Transmission of western equine encephalitis virus by Saskatchewan mosquitoes and behavior of the virus in selected laboratory vertebrates in relation to epidemiological studies
Date
1971
Authors
Journal Title
Journal ISSN
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Publisher
ORCID
Type
Degree Level
Doctoral
Abstract
For laboratory experiments on transmission of Western equine encephalitis (WEE) virus by Saskatchewan mosquitoes, a local virus isolate of human origin, strain 1540-1544 Regina 66,
was selected. The half-day old
chick which was used as the principal vertebrate host in
these experiments, showed a consistent response to subcutaneous inoculation with this strain of WEE virus. After infection with moderate doses, viremia was maximal at 18 to 24 hours, when virus concentration was within
the range of 108 to 109 intracerebral three-week old mouse LD50 (ic mouse LD50 ) per 0.03 ml of blood. Most infected
chicks showed inactivity followed by prostration before death; very few displayed overt clinical signs of neurologic disturbance. In comparative titrations, chicks were more sensitive than weanling mice for detection and titration
of virus.
An indirect fluorescent antibody test applied directly to brain sections from WEE infected mice, was satisfactory for diagnostic purposes. Sections were
fixed in Carney's fluid and transferred through chloroform
to paraffin. The use of Evans blue, of absorbed antisera
to WEE virus, and the inclusion of negative and positive
control preparations, ensured specificity of staining
reactions in the tests. The technique could not be applied
to brain sections from chicks infected subcutaneously,
despite the presence of large amounts of virus in this
tissue. Confirmation of WEE infection in chicks which
were bitten by infected mosquitoes was achieved by
passage of virus to mice, from which brain sections
served for diagnosis by immunofluorescence. These initial
investigations formed the basis for the design and interpretation
of detailed quantitative transmission experiments
with mosquitoes.
Nine species of local mosquitoes were examined for their ability to transmit WEE virus by bite. Culex
tarsalis was a very efficient transmitter at 69 and 75°F.
This mosquito transmitted from four days after infection
and remained infective for up to 44 days, which was the
longest period of observation. Throughout this period,
infection rates remained at 100% or close to this level
at both temperatures, but at 69°F, transmission rates
declined after about two weeks. At the higher temperature,
almost all infected mosquitoes transmitted after the first
week of incubation, and this trend continued throughout.
The minimum concentration of virus required to infect 50%
of C. tarsalis
was 102.5 ic mouse LD50 per 0.03 ml of donor
blood, and for the same level of infection in Culiseta
inornata,
a concentration of 104.0 was necessary. The
latter did not transmit WEE virus by bite. However,
infection rates were maintained between 63 and 88% from
the second to the sixth week of incubation, and the
quantities of virus in individual mosquitoes were comparable
to those in C. tarsalis. Anopheles earlei and Aedes vexans failed to support viral growth, while Aedes campestris, A. flavescens and A. spencerii maintained infection for close to two weeks at 69°F. Infected A. dorsalis did not survive beyond the first week of incubation, at which time, individual mosquitoes had minimal amounts of virus. Apart from c. tarsalis, A. fitchii was the only other mosquito which
transmitted the infection, and it did so after an extrinsic
incubation period of nine days.
All the mosquitoes, except for C.
tarsalis, were reluctant to feed on the chick. In an attempt to provide
an alternative laboratory host for future transmission studies, a short investigation was undertaken to determine
the susceptibility of the Mongolian gerbil (Meriones unguiculatus)
to peripheral infection with WEE virus.
Four-week old animals responded by exhibiting uniform
viremia patterns to a standard dose of virus, and small
doses regularly caused death. Resistance to infection
increased with age but ten-week old gerbils were as
susceptible as three-week old mice. Brains from gerbils
which died within ten days after infection, contained
large amounts of virus.
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Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Veterinary Microbiology
Program
Veterinary Microbiology