Allowing the 1990 Reston monkey outbreak to run its course

Allowing the 1990 Reston monkey outbreak to run its course was a joint decision by the United States Army, the Centers for Disease Control and Prevention, and Hazleton Research Products to leave infected monkeys isolated in their rooms rather than repeat the previous eradication operation. The choice made the monkey house an unintended natural experiment: Ebola Reston virus moved through a dense animal population, while its effects on exposed people remained strikingly different from its effects on the monkeys.

Background and decision

In December 1989, the Army had intervened in the Reston monkey house, and the building was returned to Hazleton Research Products after the Army certified it had been cleared. Hazleton then resumed importing crab-eating macaques from the same supplier near Manila, animals trapped in the rain forests of Mindanao. In mid-January 1990, less than a month after the restocking, monkeys in Room C began dying with nosebleeds. Dan Dalgard alerted Peter Jahrling, and the cause was again identified as Ebola Reston virus.

The first outbreak had produced no human casualties. In light of that experience, the Army, the Centers for Disease Control and Prevention, and Hazleton jointly chose to isolate the affected monkeys and let the outbreak subside without sending the Army back into the building in protective suits. Dalgard hoped that some of the monkeys might survive; Hazleton also wanted to avoid another military intervention. The decision was not simply to ignore the outbreak: the monkeys were to be left alone within the facility while the infection’s course became observable.

Spread through the monkey house

The virus moved rapidly from room to room in the confined building. By January 24, it had reached Room B; over the following weeks it appeared in Rooms I, F, E, and D. The account links its movement to the building’s air-handling ducts, making the facility’s shared airflow a route by which infection could pass between separated groups of monkeys.

The illness appeared to change in character as it spread. In addition to nosebleeds, infected monkeys developed cold-like respiratory symptoms, with abundant mucus and severe lung disease. The Institute’s scientists suspected that the virus might have altered from the strain seen in December. In rooms where a monkey developed a nosebleed, about 80 percent of the animals generally died shortly afterward. The pattern suggested both extraordinary infectivity among monkeys and the possibility that Ebola could adapt quickly in a new host population; it did not establish exactly how or why the apparent change occurred.

Human infection and the outbreak’s end

In mid-February, caretaker John Coleus cut his thumb with a scalpel while examining a dead monkey’s liver, exposing himself to infected tissue. Tom Geisbert examined a piece of the liver at the United States Army Medical Research Institute of Infectious Diseases and found it heavily infected. Coleus was considered at serious risk, but he survived and later had no known lasting effects. He was not the only infected caretaker: a total of four men who had worked in the monkey house tested positive for Ebola Reston virus.

The four men did not become ill, and the virus eventually cleared from their blood. Coleus’s cut offered a direct explanation for his exposure; three of the other caretakers had not cut themselves. Researchers therefore concluded that the virus had probably entered them through contact involving the lungs, and the infections strengthened the evidence that Ebola Reston virus could spread through the air among monkeys. The precise route into each man was not known. The contrast was consequential: infection was demonstrable in people, but the severe disease seen in the monkeys did not occur in these caretakers.

By the time the outbreak had run its course, the entire monkey population in the building had died. The Army did not need to repeat its earlier clearance operation; the virus had eliminated the monkey population itself. The event thus ended without reported human illness, but not without a serious occupational-exposure concern. The caretakers’ symptomless infections left unresolved why the virus multiplied in their bodies without producing disease.

Significance and wider context

The decision exposed, at a cost to the animals, what a rapidly spreading infection could do within a confined population. The monkey house functioned like a city of separate rooms connected by shared air: once the virus gained a foothold, it crossed those boundaries and repeatedly caused heavy losses. At the same time, the four positive tests among caretakers showed that the boundary between animal outbreak and human exposure was not secure, even though the human outcome in this instance differed sharply from the animal one.

The book places this episode within a wider account of emerging viruses and damaged tropical ecosystems, while leaving the natural reservoir and the precise route by which Ebola Reston entered the monkey supply unresolved. It describes the monkeys as trapped in Mindanao’s rain forests and brought into a dense holding facility, conditions that put animals of a shared population into close contact; it does not prove that a particular environmental change caused this outbreak. Against earlier Ebola outbreaks in Sudan and Zaire, where transmission among people was associated with blood contact, contaminated needles, and chains of human contact, the Reston episode foregrounds rapid spread among monkeys through a building’s air system. Its ending therefore carries two facts together: the outbreak exhausted itself in the monkey house, and the virus infected people there without making the known caretakers sick.