The challenges posed by equine arboviruses
G. E. CHAPMAN
, M. BAYLIS
, D. ARCHER
and J. M. DALY
Epidemiology and Population Health, Institute of Infection and Global Health, University of Liverpool, Liverpool, UK
School of Veterinary Medicine and Science, University of Nottingham, Sutton Bonington, Leicestershire, UK.
*Correspondence email: email@example.com. G.E. Chapman’s present address is: Animal and Plant Health Agency, Weybridge, New Haw,
Addlestone, Surrey KT15 3NB, UK. Received: 11.03.17; Accepted: 23.02.18
Equine populations worldwide are at increasing risk of infection by viruses transmitted by biting arthropods, including mosquitoes, biting midges
(Culicoides), sandﬂies and ticks. These include the ﬂaviviruses (Japanese encephalitis, West Nile and Murray Valley encephalitis), alphaviruses (eastern,
western and Venezuelan encephalitis) and the orbiviruses (African horse sickness and equine encephalosis). This review provides an overview of the
challenges faced in the surveillance, prevention and control of the major equine arboviruses, particularly in the context of these viruses emerging in
new regions of the world.
Keywords: horse; arbovirus; encephalitis; vector; diagnosis
The rate of emergence of infectious diseases, in particular, vector-borne
viral diseases, such as dengue, chikungunya, Zika, Rift Valley fever, West
Nile, Schmallenberg and bluetongue, is increasing globally in human and
animal species for a variety of reasons . These include increased
movement of animals and people worldwide, environmental and climate
change, and human encroachment into natural habitats (taking domestic
species with them). Equine arboviruses are no exception to this trend, and
a number of authors have highlighted the potential for the introduction of
various equine arboviruses to Europe [2–4].
Arboviruses are deﬁned as viruses transmitted by biting arthropods,
which include mosquitoes, biting midges (Culicoides), sandﬂies and ticks.
Arboviruses replicate in the body of the insect and are, therefore, distinct
from viruses with a mechanical mode of transmission (e.g. equine
infectious anaemia virus is transmittedonorinthemouthpartsofinsects
without replication). The equine arboviruses discussed in this review are
listed, with their abbreviations, in Table 1. Although there are similarities in
the transmission cycles of some of these viruses, the details are virus
speciﬁc and some are particularly complex. Similarities in clinical
presentation and cross-reactivity of antibodies between related viruses can
lead to initial misdiagnosis and delayed identiﬁcation of an emerging
arbovirus. For example, the ﬁrst human cases of West Nile virus (WNV)
infection in the United States were initially thought to be caused by St
Louis encephalitis virus infection . In this review, the major challenges
presented by equine arboviruses in relation to vector roles, surveillance,
control and risk management are summarised.
Alphavirus and ﬂavivirus disease
The main ﬂavi- and alphaviruses that affect horses share some similarities:
they are transmitted by mosquitoes, and viruses of both genera cause
encephalitic disease in both horses and humans. Major ﬂaviviruses known
to affect equids include Japanese encephalitis virus (JEV), WNV and Murray
Valley encephalitis virus (MVEV) (Table 1). Major alphaviruses of horses
include eastern, western and Venezuelan equine encephalitis viruses (EEEV,
WEEV and VEEV, respectively), Ross River virus (RRV) and Getah virus
Equine morbidity and mortality information for mosquito-borne viruses
affectinghorsesispresentedinChapmanet al. . Inapparent infections
with limited clinical signs (e.g. transient pyrexia) are common with the
encephalitogenic viruses as a result of which the proportion of deaths per
diagnosed cases (case-fatality rate) can be very high. For example, the
average fatality rate reported for cases of West Nile encephalitis in the
United States between 1999 and 2006 was 30–40% . However,
retrospective estimates suggest that less than 10% of infected horses
develop encephalitis. This ﬁgure was conﬁrmed in a prospective study
involving 37 unvaccinated horses in which only 2 of 25 animals (8%) that
seroconverted developed encephalopathy, which was fatal in one case .
Although around 80% of surviving horses recover in 3–4weeks,asmall
proportion has residual neurological deﬁcits . In addition to inapparent
infections, strain-dependent differences in virulence can lead to wide
variation in morbidity and mortality or case-fatality estimates. However, in
contrast to WNV, JEV, EEEV and VEEV, morbidity rates of MVEV and RRV
are low and they rarely cause fatal disease, and GETV is often subclinical
with affected horses usually recovering completely .
Clinical signs in horses that are infected with the encephalitogenic
viruses (WNV, JEV, MVEV, EEEV, WEEV and VEEV) include a variety of
neurological abnormalities. However, there is a signiﬁcant degree
of overlap between clinical presentation of these diseases (clinical signs of
ataxia and paresis are common to all; Table 2), and it is this that can
present a challenge in terms of clinical diagnosis, particularly where there
is overlapping distribution.
Flavivirus epidemiology and ecology
The ﬂaviviruses that affect horses share characteristics in their transmission
cycles. In general, these viruses are maintained in an enzootic cycle (i.e.
they are transmitted between wild animals, usually birds) and horses (and
humans) are infected as ‘incidental’ or ‘dead-end’ hosts (Fig 1). Dead-end
hosts usually do not produce sufﬁciently high viraemia to infect
mosquitoes and, therefore, are not involved in ongoing transmission.
Although the reservoir hosts are avian, large outbreaks of JEV may be
associated with efﬁcient ampliﬁcation of virus in pigs, which also produce
high levels of viraemia .
West Nile virus has the most widespread geographical distribution
(Fig 2a) and the largest known vector and host range of all mosquito-borne
ﬂaviviruses . Its ability to occupy a wide geographical area is due to its
broad host and vector range. In contrast, JEV and MVEV have more
restricted ranges (Fig 2b, c). However, it is not clear whether host or
vector range may be more important in restricting their distribution.
Alphavirus epidemiology and ecology
Although similar to ﬂaviviruses, alphaviruses are transmitted by
mosquitoes and their life cycles tend to be more complex. The eastern,
western and Venezuelan equine encephalitic viruses are all restricted to
the the Americas EEEV affects horses, swine and humans as dead-end
hosts . It was traditionally thought to be maintained in an enzootic
cycle between passerine birds and mosquitoes. However, rodents are now
Equine Veterinary Journal 50 (2018) 436–445 © 2018 EVJ Ltd
Equine Veterinary Journal ISSN 0425-1644