Captive Primates

On at least six different occasions since its discovery in 1989, REBOV was isolated from cynomolgus macaques (Macaca fascicularis) showing signs of viral hemorrhagic fever.¹⁵ The macaques had been imported to America and Europe from a single breeding facility in the Philippines.¹⁰ In one case, 82% of animals died and many were found to be coinfected with simian hemorrhagic fever virus. The significance of the coinfection is unknown. In almost all cases, affected macaques originated from a single breeding facility in Laguna Province, the Philippines. In 1996, an investigation at that facility reported a 14% mortality rate, with viral antigen detected in 32% of symptomatic and 4% of asymptomatic monkeys.³ Three of 301 cynomolgus macaques were positive for IgG antibodies.⁷ The source of infection for the macaques was not determined, but poor husbandry practices contributed to disease dissemination. Many macaques in both U.S. and Philippine centers were subsequently destroyed, and in 1997 the Laguna facility was closed. There have been no further REBOV cases reported in primates imported to the United States since that time.

Free-Ranging Primates

Chimpanzees (Pan troglodytes) and western lowland gorillas (Gorilla gorilla gorilla) appear to be dead-end hosts for ZEBOV infection. ZEBOV antigen was detected in 16 chimpanzee and gorilla carcasses discovered during epizootics associated with large great ape declines in central Africa.^19,25 Based on temporal and spatial links between large-scale great ape mortality and confirmed ZEBOV epidemics and epizootics, case-fatality rates in great apes have been estimated at roughly 90%.²⁴ The detection of Ebolavirus-specific IgG antibodies in 31 western lowland gorillas and chimpanzees suggests that they may survive infection or may be asymptomatically infected, or that assays are cross-reacting with an as yet unidentified, less virulent, strain of EBOV.^4,14

Although the vastness and remoteness of the central African habitat makes precise great ape mortality impossible to determine, the impact of EHF on great ape populations appears dramatic.^17,24 Some field researchers have observed rapid and astonishingly high mortality in resident ape populations. Large-scale ecologic surveys carried out over the past decade have indicated dramatic declines (up to 95%) in great ape populations in some regions. In all cases, these declines were spatially and/or temporally linked with human or animal ZEBOV outbreaks, in which hunting and habitat loss were ruled out as contributing factors.¹⁰ Given the context, it is reasonable to assume that the great ape declines were associated with EHF. The western lowland gorilla was reclassified as “critically endangered” by the International Union for the Conservation of Nature (IUCN), largely as a result of the threat of EHF.

Antibodies to ZEBOV were detected in eight wild-born monkeys of four species in Cameroon, Gabon, and the Republic of Congo, indicating virus circulation in nonhuman primates.¹⁴ Monkey morbidity and mortality have been spatially or temporally linked with human ZEBOV, although tests were negative for ZEBOV.^10,25

CIEBOV was associated with the deaths of 12 chimpanzees in the Taï forest of Côte d’Ivoire in 1994.¹⁰ One chimpanzee was confirmed CIEBOV positive on immunohistochemistry (IHC) analysis, suggesting that chimpanzees are dead-end hosts.²⁶ Antibodies to CIEBOV were detected in a red colobus monkey (Procolobus badius) following the outbreak. All affected chimps were observed consuming the monkey 6 days prior to the outbreak, suggesting the monkey as the source of infection of the chimpanzees.

Wildlife morbidity and mortality associated with SEBOV and Bundibugyo ebolavirus have not been reported.

Other Potential Hosts

Relatively little is known about EBOV susceptibility of other species that may serve as reservoir, incidental or dead-end hosts. Positive serologic findings in central African domestic hunting dogs have suggested that canines may be naturally and asymptomatically infected.³ Pigs (Potamochoerus porcus), duikers (various species), porcupines (Atherurus africanus), a civet (Civettictis civetta), sitatungas (Tragelaphus spekei), genets (Genetta spp.), an elephant (Loxodonta. africana), pythons (Python sebae), antelopes, rodents, a pangolin (Manis spp.), a mongoose (unspecified species), and a raptor have been reported dead in temporal or spatial coincidence with confirmed ZEBOV outbreaks in humans or nonhuman primates.¹⁰ Of the species that were tested for ZEBOV by polymerase chain reaction (PCR) assay, only a blue duiker (Cephalophus monticola) tested positive.¹⁹ A 50% decline in duiker populations was reported, coincident with ZEBOV outbreaks in humans and great apes in the Republic of Congo between 2000 and 2003, suggesting that duikers may be dead-end hosts for ZEBOV. Unconfirmed reports have also suggested the presence of EBOV in rodents (Mastomys, Mus, and Praomys spp.). In the Philippines, REBOV was recently isolated from domestic swine exhibiting a severe respiratory syndrome that were coinfected with porcine reproductive and respiratory syndrome virus.³ Given this lack of strong evidence, additional studies on potential Ebola virus hosts are needed.