throughout the night soil collected from the urban areas. Yet a total of only 10 sporadic clinical cases appeared, of which 9 were confirmed in the laboratory. The tenth case was clinically a moderately severe cholera but the bacteriological investigations were consistently negative,
90. The appearance thereafter of one confirmed case in a rural arca with a total of 16 contact carriers, none of whom had symptoms sug- gestive of cholera, in a group of 410 persons indicated an highly infec- tious organism but of limited invasive powers.
91. The sporadic appearance of relatively few clinical cases despite the widespread distribution of cholera vibrios in the community is be- lieved to be due to the fact that the infection was re-introduced into a population well vaccinated against cholera many of whom had received vaccine on more than one occasion. Some 76% of a total population of 3,250,000 had received vaccine in August and September 1961. The following spring 53% of the whole population was again vaccinated. During the outbreak in August 1962 a further one million doses of cholera vaccine were given.
92. From October 1962 to March 31st 1963 routine laboratory in- vestigations continued, during which particular attention was given to night soil tankers. All tankers in use were sampled twice each night and not one positive culture of cholera vibrios was obtained during this time. Similarly, cases of gastro-enteritis and specimens taken from samples of water and foodstuffs which could be possible sources of persistence of the infection were all negative. Further, non-agglutinable vibrios were only recovered rarely,
93. From experience gained during the cholera outbreaks in 1961 and 1962 three important tentative conclusions have been reached. The first is that the source of persistence of cholera is the human bowel and that the symptomless contact carrier is the essential agent in the trans- mission of the disease either in an endemic area or from an endemic area to an uninfected area when epidemic conditions may arise as a result. The second is that the organism is an highly infectious one but has limited invasive powers in a well-vaccinated community. This would explain the sporadic appearance of the disease either in the endemic areas where cholera is constantly occurring and the community has some degree of herd immunity as a result, or in a well-vaccinated population where cholera vaccine has had sufficiently wide acceptance to produce a degree of artificial immunity equivalent to that produced by the natural occurrence of the disease. The third is that standard cholera vaccine
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sufficiently comprehensively applied is an important epidemiological tool which should not be discarded. The important factors here are that the vaccination cover must be sufficiently comprehensive and mass inoculation campaigns repeated at regular intervals.
94. These conclusions are necessarily tentative but future experience of cholera in the region, which is to be expected for some time to come, may well give the opportunity to study further the evidence that appears to support them.
NOTIFIABLE DISEASES
95. During 1962, there was an increase of 12.62% in the incidence of the notifiable diseases, particularly tuberculosis, measles, chickenpox and poliomyelitis. The rise in tuberculosis notifications is, however, attri- butable to the large number of cases discovered by the examination of illegal immigrants who entered the Colony during the year and a sub- stantial increase in the total notified by private practitioners. Diseases such as diphtheria, malaria and amoebiasis showed a decrease and the total mortality from all notifiable infectious diseases declined by 5.4%. 96. Free immunization against smallpox, enteric fever, diphtheria and cholera continued to be available to all members of the public at all Government Hospitals, clinics, Port Health Inoculation Centres and District Health Offices. In addition, inoculation teams visited schools. resettlement estates and other densely populated areas in conjunction with mass immunization campaigns.
97. A summary of the prophylactic immunizations given is at Appendix 6.
Amoebiasis
98. A further decrease in the incidence was recorded, a total of 195 cases with 9 deaths being notified as against 215 and 12 deaths in the preceding year. The case fatality rate remained low at 4.6%.
Bacillary Dysentery
99. There were 795 cases of bacillary dysentery with 13 deaths during the year as against 742 cases and 8 deaths in 1961.
100. Much intensive health education in the prevention of the dysen- teric infections is carried out, by Health Officers, amongst those connect- ed with the handling, preparation and sale of food. There were 148
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