43 



It is certain, from the morphology of the organism and its 

 pathogenicity to the cat — and sometimes to human beings — that 

 the amoebae and cysts found in British subjects really belong 

 to the species known as Entamoeba histolytica— alveeidy familiar 

 as the ' cause ' of amoebic dysentery and hepatic abscess in the 

 tropics. It has been shown (Wenyon and O'Connor (1917), 

 Dobell and Jepps (1918), &c.) that there are different strains of 

 this species, distinguishable by the dimensions of their cysts. 

 No detailed study of the strains found in British infections has 

 yet been made : but it is clear, from the measurements recorded 

 by several workers (see Chapter III), that similar strains exist in 

 Britain. The commonest — as in the case of the ' tropical ' infec- 

 tions—appear to be those with cysts measuring 7-9 /i or 12-13 /i 

 in mean diameter ; but there is not yet sufficient evidence to show 

 the relative frequency of the various strains in Britain. 



Up to the present I have considered the incidence of E. histo- 

 lytica infection in Britain en bloc — without distinction of age, 

 sex, locality, or occupation of the infected individuals. Something 

 on each of these heads must be added here. 



Age. — The age at which infection with E. histolytica may be 

 first acquired has not yet been established with certainty, but it 

 is undoubtedly very early. The largest series of children 

 examined are those of Matthews and Smith (1919), who examined 

 548, and of Miss Nutt, who examined 185. The first series showed 

 that 1-8 per cent, were infected with E. histolytica, the second 2-7 

 per cent. 



Matthews and Smith (1919) state that ' among fifty children, 

 under one year of age, no infections were found ' ; but one infec- 

 tion with E. histolytica was discovered by them ' in a girl just 

 three years of age '. This is probably the youngest case in all the 

 series, but Mr. Campbell has recorded (p. 31) another E. histolytica 

 infection in a child of 3 ; and it may be recalled that Moore's 

 patient (1881) died — apparently of an amoebic abscess of the liver 

 — at the age of 3f years. It is possible that in the case recorded 

 by Mr. Thacker (p. 28) the patient contracted amoebic dysentery 

 when only two years old. 



There is now definite evidence that infections with intestinal 

 protozoa may be acquired during the first year of life (cf. Giardia, 

 p. 52) ; and, although such early infection has not yet been 

 observed in the case of E. histolytica, there appears to be no 

 reason to suppose that it behaves, in this respect, differently from 

 the other common intestinal organisms. 



Infections with E. histolytica — and all other intestinal protozoa 

 — appear to be remarkably persistent, and it is probable that when 

 an infection is once acquired it lasts, in most cases, for the rest of 

 life. (Cf. Wenyon and O'Connor (1917), Dobell and Stevenson 

 (1918).) The longer a person lives, the longer he is exposed to the 

 risk of becoming infected ; and consequently one would expect 

 that, ceteris paribus, older people would show a higher degree of 

 infection than younger. This appears to be true. Matthews and 

 Smith have given the following figures bearing on this point : 



