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The Ticks of East Africa
about May to August which, it will be observed, is approximately the time when
East Coast fever records are generally high. It is in isolated localities, and
only at very short periods, that the temperature falls as low as freezing-point.
Maximum temperatures of over 90°F. (32°C.) are, however, quite common
during the dry seasons up to altitudes of 5,000 ft.
During all seasons of the year the maximum relative humidity rises above
90 per cent before sunrise ; during the rains the relative humidity in the after-
noon rarely falls below 55 per cent and in the dry seasons it frequently drops
lower than 20 per cent.
Limited observations in the field indicate that R. appendiculatus thrives
best when the mean maximum ranges between 60° and 80°F. (16° and 27°C.)
and the mean minimum from 50° to 60°F. (10° to 16°C.) At Mpwapwa
neither A. gemma nor A. variegatum showed numeral fluctuations correlated
with rainfall and humidity. The former species, less abundant than the latter,
occurred throughout 1935 and 1936 with no sharp differences except in May
and June of 1935 and in August 1936. The mean maximum for these three
months was 75°F. (24°C.) and the mean minimum about 60°F. (16°C.).
A. variegatum, however, showed very distinct periodicities. It was abundant
from September to March and absent from March to September. The ticks
appeared with a rise to 80° to 90°F. (27° to 32°C.) and they disappeared when
the mean maximum was from 73° to 77°F. (23° to 25°C.). Other localities
in Kenya showed similar fluctuations of A. variegatum, though not for the
same months, whilst others not many miles from them did not show the same
rise and fall of numbers.
Laboratory experimentation with temperatures and humidities has been
confined to their influence on R. appendiculatus. Even these are incomplete.
The observations made are, however, of interest. The replete female of the
species is able to tolerate exposure to cold (0° to 4°C.) for not more than about
11 or 12 days. She will not oviposit when the temperature is below 6°C., but
will proceed to lay eggs when returned to 19° to 20°C. after exposure to 0°
to 4°C. for 10 days. The number of eggs is, then, usually small and the per-
centage of hatchings low. The resultant larvae do not survive long even under
good conditions of temperature and humidity. Oviposition is weak at a high
(38°C.) temperature and the eggs soon shrivel up though the humidity may be
high. Larvae moult in 5-9 (usually 6) days at 38°C., in 6-10 (usually 9)
days at 20°C., in 19-21 days at 14° to 16°C., and in 23-25 days at 12° to 14°C.
Unfed ticks succumb very quickly to high temperatures and low (50 per cent)
relative humidity. They are much more resistant to cold but do not usually
survive at 6°C. for continuous periods of over four weeks. Under less rigorous
laboratory conditions, replete and unfed R. appendiculatus are able to live for
some time ; and mortality is reduced. Larvae transferred every 24 hours for
seven days from 38° to 14°C. moulted with a mortality as high as 60 per cent.
When moved from 4° to 38°C., mortality was 48 per cent, from 4° to 27°C.
the mortality was 35 per cent and from 6° to 20°C., 21 per cent.
Still less severe treatment and more equable temperatures decrease the
rate of mortality. Alternations of 12° to 16°C. and 12° to 20°C. effect a morta-
lity of about 6 per cent.; exposure to 8°C. and 25°C. resulted in about 9 per cent
deaths among engorged larvae.