TICKS AS VIRUS VECTORS IN EASTERN EUROPE/D BLASKOVIC AND J REHACEK

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• Original Classification: C
• Document Page Count: 26
• Document Creation Date: December 22, 2016
• Document Release Date: January 19, 2011
• Sequence Number: 1
• Publication Date: December 14, 1960
• Content Type: REPORT

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Ticks as virus vectors in Eastern Europe'
D. Blalkovid and J. $eM ek
Institute of Virology, Czechoslovak Academy of Sciences, Bratislava,
Czechoslovakia
During the past 20 years there has been a considerable in-
orease in ourknowled8a concerning
viruses transmitted t~ nature
to man and domestic animals by means of arthropod vectors. The
present number of these arthropod-borne (arbor) viruses amounts
to about 150, of which about 120 are partially classified. Every
year new viruses are being isolated either from vectors or wild
living animals or man.
The new virus isolations are a result not only of improved
virus isolation techniques, but also of changed opinions concern-
ing the role of both the virus circulating in nature and its
hosts, especially of its vectors. The starting positions change.
Whereas at the beginning there was basically only one problem,
namely the sick person or animal and ways how to prevent their in-
fection by known prophylactic measures (vaccination, seroprophy-
laxis), attention is being paid recently to all factors involved
in the maintenance and circulation of virus in nature.
A theoretical basis for such a oonoaption was offered by J.N.
Pavloveky (1939, 1940) in his teaohing 2 natural fooi of iafeo
tioa and landscape r +avlovs
_ ]i6~- (1956) defines a natural
abltat is a geegraphieally thll-drti ,, # tarlaad
as
fee" of infection 'pair a trar# of ~ooua
? , dartuate: ,~no~apht-.
oral typa, oeztainin habits ) .in Leh es ,.- intaaspeei#i
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from animal donor to animal recipient under conditions of the ex-
ternal environment conducive to, or preventing circulation of the
microorganism among the participants in such biooenose".ms) In
M) Biooenose is used here to refer to the "speoies-network"
which is the mutually connected assembly of living species within
the ecosystem, the latter being defined as a particular geograplgi-
cal and climatic area together with the whole community of living
organisms, bfth plant and animal, existing in this physical en-
vironment.
this definition the term-microorganism includes not only viruses,
but also bacteria and parasitic protozoa. Emphasis is laid on the
ecological relationships between the microorganism, its hosts and
the general character of the area, in which the microorganisms pat-
hogenic for man are circulating. This gives rise to the idea that
an efficient prevention of transmissible diseases, this being syno-
twns~ir/~.1 to ti4e bitt f' wecf.r,
nymous with arbor virus*
infection , w 11 consist of efficient mea-
sures taken directly in the field after ascertaining all factors
determining the ecology of a virus or of the agent pathogenic for
man or animals. In this connection we should like to note briefly
that R.N. Pavlovsky agrees with extending his conception on natural
foci of infection also to plant viruses transmitted by leathoppprs
from wild to crop plants (Valenta 1956).
In mosquito-borne virus infections of man some fiadiags on the
etiology of viruses and their hosts were utas4ed - W introdsood
into practice very early. An example is the prsve*#ieas arced. eradioaNo1
tion of yellow tever in some areas. But even in the ease of tlis
an
olassseal example ot4arber virus infection so tboroq*Ur laresti.
g'Ateij farther studios are seofssary an virus *0010 0. so same
s ues with the hafe we= Asase by different ifvsu
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?is ,; *e rap of wire of to $ook*tet)'-
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different parts of the world. They isolated many new virus species,
many of which(oannot yet be given in a causal connection) with di-
seases of man or domestic animals and which seem to await in thei
evolution and ecology their parasitic stage.
In Europe, ticks are known as vectors of viruses of the Russian
tick-borne encephalitis complex and of the virus of Crimean mn
hemorrhagic fever. The Russian tick-borne encephalitis complex in-
cludes viruses of louping-ill distributed in northern parts of
Great Britain and in Finnland (Oker-Blom 1956), of Central European
tick-borne encephalitis., of bi-phasic meningoencephalitis 'Smoro-
dintsev et al. 195}) and of Russian spring-summer (tayga) encepha-
litis (Zilber 1939).
The viruses mentioned form a group of antigenically very close-
ly related viruses, the differentiation of which to the individual
types, in general corresponding to the slight differences in patho-
genesis in monkeys (Ilyenko and Pokrovskaya 1960) and to the cli-
nical picture, was successfully solved by Clarke (1960) by absorp-
tion of sera and immunoprecipitation in agar. Because all the vi-
wev,0_ouuC
a.>Cg~,.ov and Skrynnik 1939) ,
I. persulcatus (Serdyukova and Shubladze 1041, Shubladze 1944, Du-
mina 1955), I.ricinus (Chumakov 1941, Benda 1958 a) and I. hexa-
gonus (Streissle 1960) ticks.
A regular transovarial transmission was reported especially
in the first studies on the role of ticks as vectors of tick-borne
encephalitis virus. Recently, however, transovarial transmission
in ticks from new localities is considered to be irregular (Dumina
1958, i3enda
1958
at
a). Benda (1958 a) estimates the transovarial
transmission
*e
6% and is of the opinion that under the conditions
prevailing in Czechoslovakia the transovarial transmission of the
virus in I1.ricinus ticks is infrequent in nature. Streissle (1960)
obtained in I. hexaonus-ticks a transovarial transmission rate of
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189. There are several authors who could not demonstrate in their
experiments a transovarial transmission in different tick species
(van Tongeren 1957, Gresikovd and Aehd6ek, unpublished data).
It seems that these differences are not the result of different
methods used, but that they are a consequence of the conditions
discussed above. According to Smorodintsev ,pe S8)
tom), in some foci in the U.S.S.R. 25, of ticks are
vectors and reservoirs of virus. It would seem that the virus is
more adapted to tick organs under these conditions than under the
conditions of Central Europe, where the natural foci of infection
are not so valid.
4. Transmission of the tick-borne encephalitis virus by ticks
to host animals
Transmission of the virus by ticks to host animals or man takes
place either biologically (by bite of a starving tick) or mechani-
cally by the so-called interrupted feeding.
Biological transmission is the normal way of transmitting the
virus by means of ticks. In this way the virus can be transmitted
to host animals by any of the tick instars. According to Petrish-
cheva and Levkovich (1949) a two-day-feeding of three infectious
larvae of I. ricinus or I. persulcatus is sufficient to cause en-
cephalitis in mice. One infectious I. ricinus female is sufficient
to cause fatal infection of a mouse (Benda 1958 b). The time for
which the ticks can transmit the virus is very long. In I. ersul-
gCh
ca tus it is 26 months and the virus passes through 3 neratl '1011
`Chu-
makov 1944). The greatest amounts of tick-borne encephalitis virus
are excreted by salivary glands of females, smaller amounts by
those of nypphs and the smallest by those of larvae (Benda 1958 b).
This is in contrast with the experiences of Soviet investigators
(Smorodintsev, personal communication), who reoommend to collect
engorged females in the field and to use larvae hatched from them
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for virus isolation experiments.
After a bite by an infectious tick viremia of varying dura-
tion develops in susceptible hosts (the duration presumably de-
pends on the degree of susceptibility to infection). Virernia does
not occur in animals which acquired immunity following a previous
contact with virus. In their bloodYvirus neutralizing antibodies
can be detected. After a fresh infection with even small doses
of virus, the host animal aim produces also complement fixing anti-
bodies thmmmtm in addition to virus neutralizing antibodies.
It is reasonable to assume that also in nature virus transmis-
sion to an animal host can occur by an interrupted feeding. When
catching ticks, also partially engorged individuals are caught.
Tinder experimental conditions, virus transmission by interrupted
feeding was demonstrated in I. persulcatus males and females, in
the case of the females after a 4-hour-feeding on sick mice or
mein, from whom the ticks were transferred after 3, 6, 7 or 8 days
on healthy mice, on which they fed until completely engorged.(Chu-
makov 1939, Shubladze and Serdyukova 1938). Virus transmission by
interrupted feeding was also demonstrated in D. silvarum (Skrynnik
and Ryzhov 1940). The Central European strain of tick-borne ence-
phalitis virus could also be transmitted by interrupted feeding
of I. ricinus females to goats (Gre6fkovI and ehd6ek 1959).
5. Hibernation of tick-borne encephalitis virus in ticks
One of the main problems of the biology of tick-borne encepha-
litis virus in nature is the persistence of virus during interepi-
demic periods or during those in which the ticks are inactive, i.e.
in winter. Levkovioh and Skrynnik (1940) collected different tick
instars in the spring and tested them for presence of virus. Thus
they succeeded in obtaining several strains of the tick-borne ence-
phalitis virus in the Khabarovsk region. Experimental studies on
transovarial transmission or on the persistence of virus for vary-
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ing periods of time and at different, including low, temperatures
offer unequivocal evidence of that the tick-borne encephalitis vi-
rus hibernates in ticks. In our Institute, lehd ek 01960) observ-
ed virus survival in engorged I. ricinus larvae for 102 days of
hibernation. He could recover the virus from the larvae on the 6th
and from the Iatched nymphs from the 57th - 88th days after the
end of hibernation. Loew (1960) observed virus survival in ticks
after 8 months of winter rest in terraria with optimal biological
conditions.
III. Crimean hemorrhagic fever
Crimean hemorrhagic fever occurs in the Crimea and was also
found in Bulgaria and Central Asian Soviet Republics. The virus
was isolated from filtrates of Hyaloma plumbeum piumbeum ticks
from the field. Transmission to man takes place only by tick bite.
However, infections were reported of nurses who came into direct
contact with patients' blood. In 1944 in the Crimea tick bite was
reported in the anamnesis of 87.8% of patients and in the rest it
was not excluded. The distribution area of the disease corresponds
with that of H. plumbeum plumbeum and H. anatolioum ticks. H. plum-
beum larvae and nymphs were found the most often in hares, but no
virus could be isolated from their blood.
The period of activity of H. plumbeum ticks begins in April,
reaches a maximum in July and August and drops in September. The
ticks are living in the steppe on the ground, grasses and shrubs.
Both adults and nymphs of the ticks can feed on man. Transovarial
.transmission was demonstrated in H. plumbeum ticks. The virus can
overwinter in the ticks.,(The above data are quoted after Chumakov
1957 and Gapochko et al. 1957).
I. have presented some data on the relationship of viruses
eiroulating in nature to ticks acting as their vectors or, eventual-
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4
ly, as their reservoirs. We are aware that we could not exhaust
all the problems in their whole width; we only pointed out some
of them, in which we are interested the most. We think that the
data reported will help to elucidate the ecology of tick-borne
viruses in nature with the aim to interrupt the circulations of
these agents in order to prevent the dan,ccer of infection of man
and of economically important animals.
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R e f e r e n c e s
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"#'
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Arch. biol . "dauk 52
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r -a
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