For the Health of It
Vector-borne Diseases and
the Emergent Threat They Pose
Reprinted with permission from the July 2016 edition of Equine Disease Quarterly.
Vector-borne diseases represent
a singularly serious threat to the
health of humans and domestic
livestock species in countries or
regions of the world in which they
occur. Historically, many such
diseases were frequently regarded
as geographically restricted in their
global distribution and not considered a risk to human and animal
populations in far-distant countries
in other continents or possibly
other hemispheres. Major disease
migrations in the last 20 years,
however, have undermined that
sense of security. No longer can
the future distribution of specific
infectious agents be predicted with
confidence. This was most recently
exemplified by the explosive and
unexpected spread of Chikungunya and Zika viruses, both human
pathogens, from where they were
originally identified in Africa many
years ago. Concerns are further
highlighted by the risk of spreading
yellow fever from Angola, Republic
of Congo, and Uganda to European Union member states and even
further afield to inter-tropical zones
in the Americas and Asia.
The most significant group of
emerging human and animal diseases is caused by arboviruses such
as West Nile, Chikungunya, and
Zika; they are single stranded RNA
viruses which have spontaneous
mutation rates as high as one base
per 1,000 bases for each replication
cycle. Arboviruses are transmitted
28 • Walking On
in nature by arthropod vectors.
With the exception of African
swine fever virus, all arboviruses
of medical or veterinary medical
importance belong to one of the
following four families: Bunyaviridae, Flaviviridae, Reoviridae, and
Togaviridae. They are maintained
in nature by cycling between a host
(mammal, bird, reptile, amphibian)
that is infected with a particular
virus and a vector (mosquito, tick,
sandfly, midge) that is a carrier and
transmits the virus to other hosts.
Some of the most important viral
diseases of humans are caused by
arboviruses, many belonging to
the Flaviviridae and Togaviridae
families. These include the following notable examples: yellow fever,
dengue fever, Japanese encephalitis
(JE), West Nile encephalitis (WNE),
Zika virus infection, Eastern and
Western equine encephalomyelitis (EEE and WEE), Venezuelan
equine encephalomyelitis (VEE),
and Chikungunya virus infection.
Arboviruses are also the cause of a
number of highly significant equine
diseases, the most important of
which are African horse sickness (Reoviridae), VEE, EEE, and
WEE (Togaviridae), JE, WNE, and
Murray Valley encephalitis (Flaviviridae). It is evident from the foregoing that many of the listed equine
diseases are caused by zoonotic
pathogens.
Of major concern in assessing
the health impact of arboviral dis-
eases is the potential of the causal
agents to evolve, giving rise to
strains of enhanced pathogenicity
for humans or animals. This is well
exemplified by the emergence of
variants of West Nile virus (lineage
2) in Europe that are highly pathogenic for horses. The same phenomenon has also been observed
with respect to human infection
with Chikungunya virus and most
recently, also, with Zika virus.
There is mounting evidence that
strains of Zika virus have acquired
marked neurotropic tendencies,
being implicated as a cause of neurologic defects in unborn infants
and an increased incidence of Guillain-Barre syndrome in people.
It is highly likely we will face
future threats from the emergence
of other arboviruses with epidemic
potential. If we are to be successful
in preventing such a threat becoming a reality, we need to identify
those viruses with the potential
for emergenc