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Questions and Answers about Ebola-Reston virus in pigs, Philippines

How was Ebola-Reston virus detected in pigs? Go to top of page
Tissues from pigs that had died from an unknown etiology were sent from the Philippines to the USDA Plum Island laboratory for diagnostic analysis. Several viruses known to infect pigs (e.g. circovirus and arterivirus) were detected in some of the tissues. Using new molecular detection assays, Ebola-Reston virus RNA was detected unexpectedly in some of the samples.  Ebola-Reston virus has not been shown to infect or cause disease in swine previously.

Although infection with Ebola-Reston virus can cause a hemorrhagic fever with high fatality in non-human primates, the Reston subtype of Ebola virus has not been shown to cause disease in humans with evidence of Ebola-Reston infection; however, infection other Ebola virus species, such as Ebola-Zaire and Ebola-Sudan, usually causes a severe hemorrhagic fever that is often fatal in humans.

In October 2008, the Centers for Disease Control and Prevention (CDC) confirmed that swine from several farms in the Philippines were infected with a filovirus, which was confirmed to be the Reston species of Ebola virus (Reston ebolavirus).
What are Filoviruses? Go to top of page

The two known filoviruses (Ebola virus and Marburg virus) belong to the family Filoviridae, one of several groups of viruses that can cause hemorrhagic fever in animals, including humans.  Filoviruses are enveloped, filamentous viruses with a single-stranded RNA genome. Five species of Ebola virus are known today.

One of the five known Ebola virus species, Ebola-Reston virus, was discovered in 1989 as the cause of an outbreak of severe illness and death among non-human primates imported from the Philippines to a quarantine facility in Reston, Virginia. Ebola-Reston virus was later detected in ill non-human primates imported from the Philippines into the United States in 1990, into Italy in 1992, and into the US and the Philippines in 1996, as well as in the Philippine export facility itself. Infection with this virus can be fatal in non-human primates; however, the four documented human infections with this virus confirmed in the US among animal handlers with extensive exposure did not result in disease. One of the infections was in an animal caretaker at the Reston primate facility who inadvertently cut himself with a virus-contaminated scalpel; he developed no significant illness suggestive of infection with Ebola virus, although Ebola-Reston virus was isolated from his blood. Antibodies to this virus were later detected in his blood (and in blood specimens from the other three other animal caretakers).  These cases of asymptomatic infection imply that humans may be susceptible to infection, but not disease, due to Ebola-Reston.  Despite this, Ebola-Reston is still considered a high-hazard pathogen for humans because the possibility still exists that it may indeed cause human disease, and may be transmissible person-to-person. At present, there is no vaccine for Ebola-Reston virus.

How is Ebola virus diagnosed in the laboratory? Go to top of page

Diagnostic specimens should be handled with extreme caution and sent to an appropriate laboratory, using approved shipping containers and import permits. The diagnosis depends on detecting the infectious agent during the acute phase of the disease or measuring the host’s specific immunological response during illness and convalescence.  Several approaches have been developed for the diagnosis of Ebola, using standard diagnostic methods on acute sera or post-mortem tissue specimens from infected humans and animals, as well as materials gathered during ecological investigations on possible reservoir hosts, namely fruit bats.

Sensitive tests are already available to measure the degree of infection during the acute phase of illness. The four tests described below are species-independent.

1. ELISA techniques have been developed to allow the detection of the viral antigen on inactivated specimens, such as blood, serum, or tissue suspensions.

2. Nucleic acid detection (reverse transcriptase polymerase chain reaction, RT-PCR; or real-time RT-PCR, also known by the trade name TaqMan) on inactivated material is rapid, more sensitive than antigen detection ELISA, and provides specific genetic identification of genetic fragments of the virus, allowing identification of the virus species.

3. Virus isolation is relatively simple but requires a Biosafety Level-4 laboratory and can take several days.

4. On dead animals and collected tissues, immunohistochemical staining and histopathology can localize viral antigen within formalin-fixed tissues and assist in elucidating the pathogenesis of the disease.

During the post-acute and recovery phases of the disease in pigs, tests measuring the quantity and type of antibodies (immunoglobulin M or G, or IgM and IgG) present in blood or serum are needed; these tests can be run in few hours. Unfortunately, none are available commercially; the swine-specific reagents for Ebola Reston IgG and IgM will need to be developed, some of them in the Biosafety Level-4 laboratory at CDC in Atlanta. Once developed, these sensitive tests will help to guide decisions on the public health response and recommendations.

Can Ebola-Reston virus cause disease in humans? Go to top of page

The four well-documented human infections caused by Ebola-Reston virus in the US in 1990 resulted in no clinical illness.  Thus, the Ebola-Reston virus appears to be less capable, and possibly incapable, of causing disease in humans than the other virus of the other Ebola species.  However, much more study of Ebola-Reston virus is needed before definitive conclusions can be made about the pathogenicity and virulence of this virus in humans.

How is Ebola-Reston virus spread? Go to top of page

Evidence from prior outbreaks indicates that Ebola-Reston is highly transmissible by percutaneous exposure (injection) or by mucous membrane (eg., eye or respiratory tract) exposure to droplets of infected body fluids and tissues from infected animals.  As with other Ebola virus species, isolation of infected animals, and contact and droplet precautions (gowns, gloves, masks, eye protection) are indicated to prevent transmission.  During the outbreaks in U.S. monkey quarantine facilities in 1989 and 1990, there was transmission to animals in separate rooms that may have been due to small-particle aerosols; however, this mode of transmission has not been proven, and other possible explanations for these infections exist.

Veterinarians and those working with pigs are advised to wear protective clothing (gowns, gloves, masks, eye protection) when working around sick pigs suspected of harboring the virus. Meat from sick pigs should not enter the food chain.

Was the virus that infected the pigs in the Philippine farms similar to previous strains of Ebola-Reston virus? Go to top of page

Genetic material of the virus from pigs from the Philippine farms has been examined and is nearly identical to the Ebola-Reston virus strains found in 1989, 1990, 1992, and 1996.  This means that these virus strains are all closely related and are the same virus species.  Viruses usually have differences in their genetic material when various strains are compared.  Thus, the changes found are not of any particular importance and are not unexpected. 

What is the natural reservoir of filoviruses? Go to top of page

The reservoir and natural cycle of filoviruses remain unknown, although fruit bats are strongly suspected to be the reservoir species.  Non-human primates are even more sensitive to infection with filoviruses than humans. It seems likely that these animals serve as indicators of filovirus activity, as so-called “incidental hosts,” but there is no evidence linking them to the maintenance cycle of the virus. It is too early to define the role of pigs, although it is more likely that they also serve as indicator of filovirus activity, as incidentally-infected hosts.

What is CDC's role in the investigation in the Philippines? Go to top of page

At the request of the USDA laboratory in Plum Island, CDC has confirmed the presence of the genetic material of Ebola-Reston in the pig tissues from the Philippines. The CDC investigators are working with the USDA to help to definitively characterize the virus and the extent of infection among the specimens already received. Based on its previous experience with Ebola-Reston in the US and the Philippines, CDC will offer epidemiological and laboratory assistance to the USDA and the Philippine Veterinary and Public Health institutions.

Related Links

Ebola Hemorrhagic Fever Home page

List of Known Cases and Outbreaks of Ebola Hemorrhagic Fever

 Filoviruses Fact Sheet
 Infection Control Manual
 Outbreak Postings (since 2000)
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This page last reviewed December 19, 2008

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