The Reston, Virginia Outbreak
On October 2, 1989, 100 cynomolgus macaques (Macaca fascicularis) from Ferlite Farms in Mindanao Island, Philippines were flown from Manila, through Amsterdam to New York, and then transported by truck to Hazleton Research Products' (HRP) Reston Primate Quarantine Unit in Reston, Virginia. These monkeys were placed in Room F of the Reston Unit on October 4. HRP's Reston Unit already had approximately 500 cynomolgus monkeys when this shipment arrived. There had not been any African species quarantined in the Reston unit for many years, ergo it is not possibile that the monkeys contracted Ebola from fomites contaminated by a prior shipment of monkeys. Because of the 1976 Marburg incident, all primates imported into the United States must be quarantined for 30 days to insure that they are disease free before they are released. In any transcontinental shipment of animals, a high attrition rate is to be expected due to this experience. However, this particular shipment of nonhuman primates had a far larger number of deaths in Room F than would normally have been expected. The HRP veterinarian conducted a few necropsies of the dead monkeys from this shipment in Room F and, based on the clinical symptomatology and on gross anatomy, made an initial diagnosis of simian hemorrhagic fever (SHF). SHF is a terrible disease in monkeys (fortunately it does not infect humans) and is easily transmitted amongst them. The HRP vet sent samples of the dead monkey tissue to United States Medical Research Institute of Infectious Diseases (USAMRIID) for conclusive diagnosis. SHF was isolated in the tissue cultures that HRP sent to USAMRIID. Before USAMRIID finished their diagnosis, HRP made the decision to euthanize all of the remaining monkeys in Room F to prevent possible further spread. During the 10 days following the euthanization of the monkeys in Room F, there were sporadic deaths in the remaining monkey population at Reston. The pattern of the deaths nor the pathology in the dead monkeys was indicative of SHF. The HRP vet became alarmed by this. Meanwhile, USAMRIID was conducting additional tests on the monkey tissue cultures and discovered that Ebola was also responsible for the Reston monkey deaths from an electron micrograph of damaged tissue from one of the dead Reston monkeys. Unfortunately, the pathogen was not contained by the euthanization of the monkeys in Room F. 29 additional monkeys in Room H had also died. The monkeys in Room H were from a separate shipment (but from the same supplier, Ferlite Farms) that had arrived at the Reston Unit on November 8.
Were the Room H monkeys contracting the pathogen from the Reston Quarantine Unit or were they infected with the pathogen back in the Philippines? The Room F monkeys were euthanized on November 16. The Room H shipment of monkeys arrived on November 8. Both the Room F and Room H cynomolgus monkey shipments came from Ferlite Farms in the Philippines. Ferlite Farms was experiencing a hemorrhagic disease outbreak concurrently. It is likely that the Room H monkeys were sub-clinically harboring EBO on arrival. EBO has an incubation period ranging from five to seven days in nonhuman primates (personal correspondence, Anderson). The Room H monkeys arrived at the Reston unit while the Room F monkeys were still alive.
On November 29, the Center for Disease Control (CDC) and the Virginia Department of Health met with USAMRIID, and a coherent plan of action was formulated to insure the safety of the community and the humane treatment of the Reston primates. Because of the threat that Ebola might spread to the remaining animals in the quarantine unit and that it might infect the staff, the remaining animals (~500) in Room H were euthanized on November 30, 1989. On November 28, 1989, Ferlite Farms, unknowingly, sent a shipment of EBO-infected cynomolgus monkeys to Philadelphia.
Six of the 178 people who had contact with the infected monkeys at the Reston Quarantine Unit seroconverted. All six of the individuals worked with the primates. None of the six who seroconverted developed a filovirus-related illness. Of them, four (all of whom were animal handlers at one quarantine facility) had serologic evidence of recent infection with Ebola-Reston. It is likely that one of the four infected himself when he cut his finger while performing a necropsy on an infected monkey. The mode of transmission for the other three handlers is not known. The remaining two people were seropositive at low titer and had evidence of past infection. One of these two people is a worker at a facility that temporarily houses nonhuman primates before delivery to U.S. quarantine facilities and had had regular contact with quarantined nonhuman primates for three years. The second person was an employee at Hazleton's Texas Primate Center.
CDC researchers conducted an additional study on the prevalence of seropositivity to filoviruses. They tested 550 people with varying levels of exposure to monkeys (or monkey tissues or body fluids) with an indirect immunofluorescence assay test (IFAT) and confirmed the results using Western blot. 42 of the 550 (7.6%) people tested were positive to at least one of the filoviruses (EBO-Z, EBO-S, EBO-R, EBO-CI, MBG). 26 of 266 (9.8%) import quarantine facility staff members were seropositive; 16 of the remaining 284 (5.6%) persons having contact with monkeys (or with monkey tissue or body fluids) outside of import quarantine facilities were seropositive. None of the 42 who tested seropositive reported having any illness believed to be caused by a filovirus.
In order to assess the significance of the prevalence of the seropositivity in people who have contact with monkeys and their bodily fluids/tissues, the CDC conducted a cross-sectional study on the seroprevalence of filovirus on the serum from 449 adults in outpatient primary care facilities throughout the United States. Using the same IFAT and Western blot assays, of theses 449, 12 were positive (2.7%) (Morbidity and Mortality Weekly Report 39(24): 404-05, 1990).