Ebola Virus Outbreak among Wild Chimpanzees Living in a Rain

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					Ebola Virus Outbreak among
Wild Chimpanzees Living in a
Rain Forest of Cote d’Ivoire

             Erin Goode

  ~25% of a 43 member troop of wild chimpanzees
   disappeared or were found dead in the Tai
   National Park, Cote d’Ivoire
  In November 1994, primatologists studying the
   chimp community in the park found 8 dead
   chimps, and absences of many others
  Epidemiologic survey was done to discover the
   cause of death

  This is the first outbreak of Ebola to be described
   in the wild
  This article discusses the results of investigations
   leading to the identification of a new subtype of
   the Ebola virus (EBO-CI) in chimpanzees
        Tai Chimps
  Tai National Park is the largest
   and last remnant of the tropical rain forest belt in
   West Africa (436,000ha)
  Since 1979, wild chimpanzee troop has been studied
   in the National Park
  The home range of these chimpanzees is ~27 square
   km, and located in the western park of the park.
  A similar outbreak of Ebola also occurred in
   November 1992 among same chimp troop
Location of October-November 1994
Ebola outbreak among chimps
      Ebola virus
  Ebola (EBO) epidemics occurred in the Democratic
   Republic of Congo (DRC) and Sudan in 1976
  Investigations did not find the virus in insects or
  EBO caused one lethal case in DRC in 1977 and an
   outbreak in Sudan in 1979
  A new subtype of EBO (subtype Reston: EBO-R)
   occurred in a cynomolgus monkeys colony at a
   quarantine facility in Reston, Va in 1989
  EBO-R was responsible for 3
   other outbreaks in monkeys in
   the USA in 1990 and 1 in Italy in
  Investigations traced the source
   of these EBO-R outbreaks to a
   primate exporter in the
  The mode of contamination of
   this facility has not been
Materials and Methods:
outbreak investigation
  A case of Ebola infection- a chimp from the
   community who was missing or found dead during
   Oct, Nov, or Dec of 1994
  A definite case- a dead chimp with a confirmed
   presence of EBO virus via laboratory testing
  A probable case- a chimp whose dead body was
  A possible case- a chimp who went missing, with a
   unexplained disappearance
 Cases identified from daily records of
 Date of onset- the first day clinical
  signs were recorded in the
  chimpanzees (2 cases) or day of
  disappearance (10 cases)
 This is based on the presumption that chimps
  isolate themselves after they feel sick
 Date of death determined from the condition
  of the body
Materials and Methods:
epidemiologic investigations
  Study done with data available from surveys in
   Park with the troop since 1979 for identifying risk
   factors for cases
  Study from October to December 1994
  Presence and physical status of all chimps
   recorded on a daily basis
  Behaviors noted: sexual activity of females,
   duration of meat consumption (min) in hunting
   season, and situation of group

  All social interactions with cases recorded,
   including: grooming, behavior while
   discovering dead bodies, and behavior with
   sick animals
  Contact chimp- any chimp observed with
   direct contact with a case-chimp, during
   period between 2 days prior to the onset of
   symptoms and death of the case
Materials and Methods:
specimen collection
  2 necropsies were conducted in field in
   November 1994
  First necropsy done by investigators not
   aware of proper sampling methods- samples
   not usable for classic microbiologic testing
  Second necropsy- samples of kidney, spleen,
   lung, liver, lymph nodes, and intestinal
   tissue collected from a 45 month old female
   for histologic and bacteriologic studies. Not
   tested for viruses

  Blood specimens taken from 3 live chimps
   during 1st week in Dec. 1994 for complete
   blood cell counts and serologic tests
  Blood smears prepared from blood to look for
   malaria or trypanosome parasites
Materials and Methods:
laboratory studies
  Tissues fixed and embedded in paraffin wax
  4m sections stained with hematoxylin, eosin, and
  Immunohistochemistry studies done using a pool of
   monoclonal antibodies known to cross-react with
   subtypes of EBO
  Mouse polyclonal antibodies prepared with EBO-
   CI and EBO were also used in
   immunohistochemical tests
  Part of each tissue sample used in bacteriologic

  Platelet, leukocyte, RBC, and WBC counts
  Hemoglobin and hematocrit levels determined
  ELISAs done to identify presence of IgG and IgM
   antibodies for Rift Valley and Crimean-Congo
   hemorrhagic fever viruses, hantaviruses,
   chikungunya, yellow fever, and dengue viruses
  ELISAs for IgG and IgM antibodies against EBO
   and EBO-CI viruses also tested
epidemiologic investigation
  Beginning of October 1994 chimp community had
   43 individuals: 13 infants, 4 young adults, and 26
  From October to December- 12 members died or
   went missing
  Of these: 1 definite EBO case, 7 probable cases,
   4 possible cases
  None of other 4 missing chimps seen since
Ebola virus attack rate by age and sex
 Of 12 cases, included 2 infants (virus attack
  rate = 15%) and 10 adults (attack rate = 38%)
 No significant differences in attack rates between
  males and females
 Attack rates highest among adults
 Mothers of both infant cases also died or
 First case of EBO recorded on Oct. 25, 1994 with
  last on Nov. 27th
 Geographic distribution of dead chimps shows
  clustered distribution in most commonly used area
  of the territory
risk factors
  Activities involving case-contact were
   not significant risk factors. Including
   touching dead chimps or grooming a
   case patient
  Infants had a very high risk factor
   when their mother was an EBO case
  Chimps engaging in sexual activity
   from Oct to Nov had a relative risk of
  Chimps who consumed meat had a
   relative risk of 5.2, the risk increase
   with quantity of meat ingested
Risk factors during Ebola outbreak
  Before outbreak, chimp group fed on fruit from 1
   fig tree from Oct 10-19
  Pigeons seen feeding on same tree everyday
  Chimps seen hunting twice from Oct to Nov
  First hunt occurred 7 days before outbreak-
   young red colobus monkey killed and eaten
  2 main consumers of monkey were among early
  Last 2 cases fed on adult red colobus 11 days
   before disappearing
  Liver lesions contained many small sites of
  Spleen showed extensive areas of necrosis in the
   red pulp
  Single, large, inclusion bodies in cytoplasm of
   macrophages of red pulp of spleen, some hepatic
   Kupffer cells, and hepatocytes
  In mesenteric lymph node, cortical pulp showed
   pyknosis and necrosis of centrofollicular areas

  Several macrophages in lymph nodes contained big
    inclusion bodies- that were compatible with viral
  EBO-specific immunohistochemistry of
   liver, spleen, lymph nodes, and lung positive
  Large distribution of virus in all organs
  Mf, esp. vascular Mf, in spleen and liver
   were the immunopositive cells
  Bacterial cultures and serologic tests for Rift Valley,
    Crimean-Congo fevers, and chikungunya, hantaviruses

  3 live chimps had IgG but not IgM antibodies
   against yellow fever and dengue viruses
  3 live chimps sampled had no antibodies
   against new subtype EBO-CI
  Immunohistochemistry diagnosed EBO infection in
   1 chimp who died in Nov 1994
  During this epidemic, the other dead and missing
   most likely died from same pathogen
  Findings indicate a short illness followed by
   sudden death
  3 surviving chimps with high risk factors for EBO,
   from eating meat, were negative for EBO-CI
  Indicates they were never infected- supports idea
   that case fatality rate is near 100% in infected

  Lesions described similar to lesions observed in
   experimentally infected monkeys
  Contrary to reports of naturally infected humans
   and experimentally infected monkeys- no
   hemorrhagic, thrombotic, or vascular lesions
  But pathologic features of EBO-CI could be
   different from other EBO infections
  Results of risk factors are compatible with
   dissemination patterns in human EBO outbreaks
Discussion con’t

  Chimps not at risk for EBO-CI from touching or
   grooming infected animal
  But very close contact is a risk factor, like sexual
   activities (low risk) or mother-infant contact (high
  During this outbreak, infection seemed to have
   originated from a point source and spread through
  Data shows that highest risk factor for infection
   was meat consumption between Sept and Oct
Colobus connection?
  Chimpanzees may be infected from eating their
   prey- 85% of which is red colobus
  1st colobus hunted could have been cause for early
   cases, and 2nd colobus for last 2
  But last 2 infected were in contact with the
   definite case and therefore may be secondary
  If colobus are EBO source and carriers, epidemics
   should occur throughout year/hunting season but
   it does not
Colobus con’t
  Red colobus are widely distributed in Tai
   Forest, 1/3 of monkeys in Park
  Colobus may be intermediate hosts, being
   contaminated at the true reservoir in Oct-Nov
   at end of chimpanzee hunting season
  Red colobus are strictly vegetarian spending
   most of time in canopy and emergent trees
  Their niche might be where EBO virus
   reservoir goes into hiding
  Could be contaminated via virus in food, other
   animals, or contact with small mammal
Fig tree of death?
  Before beginning of outbreak, chimp
   community spent large amounts of
   time in a fruiting fig tree (F. goliath)
  Observers saw many birds in the
   tree during day, and argue that rodents, fruit
   bats, and other species were feeding there at
  This fig tree was a focal point for many species
   and could have allowed transmission of the virus
   between species
the human role

  Habitat has been constantly modified by human
   migration from northern regions of the forest belt
  Process has increased dramatically over last 6
   years since start of Liberian civil war
  Influx of refugees doubled local populations from
   early 1992 to early 1993, then again in 1994
  Causes increased deforestation –crop activities
   and poaching in the park, leading to a large
   farmland and broken forest only 2km from chimp
   home range
results of human interference
  Emergence of infectious diseases are often linked
   to ecologic changes
  Environmental and climatological disturbances
   recorded in Tai could have combined to change
   parts of the EBO reservoir or parts of its behavior
  Outbreak occurred at end of rainy season in Tai,
   which includes larger numbers of small mammals
   and insects which could explain outbreaks at that
further studies
  Studies to investigate the natural EBO virus
  Study would give better understanding of the
   transmission mechanisms of EBO within and
   between species
  More research on the capacity for EBO to leave
   the forest and emerge in human populations
  There is a current EBO outbreak
   among critical chimpanzee and gorilla
   populations throughout Central Africa
  Primatologists say that nearly 2/3 of a gorilla
   population wiped out in Lossi Sanctuary in
   Republic of Congo. From 1,200 gorillas to
   450. Leads to renewed fear for already
   critically endangered species, including
   mountain gorillas
  This massive gorilla and chimp disaster has
   drawn the attention of the WHO & UN
  Some slides quoted from: Formenty, Pierre et al. Ebola
   Virus Outbreak among Wild Chimpanzees Living in a Rain
   Forest of Cote d’Ivoire. Journal of Infectious Diseases.
   Volume 179, February 1999. Pp S120-S126.
  Tsoumou, Christian. Ebola kills 100 in Congo, wipes out

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