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77
Acute Encephalitis
No case of acute encephalitis was notified during the
year.
Measles
Of the 457 cases notified during 1960, 8 were admitted to
hospital for treatment.
No death occurred under this heading.
Attempts have been made from time to time to explain the
biennial periodicity of measles, and among others have been
theories that it is dependent on the proportion of susceptibles to
immunes, the topography and rate of spread in an urban unit, the
dispersal or concentration of the population, the possibility of
latent immunisation arising from sub-clinical attacks, or the
periodic fluctuation in the infecting power of the virus.
One of the most complete studies of the problem is a
fifteen year review of cases occurring in the Isle of Wight by
Dr. C.G.M. Nicol of the Ministry of Health. He points out that,
after the first months of life, susceptibility to measles is probably
nearly one hundred per cent. It is not "immunity" which protects
the under-fives collectively, but absence of exposure to infection.
With school entry the chances of thorough exposure to infection
become greatly increased. He concludes that it is the fiveand
six-year old groups which provide the material for the spread of
epidemics of measles, and themselves bring each epidemic to an end
when a sufficient proportion has had the disease.
As long as the percentage who have had measles remains
under 30, there is little evidence of a general group protective
effect. Once the percentage reaches 40, there does seem to be an
over-all protective effect both in interrupting the course of the
outbreak and in leaving some residual protection to be carried
forward to the following year. If the percentage of recorded cases
passes 70, the residual effect appears to last for two years or
possibly longer.
The following is taken from Dr. Nichol's article in the
Ministry of Health's Bulletin for July, 1956:-
"Another possible explanation, and a very simple one, is
that each local epidemic ends by the combined effect of several
rather obvious factors; the number of close contacts of each case
is limited, even among school children, and becomes very sharply
limited as soon as the child is isolated at home; the spread of the
disease through a school or classroom begins to empty the classes,
reduce the degree of crowding together and the ease of spread by
propinquity and sooner or later each case of measles is ringed round
by a circle of immunes, when the outbreak must end from lack of
sufficient susceptibles in close enough contact with sources of
infection. If this explanation is correct, it follows that the
higher the proportion of immunes already present in the age group,
the sooner will the chain of spread be broken. From the Isle of
Wight data, it appears that 40 per cent. immunity in the 5and
6-year old groups is enough to interrupt the paths of spread of
measles."
"If this surmise is correct, each epidemic as it ends
still leaves many susceptibles whom it has failed to reach. Some
of these develop measles in later years, occasionally in old age the oldest notified case in the Island series was aged 72 but an
appreciable number in each age group appear to escape the disease
(or to escape notification)."