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Image Adapted from Encyclopedia Britannica
Background on Tai National Park
The Tai National Park (436,000ha) is the last and largest of the tropical rain forest belt in West Africa and is located in the southwest Cote d'Ivoire near the Liberian border. It's classified as a Biosphere Reserve and is unique for its biological diversity and numerous endemic species. Hundreds of chimpanzees and Western Red Colobus monkeys (Colobus badius) live in the Tai Forest. Emergent trees 40-60 meters tall tower over canopy trees 30 meters tall. Average temperature is 25°C with constant humidity. Average rainfall is 1900mm per year. There are two periods of light rainfall: December to February (<70 mm/month) and August (140 mm).
General Background on Chimpanzees
Since 1979, behavioral and ecological studies of the chimpanzees in the Tai Forest have been conducted by a group of ethologists. These chimpanzees have a home range of about 27km². Chimpanzees dwell both on land and in trees. Their diet is primarily vegetarian, but they also eat arthropods and some mammals (young baboons, bush pigs, and Colobus monkeys). They sleep in trees in nests made of branches and leaves. Chimpanzees are extroverted nonhuman primates living in troops ranging from 15-80 members. Adult chimpanzees groom each other's fur and the fur of young chimpanzees' to remove external parasites.
Chimpanzees Infected with Ebola Cote d'Ivoire Of the many troops of chimpanzees in the Tai Forest, one specific troop has been studied by ethologists. This particular troop was comprised of 80 chimpanzees in 1987 but has been reduced to 32. This troop experienced two episodes of severe mortality: November 1992 (8 deaths) and October-November 1994 (12 deaths). The corpses of two female chimpanzees, a 13-year-old and a 45-month-old, were found on the forest floor on November 1, 1994. The necropsy of the 13-year-old showed that blood within the heart was noncoagulated and brown. There were no obvious lesions seen in the viscera. No tissue samples were taken from this chimpanzee. The 45-month-old's rib cage was full of liquid blood and her lungs were dark red. Tissues from her different organs were collected for pathology studies. Pathological studies indicated results similar to those of human cases during the 1976 Ebola Sudan and Zaire outbreaks. Later in November of 1994, several dead chimpanzees from this troop were found with obvious signs of homorrhage, but the corpses were too decayed to collect any useful tissue samples for analysis. On November 16, 1994, one recently dead chimpanzee was found and necropsied by three scientists in the field. This chimpanzee tested positive for Ebola using IFA and ELISA tests done by the Pasteur Institute. Twelve fatal cases of Ebola hemorrhagic fever (EHF) in chimpanzees occurred during October-November of 1994. Four cases were identified from October 24th to 30th. Three days later, six cases were recovered during a wave that lasted 12 days. The last two cases in the third wave of EHF were 10 days after the end of the second episode. The chimpanzee corpses were found in a clustered distribution within a 1.5 km radius inside the chimpanzees' home range. This zone also corresponds to the most frequently used core area of the chimpanzees' territory. Male and female chimpanzees were affected. Chimpanzees with an age of 10 years or more were more susceptible to being attacked by Ebola. The three waves in the epidemic curve indicates that the chimpanzees were contaminated by a point source or by intermittent point sources. Normal forms of contact amongst the chimpanzees (grooming a chimpanzee with EHF, touching a corpse with EHF, taking care of a sick chimpanzee) was not a risk factor. Since normal contact was not a risk factor, the researchers believe that the epidemic was not spread amongst them by simple contact. The researchers found the highest risk factor during September-October to be the consumption of meat, and they believed the risk increased with the quantity of meat consumed. The researchers' analysis of their data indicated that the chimpanzees might have been infected from the mammal prey they had eaten during this period of time. Western Red Colobus monkeys are their main prey. Chimpanzees hunt about once per day and hunt monkeys about once per week. On October 19, 1994, six days before the first mortality wave, a hunting party killed and ate a young Red Colobus. The chimpanzees who were among the primary consumers of this monkey were among the victims of the first wave. A hunting party killed and ate an adult Red Colobus on Nobember 17th, which would correspond to the third wave. Two of the chimpanzees who had eaten parts of this Colobus disappeared seven days later. The male chimpanzees' hunting patterns were not being observed during the time period that would correspond to the second wave.
The Infected Ethologist
A 34-year-old female Swiss ethologist, one of the three scientists performing the necropsy on the chimpanzee found on November 16, 1994, contracted Ebola, presumably from the necropsy. During the necropsy of the chimpanzee, she wore household latex gloves that were in poor condition. During the necropsy, she noted no wounds or punctures. Transmission of Ebola to the ethologist probably occurred as a result of contact with the chimpanzee blood either by the projection of droplets onto the face, particularly mucous membranes, or on the hand. She developed a "dengue-like" syndrome on November 24th (eight days after performing the necropsy) that was later diagnosed as EHF. After checking herself into the Abidjan hospital, she was transported to Switzerland for treatment. She received a rigorous treatment of fluid and electrolyte replacement therapy. Despite the lack of strict containment measures, no secondary transimission occurred. The lack of secondary transmission even though strict containment measures were not taken supports the theory that Ebola is not respiratory/aerosol transmitted and transmission requires direct contact with the patient or the patient's bodily secretions/fluids. On day 15 of her hospitalization, she was discharged from the hospital. She did not fully recover until 6 weeks after her infection.
References:
- "Chimpanzee" Encyclopędia Britannica Online
- Le Guenno, Bernard, P. Formenty, and C. Boesch. "Ebola Virus Outbreaks in the Ivory Coast and Liberia, 1994-1995." Current Topics in Microbiology and Immunology Volume 235. Springer Verlag: New York. 1999. 77-84.