Getah virus infection. Mair, T.S. and Timoney, P.J. - Bell Equine ...
Getah virus infection. Mair, T.S. and Timoney, P.J. - Bell Equine ...
Getah virus infection. Mair, T.S. and Timoney, P.J. - Bell Equine ...
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<strong>Getah</strong> <strong>virus</strong> <strong>infection</strong> 155<br />
GETAH VIRUS INFECTION<br />
T. S. <strong>Mair</strong>* <strong>and</strong> P. J. <strong>Timoney</strong> †<br />
<strong>Bell</strong> <strong>Equine</strong> Veterinary Clinic, Mereworth, Maidstone, Kent ME18 5GS; <strong>and</strong> † Gluck <strong>Equine</strong> Research<br />
Center, University of Kentucky, Lexington, Kentucky 40546-0099, USA.<br />
Keywords: horse; <strong>Getah</strong> <strong>virus</strong>; arbo<strong>virus</strong>; Semliki Forest <strong>virus</strong>es<br />
Summary<br />
<strong>Getah</strong> <strong>virus</strong> is an arbo<strong>virus</strong> that was first isolated<br />
from mosquitoes in Malaysia in 1955. Outbreaks of<br />
<strong>infection</strong> by <strong>Getah</strong> <strong>virus</strong> have been recorded in horses<br />
in Japan <strong>and</strong> India. Infection can occur in a wide range<br />
of vertebrates, <strong>and</strong> the <strong>virus</strong> is probably maintained<br />
in a mosquito-pig-mosquito cycle. Clinical disease in<br />
horses is generally mild, characterised by pyrexia,<br />
oedema of the limbs, urticaria <strong>and</strong> a stiff gait. An<br />
inactivated whole-<strong>virus</strong> vaccine is available in Japan.<br />
Introduction<br />
<strong>Getah</strong> <strong>virus</strong> is an arbo<strong>virus</strong> that was first isolated from<br />
mosquitoes (Culex gelidus) in Malaysia in 1955 (Berge<br />
1975). However, it was not until 1978 that the <strong>virus</strong><br />
was shown to be responsible for a mild disease among<br />
racehorses in Japan (Kamada et al. 1980; Sentsui <strong>and</strong><br />
Kono 1980a; <strong>Timoney</strong> 2004). Subsequent outbreaks of<br />
<strong>Getah</strong> <strong>virus</strong> <strong>infection</strong> have been documented in<br />
racehorses in Japan in the 1970s <strong>and</strong> 1980s (Sentsui<br />
<strong>and</strong> Kono 1985) <strong>and</strong> among breeding horses in India<br />
in 1990 (Brown <strong>and</strong> <strong>Timoney</strong> 1998). Infection with the<br />
<strong>virus</strong> also occurs in other domestic animals, including<br />
pigs where it has been associated with illness <strong>and</strong><br />
death in young piglets aged up to 18 days. Serological<br />
evidence of <strong>infection</strong> has been found in a large number<br />
of vertebrate species (mammals, birds <strong>and</strong> reptiles) in<br />
which the <strong>infection</strong> appears to be subclinical (Doherty<br />
et al. 1966). There is no evidence that the <strong>virus</strong> can<br />
infect man (Fukunaga et al. 2000).<br />
Aetiology<br />
<strong>Getah</strong> <strong>virus</strong> is a positive-str<strong>and</strong>ed RNA <strong>virus</strong><br />
belonging to the genus Alpha<strong>virus</strong> of the family<br />
Togaviridae (Calisher et al. 1980; <strong>Timoney</strong> 2004). It<br />
*Author to whom correspondence should be addressed.<br />
is classified in the Semliki Forest complex along with<br />
Semliki Forest, Bebaru, Ross River, Chikungunya,<br />
O’nyong-nyong, Una <strong>and</strong> Mayaro <strong>virus</strong>es (van<br />
Regenmortel et al. 2000). Several subtypes of <strong>Getah</strong><br />
<strong>virus</strong> are recognised, including Sagiyama, Ross River<br />
<strong>and</strong> Bebaru <strong>virus</strong>es (Calisher <strong>and</strong> Walton 1996).<br />
<strong>Getah</strong> <strong>virus</strong> possesses haemagglutinating <strong>and</strong><br />
complement fixing antigens, which have been used to<br />
develop diagnostic tests for the <strong>virus</strong>. These tests in<br />
addition to the neutralisation test have been used to<br />
investigate antigenic relationships between different<br />
strains <strong>and</strong> subtypes of <strong>virus</strong> (Fukunaga et al. 2000).<br />
Epidemiology<br />
<strong>Getah</strong> <strong>virus</strong> is transmitted by mosquitoes, <strong>and</strong> is<br />
widely distributed in southeast Australasia <strong>and</strong><br />
surrounding countries, including Australia, Borneo,<br />
Cambodia, China, Indonesia, Japan, Korea,<br />
Malaysia, Mongolia, the Philippines, Russia,<br />
Thail<strong>and</strong>, Sarawak, Siberia, Sri Lanka <strong>and</strong> Vietnam<br />
(Calisher <strong>and</strong> Walton 1996; Fukunaga et al. 2000;<br />
<strong>Timoney</strong> 2004). Natural <strong>infection</strong> in wildlife is<br />
believed to be subclinical (Fukunaga et al. 2000). In<br />
tropical areas of Asia, pigs appear to be important in<br />
maintaining a reservoir of <strong>infection</strong> via a mosquitopig-mosquito<br />
cycle (Chanas et al. 1977). Pigs, <strong>and</strong><br />
possibly other vertebrates, may play an important<br />
role as amplifying hosts for the <strong>virus</strong> (Fukunaga<br />
et al. 2000).<br />
Serological surveys of horses in Japan have shown<br />
that the <strong>virus</strong> is widespread in this country, with<br />
seropositive rates up to 93% (Imagawa et al. 1981;<br />
Sentsui <strong>and</strong> Kono 1980b; Brown <strong>and</strong> <strong>Timoney</strong> 1998;<br />
Sugiura <strong>and</strong> Shimada 1999). The highest rates of<br />
seropositivity occurred in older horses in the cooler<br />
regions of northern Japan. Seroprevalence was<br />
reported as 17% in India <strong>and</strong> 25% in Hong Kong<br />
(Kamada et al. 1991; Shortridge et al. 1994). Many
156 Infectious Diseases of the Horse<br />
cases of <strong>Getah</strong> <strong>virus</strong> <strong>infection</strong> in horses are subclinical<br />
(Fukunaga et al. 2000).<br />
The principal vectors for the transmission of<br />
<strong>Getah</strong> <strong>virus</strong> to horses are mosquitoes (species of Culex<br />
or Aedes) (Calisher <strong>and</strong> Walton 1996; Fukunaga<br />
et al. 2000), although horse to horse transmission<br />
could also occur (Sentsui <strong>and</strong> Kono 1980a). High<br />
levels of <strong>virus</strong> can be found in nasal secretions of<br />
horses experimentally infected by <strong>Getah</strong> <strong>virus</strong> by the<br />
nasal route (Kamada et al. 1991), thereby permitting<br />
horse to horse spread in animals in close contact.<br />
Nonvector spread of the <strong>virus</strong> was believed to have<br />
contributed to the outbreak of <strong>Getah</strong> <strong>virus</strong> in India in<br />
1990 (Brown <strong>and</strong> <strong>Timoney</strong> 1998).<br />
FIGURE 1: Lower limb oedema due to <strong>Getah</strong> <strong>virus</strong> <strong>infection</strong>.<br />
Clinical signs<br />
Clinical <strong>infection</strong> caused by <strong>Getah</strong> <strong>virus</strong> has been<br />
observed almost exclusively in the horse. Outbreaks<br />
of the disease in horses have been sporadic, <strong>and</strong> have<br />
not been associated with any mortality (Fukunaga<br />
et al. 2000). The morbidity rate in one outbreak of<br />
<strong>infection</strong> in racehorses was 38% (Kamada et al. 1980;<br />
Sentsui <strong>and</strong> Kono 1980a), with slow <strong>and</strong> irregular<br />
spread of <strong>infection</strong>.<br />
The clinical signs associated with <strong>Getah</strong> <strong>virus</strong><br />
<strong>infection</strong> in horses vary depending on the strain of<br />
<strong>virus</strong> <strong>and</strong> viral dose (<strong>Timoney</strong> 2004). Some <strong>infection</strong>s<br />
only produce a febrile response with associated<br />
inappetence <strong>and</strong> depression (Sentsui <strong>and</strong> Kono<br />
1980a). The febrile stage lasts 1–4 days. Other signs<br />
seen in infected horses include lower limb oedema<br />
(Fig 1), swelling of the subm<strong>and</strong>ibular lymph nodes<br />
(Fig 2), urticaria (especially on the neck, shoulders<br />
<strong>and</strong> hind quarters) (Fig 3) <strong>and</strong> a stiff gait (Kamada<br />
et al. 1980; Sentsui <strong>and</strong> Kono 1980a; Fukunaga et al.<br />
1981a; Brown <strong>and</strong> <strong>Timoney</strong> 1998). Mild colic, icterus<br />
<strong>and</strong> scrotal oedema have also been reported (<strong>Timoney</strong><br />
2004). Limb oedema <strong>and</strong> urticaria usually appear<br />
several days after the onset of pyrexia. Mild anaemia<br />
<strong>and</strong> a transient lymphopenia may be present<br />
(Fukunaga et al. 2000). Serum alkaline phosphatase<br />
can be elevated during the acute phase of the<br />
<strong>infection</strong>, whereas serum lactic dehydrogenase <strong>and</strong><br />
glutamine pyruvic transaminase rise during<br />
convalescence (Calisher <strong>and</strong> Walton 1996). Most<br />
affected horses make a full clinical recovery within<br />
one week, although a small number may require up<br />
to 2 weeks to recover (<strong>Timoney</strong> 2004). Abortion is not<br />
a feature of the disease, <strong>and</strong> foals born to mares that<br />
FIGURE 2: Subm<strong>and</strong>ibular lymphadenopathy associated with<br />
<strong>Getah</strong> <strong>virus</strong> <strong>infection</strong>.<br />
FIGURE 3: Urticaria due to <strong>Getah</strong> <strong>virus</strong> <strong>infection</strong>.
<strong>Getah</strong> <strong>virus</strong> <strong>infection</strong> 157<br />
have had the disease during gestation are normal<br />
(Brown <strong>and</strong> <strong>Timoney</strong> 1998). In experimental<br />
<strong>infection</strong>s, the clinical signs are similar, but infected<br />
horses also commonly develop a serous nasal<br />
discharge (Kamada et al. 1991).<br />
Diagnosis<br />
In view of its clinical similarity to a range of other<br />
infectious <strong>and</strong> noninfectious equine diseases, a<br />
provisional clinical diagnosis must be confirmed<br />
through laboratory examination. Included in a<br />
differential diagnosis of <strong>Getah</strong> <strong>virus</strong> <strong>infection</strong> are<br />
equine viral arteritis, equine rhinopneumonitis,<br />
equine encephalosis, equine influenza, equine<br />
infectious anaemia, African horse sickness fever,<br />
purpura haemorrhagica <strong>and</strong> hoary alyssum toxicosis.<br />
Virus isolation or demonstration of viral nucleic acid<br />
by reverse-transcription polymerase chain reaction<br />
may be performed on nasal swabs, unclotted blood or<br />
saliva of acutely infected animals (Kamada et al.<br />
1980; Sentsui et al. 1980; Fukunaga et al. 1981a). The<br />
<strong>virus</strong> can be cultivated in a range of equine <strong>and</strong><br />
nonequine cell culture systems, especially Vero <strong>and</strong><br />
RK-13 cell lines, as well as in suckling mice<br />
inoculated by the intracerebral route. Highest rates<br />
of <strong>virus</strong> isolation have been achieved from plasma<br />
(Fukunaga et al. 1981b). Detection of <strong>Getah</strong> <strong>virus</strong> is<br />
optimal where specimens are collected as early as<br />
possible after the onset of fever.<br />
Serum antibodies to <strong>Getah</strong> <strong>virus</strong> can be detected<br />
by haemagglutination inhibition, complement<br />
fixation, enzyme-linked immunosorbent assay<br />
(ELISA) <strong>and</strong> neutralisation tests (Kamada et al.<br />
1980; Sentsui <strong>and</strong> Kono 1980; Sentsui <strong>and</strong> Kono<br />
1985; Brown <strong>and</strong> <strong>Timoney</strong> 1998). If possible, paired<br />
samples taken during the acute <strong>and</strong> convalescent<br />
stages of the disease should be assayed. The<br />
neutralisation test is considered the most specific<br />
test at differentiating <strong>Getah</strong> <strong>virus</strong> <strong>infection</strong> from<br />
other antigenically related alpha<strong>virus</strong>es (Chanas<br />
et al. 1977).<br />
Control<br />
Control of <strong>Getah</strong> <strong>virus</strong> in endemic areas relies on<br />
control of the mosquito vector (Fukunaga et al. 2000;<br />
<strong>Timoney</strong> 2004). This can be achieved by eliminating<br />
or reducing mosquito breeding sites, <strong>and</strong> use of<br />
larvicides <strong>and</strong> adulticides. The risk of exposure of<br />
horses to <strong>virus</strong>-infected mosquitoes can be reduced by<br />
housing them during dusk <strong>and</strong> at night.<br />
An inactivated whole-<strong>virus</strong> vaccine is available in<br />
Japan. Horses receive an annual booster dose of the<br />
vaccine in May or June, prior to the onset of the<br />
mosquito season. No epidemics of <strong>Getah</strong> <strong>virus</strong><br />
<strong>infection</strong> have been recorded in horses in Japan since<br />
the current vaccination programme was implemented<br />
in 1979 (<strong>Timoney</strong> 2004).<br />
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