Ebola virus outbreaks linked to infected wild animal carcasses

All recent Ebola virus outbreaks in humans in forests between Gabon and the Republic of Congo were the result of handling infected wild animal carcasses, according to a study by the Wildlife Conservation Society (WCS) and its regional partners.

Appearing in the February edition of the journal Emerging Infectious Diseases, the study found that many animal carcasses tested for Ebola between 2001 and 2003 produced positive results, and found direct links between the deadly disease in animal populations and humans.

"This research proves that hunting and consumption of great apes represent a serious health risk for people in Central Africa, and a risk that can be avoided," said Dr. William Karesh, field veterinarian for the Wildlife Conservation Society and a co-author on the paper. "What we need now is improved awareness of this risk in communities where bushmeat is still a source of sustenance and continued monitoring of wildlife in the region. We have identified a 'win – win' opportunity by using this information to both protect endangered apes from illegal hunting and to protect humans from deadly outbreaks."

The paper provides definitive proof for the assumption that Ebola moves from wildlife populations to humans through the consumption or handling of carcasses or bushmeat.

Specifically, the researchers found that Ebola infections in wild animals such as gorillas, chimpanzees, and occasionally duikers (a diminutive antelope species), move across the human-wildlife divide through hunters taking either sick animals or carcasses for meat. Hunters can then spread the disease to families and hospital workers, creating the conditions for an epidemic in the process.

Between August 2001 and June 2003, researchers noted that wildlife outbreaks occurred prior to five human outbreaks in the same relative locations. During this same period, 98 animal carcasses were discovered in the region straddling northeast Gabon and the northwest Republic of Congo. Of these carcasses, 21 gorilla, chimp and duiker carcasses were tested for the Ebola virus, with 14 samples being found positive. In 11 cases, instances of human infection were directly linked to gorilla, chimpanzee and duiker carcasses.

To prevent future outbreaks from becoming health crises, health officials and wildlife researchers must continue to work together in monitoring the region's wildlife for signs of Ebola. Shortly after August 2001, participating agencies founded the Animal Mortality Monitoring Network in collaboration with Gabonese and Congolese Ministries of Forestry and Environment to predict and prevent outbreaks. On two occasions since then, the network alerted health authorities about potential human outbreaks weeks before they occurred.

"The signs of potential outbreaks often occur in remote areas, where roads are barely usable and communications limited," added Karesh. "Conservation organizations such as WCS are therefore critical to the early detection of the conditions that can lead to deadly health emergencies."

Ebola hemorrhagic fever (Ebola HF) is a severe, often-fatal disease in humans and nonhuman primates (monkeys, gorillas, and chimpanzees) that has appeared sporadically since its initial recognition in 1976.

The disease is caused by infection with Ebola virus, named after a river in the Democratic Republic of the Congo (formerly Zaire) in Africa, where it was first recognized. The virus is one of two members of a family of RNA viruses called the Filoviridae. There are four identified subtypes of Ebola virus. Three of the four have caused disease in humans: Ebola-Zaire, Ebola-Sudan, and Ebola-Ivory Coast. The fourth, Ebola-Reston, has caused disease in nonhuman primates, but not in humans.

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Where is Ebola virus found in nature?

The exact origin, locations, and natural habitat (known as the "natural reservoir") of Ebola virus remain unknown. However, on the basis of available evidence and the nature of similar viruses, researchers believe that the virus is zoonotic (animal-borne) and is normally maintained in an animal host that is native to the African continent. A similar host is probably associated with Ebola-Reston which was isolated from infected cynomolgous monkeys that were imported to the United States and Italy from the Philippines. The virus is not known to be native to other continents, such as North America.

Where do cases of Ebola hemorrhagic fever occur?

Confirmed cases of Ebola HF have been reported in the Democratic Republic of the Congo, Gabon, Sudan, the Ivory Coast, Uganda, and the Republic of the Congo. An individual with serologic evidence of infection but showing no apparent illness has been reported in Liberia, and a laboratory worker in England became ill as a result of an accidental needle-stick. No case of the disease in humans has ever been reported in the United States. Ebola-Reston virus caused severe illness and death in monkeys imported to research facilities in the United States and Italy from the Philippines; during these outbreaks, several research workers became infected with the virus, but did not become ill.

Ebola HF typically appears in sporadic outbreaks, usually spread within a health-care setting (a situation known as amplification). It is likely that sporadic, isolated cases occur as well, but go unrecognized. A table showing a chronological list of known cases and outbreaks is available.

How is Ebola virus spread?

Infections with Ebola virus are acute. There is no carrier state. Because the natural reservoir of the virus is unknown, the manner in which the virus first appears in a human at the start of an outbreak has not been determined. However, researchers have hypothesized that the first patient becomes infected through contact with an infected animal. Treating patients with Ebola HF during outbreak of the disease in Kikwit, Democratic Republic of the Congo, in 1995. After the first case-patient in an outbreak setting is infected, the virus can be transmitted in several ways. 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.

Nosocomial transmission refers to the spread of a disease within a health-care setting, such as a clinic or hospital. It occurs frequently during Ebola HF outbreaks. It includes both types of transmission described above. In African health-care facilities, patients are often cared for without the use of a mask, gown, or gloves. Exposure to the virus has occurred when health care workers treated individuals with Ebola HF without wearing these types of protective clothing. In addition, when needles or syringes are used, they may not be of the disposable type, or may not have been sterilized, but only rinsed before reinsertion into multi-use vials of medicine. If needles or syringes become contaminated with virus and are then reused, numerous people can become infected.

Ebola-Reston appeared in a primate research facility in Virginia, where it may have been transmitted from monkey to monkey through the air. While all Ebola virus species have displayed the ability to be spread through airborne particles (aerosols) under research conditions, this type of spread has not been documented among humans in a real-world setting, such as a hospital or household.

What are the symptoms of Ebola hemorrhagic fever?

The incubation period for Ebola HF ranges from 2 to 21 days. The onset of illness is abrupt and is characterized by fever, headache, joint and muscle aches, sore throat, and weakness, followed by diarrhea, vomiting, and stomach pain. A rash, red eyes, hiccups and internal and external bleeding may be seen in some patients.

Researchers do not understand why some people are able to recover from Ebola HF and others are not. However, it is known that patients who die usually have not developed a significant immune response to the virus at the time of death.

How is Ebola hemorrhagic fever clinically diagnosed?

Diagnosing Ebola HF in an individual who has been infected only a few days is difficult because early symptoms, such as red eyes and a skin rash, are nonspecific to the virus and are seen in other patients with diseases that occur much more frequently. However, if a person has the constellation of symptoms described above, and infection with Ebola virus is suspected, isolate the patient and notify local and state health departments and the CDC.

What laboratory tests are used to diagnose Ebola hemorrhagic fever?

Antigen-capture enzyme-linked immunosorbent assay (ELISA) testing, IgM ELISA, polymerase chain reaction (PCR), and virus isolation can be used to diagnose a case of Ebola HF within a few days of the onset of symptoms. Persons tested later in the course of the disease or after recovery can be tested for IgM and IgG antibodies; the disease can also be diagnosed retrospectively in deceased patients by using immunohistochemistry testing, virus isolation, or PCR.

How is Ebola hemorrhagic fever treated?

There is no standard treatment for Ebola HF. Patients receive supportive therapy. This consists of balancing the patient’s fluids and electrolytes, maintaining their oxygen status and blood pressure, and treating them for any complicating infections.

How is Ebola hemorrhagic fever prevented?

The prevention of Ebola HF in Africa presents many challenges. Because the identity and location of the natural reservoir of Ebola virus are unknown, there are few established primary prevention measures.

If cases of the disease do appear, current social and economic conditions often favor the spread of an epidemic within health-care facilities. Therefore, health-care providers must be able to recognize a case of Ebola HF should one appear. They must also have the capability to perform diagnostic tests and be ready to employ practical viral hemorrhagic fever isolation precautions, or barrier nursing techniques. These techniques include the wearing of protective clothing, such as masks, gloves, gowns, and goggles; the use of infection-control measures, including complete equipment sterilization; and the isolation of Ebola HF patients from contact with unprotected persons. The aim of all of these techniques is to avoid any person’s contact with the blood or secretions of any patient. If a patient with Ebola HF dies, it is equally important that direct contact with the body of the deceased patient be prevented.

CDC has developed a set of tools to meet health-care facilities' needs. In conjunction with the World Health Organization, CDC has developed practical, hospital-based guidelines, entitled Infection Control for Viral Haemorrhagic Fevers In the African Health Care Setting. The manual describes how to recognize cases of viral hemorrhagic fever, such as Ebola HF, and prevent further nosocomial transmission by using locally available materials and few financial resources. Similarly, a practical diagnostic test that uses tiny samples from patients’ skin has been developed to retrospectively diagnose Ebola HF in suspected case-patients who have died.

What challenges remain for the control and prevention of Ebola hemorrhagic fever?

Scientists and researchers are faced with the challenges of developing additional diagnostic tools to assist in early diagnosis of Ebola HF and conducting ecological investigations of Ebola virus and its possible reservoir. In addition, one of the research goals is to monitor suspected areas to determine the incidence of the disease. More extensive knowledge of the natural reservoir of Ebola virus and how the virus is spread must be acquired to prevent future outbreaks effectively.