals some bacteria were also introduced. Thus, there was always some doubt regarding the actual capacity of the amoebae to form an abscess.
Some workers produced amoebic liver abscesses by injecting bacteria and compared them with those produced by introducing both amoebae and the bacteria. The abscesses produced by injecting only bacteria were solid.
Others inoculated the animals with bacteria with the idea of developing immunity against them.^2,3 When the mixture of amoebae and bacteria was later injected for producing an amoebic liver abscess the number of bacteria in the abscess was much less. Still others gave appropriate antibiotics or passed the culture through seitz filters¹⁸ without much effect.¹⁷

Immunity of the animals
When an attempt was made to produce amoebic liver abscesses in animals who were on prednisolone and azathioprine, it was found that the rate of success was better and fewer amoebae were required.²²
Other methods for suppressing the immunological response of the animals have also been employed. e.g. irradiation.
Maegraith observed that it was easier to produce amoebic liver abscesses in animals who had chronic intestinal lesions indicating that prior sensitization of the animals by amoebic antigen was desirable.²⁰

References

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