Replication, Pathogenicity, Shedding, and Transmission
of Zaire ebolavirus in Pigs
http://jid.oxfordjournals.org/content/204/2/200.full
Abstract
(See the editorial commentary by Bausch, on pages 179–81.)
Background. Reston ebolavirus was recently detected in pigs in the Philippines.
Specific antibodies were found in pig farmers, indicating exposure to the virus. This
important observation raises the possibility that pigs may be susceptible to Ebola
virus infection, including from other species, such as Zaire ebolavirus (ZEBOV), and
can transmit to other susceptible hosts.
Methods. This study investigated whether ZEBOV, a species commonly reemerging
in central Africa, can replicate and induce disease in pigs and can be transmitted to
naive animals. Domesticated Landrace pigs were challenged through mucosal
exposure with a total of 1 ×106 plaque-forming units of ZEBOV and monitored for
virus replication, shedding, and pathogenesis. Using similar conditions, virus
transmission from infected to naive animals was evaluated in a second set of pigs.
Results. Following mucosal exposure, pigs replicated ZEBOV to high titers (reaching
107 median tissue culture infective doses/mL), mainly in the respiratory tract, and
developed severe lung pathology. Shedding from the oronasal mucosa was detected
for up to 14 days after infection, and transmission was confirmed in all naive pigs
cohabiting with inoculated animals.
Conclusions. These results shed light on the susceptibility of pigs to ZEBOV
infection and identify an unexpected site of virus amplification and shedding linked
to transmission of infectious virus.
Outbreaks of Ebola hemorrhagic fever in endemic areas, as well as the introductions
of single cases into nonendemic countries, are unpredictable and always a matter of
considerable concern to public health authorities. Ebola viruses are prime examples
of “emerging/reemerging” pathogens causing the most severe hemorrhagic fever
found in human and nonhuman primates [1–4]. In fatal cases, the immune response
is insufficient to provide the rapid protection required to control fulminant Ebola
virus replication. The species Reston ebolavirus (REBOV) has never been associated
with human disease, despite multiple documented exposures [5, 6]. Alternatively,
Zaire ebolavirus (ZEBOV) is associated with case fatality rates as high as 90% in
humans [1, 7].
Currently, we are just beginning to understand the pathogenic mechanisms that lead
to severe disease and death [1–4, 8–10]. The most reliable animal model for
studying ebolavirus replication and its associated disease is the nonhuman primate
[11, 12]. The difficulties of working with nonhuman primates in a high-containment
laboratory have been limiting the speed at which researchers can fully elucidate the
complex physiopathology and fulminating shock induced by ebolavirus. Although the
nonhuman primates are the only species known to succumb to strains of ebolavirus
isolated from human cases, sequential infection of mice or guinea pigs has
generated variants that are also lethal to these 2 animal species [13, 14].
Accumulating evidence is identifying fruit bats as the natural reservoir of ebolavirus
[15, 16]. One current hypothesis is that bats transmit ebolavirus to an amplifying
host, animal or human, and that high virus load reached in the first amplifying host
favors transmission to and between humans mainly through contact with body fluids.
Several human cases of ebolavirus infection have been associated with the
butchering of infected bush meat derived from nonhuman primates [17, 18].
Recently, REBOV was detected in pigs in the Philippines and antibody to the virus
was found in several pig farmers, supporting the concept of ebolavirus replication in
pigs and zoonotic transmission between pigs and humans [19–21].
The identification of animal species that can replicate ebolavirus and transmit the
virus to other animals and/or to humans is critical to the development of
preventative measures to avert outbreaks in humans. The present work investigated
the susceptibility of domestic pigs to highly pathogenic ZEBOV. Virus replication,
pathogenicity, shedding, and transmission to naive animals were evaluated in 2
independent studies. The first study focused on virus replication, pathogenicity, and
shedding, whereas the second study was designed to evaluate shedding and
transmission of the virus from inoculated to contact pigs.
---------------
Genomic surveillance elucidates Ebola virus origin
and transmission during the 2014 outbreak
http://www.sciencemag.org/content/345/6202/1369.full
ABSTRACT
In its largest outbreak, Ebola virus disease is spreading through Guinea, Liberia,
Sierra Leone, and Nigeria. We sequenced 99 Ebola virus genomes from 78 patients
in Sierra Leone to ~2000× coverage. We observed a rapid accumulation of interhost
and intrahost genetic variation, allowing us to characterize patterns of viral
transmission over the initial weeks of the epidemic. This West African variant likely
diverged from central African lineages around 2004, crossed from Guinea to Sierra
Leone in May 2014, and has exhibited sustained human-to-human transmission
subsequently, with no evidence of additional zoonotic sources. Because many of the
mutations alter protein sequences and other biologically meaningful targets, they
should be monitored for impact on diagnostics, vaccines, and therapies critical to
outbreak response.
Patterns in observed intrahost and interhost variation provide important
insight about transmission and epidemiology. Groups of patients with
identical viruses or with shared intrahost variation show temporal
patterns suggesting transmission links (fig. S10). One iSNV (position
10,218) shared by 12 patients is later observed as fixed within 38
patients, becoming the majority allele in the population (
Fig. 4C) and
defining a third Sierra Leone cluster (
Fig. 4, A and D, and fig. S8).
Repeated propagation at intermediate frequency suggests that
transmission of multiple viral haplotypes may be common. Geographic,
temporal, and epidemiological metadata support the transmission
clustering inferred from genetic data (
Fig. 4, D and E, and fig. S11) (
6).
The observed substitution rate is roughly twice as high within the 2014
outbreak as between outbreaks (
Fig. 4F). Mutations are also more
frequently nonsynonymous during the outbreak (
Fig. 4G). Similar
findings have been seen previously (
15) and are consistent with
expectations from incomplete purifying selection (
16–
18).
Determining
whether individual mutations are deleterious, or even adaptive, would
require functional analysis; however, the rate of nonsynonymous
mutations suggests that continued progression of this epidemic
could afford an opportunity for viral adaptation (Fig. 4H),
underscoring the need for rapid containment.
As in every EVD outbreak, the 2014 EBOV variant carries a number of
genetic changes distinct to this lineage; our data do not address whether
these differences are related to the severity of the outbreak. However,
the catalog of 395 mutations, including 50 fixed nonsynonymous
changes with 8 at positions with high levels of conservation across
ebolaviruses, provides a starting point for such studies (table S4).
To aid in relief efforts and facilitate rapid global research, we have
immediately released all sequence data as it is generated. Ongoing
epidemiological and genomic surveillance is imperative to identify viral
determinants of transmission dynamics, monitor viral changes and
adaptation, ensure accurate diagnosis, guide research on therapeutic
targets, and refine public health strategies. It is our hope that this work
will aid the multidisciplinary international efforts to understand and
contain this expanding epidemic.
.
