During the summer of 1932-1933 our work on the natural transmission
of horsesickness was continued at Onderstepoort.
The summer was extraordinarily dry, the total rainfall from
November, 1932, to April, 1933, being only 11.43 inches.
Notwithstanding the dry character of the season horsesickness
was extremely severe throughout the country during the second half
of March and in April.
At Onderstepoort, animals which were always stabled at night
also contracted the disease.
In contra distinction to the general opinion, therefore, there
appears to exist no very close relationship between rainfall and the
occurrence of horsesickness and stabling at night does not afford a sufficient degree of protection against infection. Two extremely
important epidemiological facts so far as the planning of experimental
work was concerned thus proved to be unreliable.
An insect survey carried out in the field at Onderstepoort confirmed
the results obtained during the previous season as to the
important field species of Aedes, viz. A. caballus, A. lineatopennis,
A. hirsutus, A. dentatus and Mucidus mucidus.
An extensive search revealed the unexpected result that larvae
of Anophelines, especially of Anopheles gambiae, A. pretoriensis,
A. rufipes and A. mauritianus were present in fair numbers during
the driest part of the season in a river bed and in a marshy area
formed by accidental leakage of water. Rain of even medium
intensity destroyed the breeding places. Anophelines appear to find
suitable breeding conditions in very wet or very dry seasons and had
thus to be regarded as potential transmitters.
The occurrence of Anopheles gambiae, an important malaria
carrier, at Onderstepoort during a very dry season is of special
The experimental technique was the same as that worked out
during the previous season and already described in a separate paper.
The strains of virus used for infecting the mosquito were derived
from a number of spontaneous cases. Only fresh material or early
generations were used. In some experiments the virus horses were
infected with different strains at the same time.
In all, 31 experiments were carried out with mosquitoes in which
experimentally infected insects were either injected subcutaneously
into or refed after varying intervals on susceptible horses. 591 Specimens
were injected after 5 to 54 clays, and 615 mosquitoes refed after
periods of from 5 to 36 days.
One experiment only was positive in which 8 Anopheles gambiae,
8 A. mauritianus and 7 A. pretoriensis were injected in the
form of an emulsion 7 days after having fed on a virus horse. The
other experiments were all negative. The negative results comprised
the injection of: 21 Aedes caballus after 7 days, 27 A. lineatopennis
after 16-39 days, 398 A. caballus and A. lineatopennis (mixed) after
31-39 days, 8 A. nigeriensis after 7-9 days, 50 A. argenteus after 7-30
days, 3 Culex annulioris after 30 days, 16 Mucidus mucidus after
5-24 days, 4 Anopheles gambiae after 27-54 days, 5 A. mauritianus
after 39-54 days and 19 A. pretoriensis after 5-54 days.
The negative results obtained by feeding mosquitoes consisted of
the feeding of 137 Aedes caballus after 5-20 days, 28 A. lineatopennis
after 11-18 days, 5 A. caballus and A. lineatopennis (mixed) after 7
days, 88 A. hirsutus after 11-16 days, 16 A. dentatus after 7-19 days,
55 A. argenteus after 10-23 days, 3 Culex annulioris after 19 days, 26
Mucidus mucidus after 7-22 days, 36 Anopheles gambiae after 7-36
days, 60 A. mauritianus after 7-35 days, 70 A. pretoriensis after 7-36
days, 76 A. rufipes after 7-36 days and 15 A. squamosus after 7-35
During the previous season, in a total of 35 experiments, 1,434
specimens of Aedes, Culex and Anopheles had been injected after
intervals of from ½ to 65 days and 704 Aedes and Culex had been
refed after 1 minute to 65 days. During the two seasons, together
2,025 mosquitoes had been injected and 1,319 specimens refed after
intervals of up to 65 days.
Positive results had been obtained occasionally by the injection
of relatively large numbers of mosquitoes at intervals of up to 7 days
but never after this period or by feeding.
The important species of all the promising groups of mosquitoes,
viz., Aedes, Mucidus and Anopheles have been controlled with
sufficient material for transmission purposes to justify the conclusion
that, mosquitoes are not vectors of horsesickness.
It is of interest to note that even in the case of Aedes argenteus,
the vector of yellow fever and dengue, the horsesickness virus displayed
no tendency to persist for any length of time.
Finally, in two experiments, the eggs of 10 Rhipicephalus appendiculatus and 1 Hyalomma aegyptium, which had engorged on a
spontaneous case of horsesickness, were injected without producing a
With the present state of our knowledge we are unable to indicate
any probable vectors of horsesickness the transmitting capacity
of which might be investigated, once mosquitoes have been excluded.
Further information will have to be obtained by means of insect
surveys in the field and the new epidemiological facts established
during this season will have to be taken into consideration.
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