Ebola Virus - Credit CDC
With Uganda’s second outbreak of Ebola Sudan in recent months announced earlier this week, along with their ongoing outbreak of Marburg Hemorrhagic Fever, the news that researchers in Canada have documented the transmission of Ebola Zaire from pigs to monkeys – without direct contact - has captured a good deal of attention this week.
This transmission occurred in an artificial laboratory setting, with prolonged-close, but-not-direct-contact, and doesn’t necessarily represent how things happen in the `real world’.
Nonetheless, it is notable since the conventional wisdom of how Ebola viruses are spread has long been:
People can be exposed to Ebola virus from direct contact with the blood and/or secretions of an infected person. Thus, the virus is often spread through families and friends because they come in close contact with such secretions when caring for infected persons. People can also be exposed to Ebola virus through contact with objects, such as needles, that have been contaminated with infected secretions. – CDC Special Pathogens Branch
At this point it is important to note that there is nothing in this new research to suggest that the Ebola virus has suddenly gone `airborne’.
But we’ll get to that a little later.
We’ve known since 2008 that pigs can carry some Ebola viruses (see When Viruses Jump Species), usually without showing ill effects.
FAO/OIE/WHO offer assistance to the Philippines
Manila/Roma, 23 December 2008 - Following the detection of the Ebola-Reston virus in pigs in the Philippines, FAO, the World Organization for Animal Health (OIE) and the World Health Organization (WHO) announced today that the government of the Philippines has requested the three agencies send an expert mission to work with human and animal health experts in the Philippines to further investigate the situation.
Roughly a month later, we learned that several farm workers in contact with infected pigs tested positive for antibodies to the Ebola-Reston virus. None displayed any signs of illness.
3 February 2009 - On 23 January 2009, the Government of the Philippines announced that a person thought to have come in contact with sick pigs had tested positive for Ebola Reston Virus (ERV) antibodies (IgG). On 30 January 2009 the Government announced that a further four individuals had been found positive for ERV antibodies: two farm workers in Bulacan and one farm worker in Pangasinan - the two farms currently under quarantine in northern Luzon because of ERV infection was found in pigs - and one butcher from a slaughterhouse in Pangasinan. The person announced on 23 January to have tested positive for ERV antibodies is reported to be a backyard pig farmer from Valenzuela City - a neighbourhood within Metro Manila.
Ebola-Reston is the only one of five known Ebola viruses that is not pathogenic in humans. It can kill simians, and its ability to infect pigs is worrisome given how similar human and porcine immune systems are to each other.
The natural reservoir for Ebola viruses are believed to be fruit bats of the Pteropodidae family. While the route of initial transmission to a human host is often undetermined, the assumption is that it is usually linked to the consumption of infected bushmeat (probably an intermediate host).
Although Ebola-Reston has been documented in pigs in the Philippines, it isn’t clear what role – if any - pigs play in the ecology of Ebolaviruses virus in Africa.
In 2011 researchers showed that pigs were also highly susceptible to Ebola-Zaire, which can be up to 90% fatal in humans.
This from the Journal of Infectious Diseases.
Gary P. Kobinger, Anders Leung, James Neufeld, Jason S. Richardson, Darryl Falzarano, Greg Smith, Kevin Tierney, Ami Patel and Hana M. Weingartl
This week’s study, which appears in Scientific Reports, takes this information one step further. Researchers placed four macaques in a wire cage inside a pig pen where pigs, infected with Ebola-Zaire, were kept.
Although sharing common living space, they were separated by the wire cage. Yet after 2 weeks of shared confinement, all four macaques had contracted the virus.
Hana M. Weingartl,Carissa Embury-Hyatt,Charles Nfon,Anders Leung,Greg Smith& Gary Kobinger
Article number: 811 doi:10.1038/srep00811
Here we show ZEBOV transmission from pigs to cynomolgus macaques without direct contact. Interestingly, transmission between macaques in similar housing conditions was never observed. Piglets inoculated oro-nasally with ZEBOV were transferred to the room housing macaques in an open inaccessible cage system. All macaques became infected. Infectious virus was detected in oro-nasal swabs of piglets, and in blood, swabs, and tissues of macaques. This is the first report of experimental interspecies virus transmission, with the macaques also used as a human surrogate. Our finding may influence prevention and control measures during EBOV outbreaks.
Researchers speculate that:
. . . transmission of ZEBOV could have occurred either by inhalation (of aerosol or larger droplets), and/or droplet inoculation of eyes and mucosal surfaces and/or by fomites due to droplets generated during the cleaning of the room. Infection of all four macaques in an environment, preventing direct contact between the two species and between the macaques themselves, supports the concept of airborne transmission.
While there is some degree of ambiguity here, the idea that large droplet transmission over a very short distance occurred in this setting is not unreasonable. The researchers conclude by stating (reparagraphed for readability):
The present study provides evidence that infected pigs can efficiently transmit ZEBOV to NHPs in conditions resembling farm setting.
Our findings support the hypothesis that airborne transmission may contribute to ZEBOV spread, specifically from pigs to primates, and may need to be considered in assessing transmission from animals to humans in general.
The present experimental findings would explain REBOV seropositivity of pig farmers in Philippines that were not involved in slaughtering or had no known contact with contaminated pig tissues.
The results of this study also raise a possibility that wild or domestic pigs may be a natural (non-reservoir) host for EBOV participating in the EBOV transmission to other species in sub-Saharan Africa.
According to a report by Ed Yong in The Scientist (see Ebola from Pigs to Monkeys), these researchers are now planning a trip to Africa to do serological testing of pigs in areas that have experienced Ebola outbreaks.