The 1990 Reston Ebola virus outbreak was a second wave of infection in the monkey house near Washington, D.C. After the first outbreak, the facility had been certified as decontaminated and returned to Hazleton Research Products. When infected monkeys began dying there again in January, the recurrence showed that the earlier crisis had not ended the possibility of Ebola Reston returning to the building.

Recurrence in the Monkey House

Hazleton restocked the facility with crab-eating macaques trapped in the rain forests of Mindanao in the Philippines. Less than a month later, monkeys in Room C began dying, and Dan Dalgard alerted Peter Jahrling that the facility appeared to be affected again. The virus was identified as Ebola, and the account links this outbreak to the newly imported animals.

Because the first outbreak had caused no human casualties, the Army, the Centers for Disease Control and Prevention, and Hazleton jointly chose to isolate the monkeys and allow the infection to run its course. Dan Dalgard hoped that some animals might survive, while Hazleton wanted to avoid another Army intervention in protective suits. Their decision also turned the outbreak into an observation of what Ebola could do in a confined monkey population: rather than immediately remove the animals, they would watch the disease spread among them.

A Changing Disease Pattern

In the confined monkey population, Ebola Reston spread rapidly from room to room and appeared to present differently from the disease seen during the previous outbreak. Infected animals developed substantial nasal discharge, sometimes with blood, and pneumonia; their lungs were badly affected. When an animal with a nosebleed appeared in a room, about 80 percent of the monkeys there generally died soon afterward. The scale and speed of the losses made the outbreak a stark demonstration of how efficiently the virus could move through a densely housed population.

The pattern raised the possibility that the virus had changed. Institute scientists suspected that they were seeing a mutant strain, distinct in some way from the virus encountered the previous December. Its coldlike symptoms suggested that it might be adapting to a new host and changing its effects, but the account presents this as a concern and hypothesis, not a confirmed explanation. The virus’s apparent ability to alter its character over a short interval deepened uncertainty about what form it might take in a later outbreak.

The spread also exposed the importance of the building’s shared air. By January 24, infection had reached Room B, and in the following weeks it entered Rooms I, F, E, and D. The account describes Ebola moving through the building’s air-handling ducts, linking separate rooms into a single outbreak. What had been intended as an opportunity to observe infection in a confined monkey population thus became evidence of how the facility’s layout and airflow could carry the disease beyond its first affected room.

Human Exposure and Uncertain Transmission

In mid-February, a Hazleton caretaker identified as John Coleus cut his thumb with a scalpel while examining a dead monkey. The blade had been used to cut into the animal’s liver, a tissue in which Ebola commonly accumulated. A sample of the liver was sent to the United States Army Medical Research Institute of Infectious Diseases, where testing showed that it contained a very high concentration of virus. Researchers feared that Coleus had received a dangerous exposure. The Centers for Disease Control and Prevention did not place him in isolation, and he continued his usual activities, including visiting bars with friends. Coleus also underwent surgery while still within the incubation period. The account records no excessive bleeding during the procedure; he recovered and suffered no lasting effects from the exposure. His survival did not resolve the question of how the other caretakers had encountered the virus. All four men who worked in the monkey house eventually tested positive for Ebola Reston, although only Coleus was known to have cut himself. The virus had entered and multiplied in each man’s body, but none developed recognized illness, and it later cleared from their blood. The contrast between infected monkeys and symptomless workers became one of the outbreak’s most consequential findings. Since three of the four men had no known cut, researchers considered infection through the lungs the likeliest explanation, raising the possibility that the virus had spread through the air. That remained an inference rather than a fully established account of each man’s exposure. The episode nevertheless sharpened concern that a virus producing severe disease in monkeys could enter people without causing visible illness, leaving both its route of transmission and its effects in humans uncertain.

Significance and Unresolved Risk

The outbreak’s aftermath resisted a simple verdict of either safety or catastrophe. Ebola Reston had devastated the monkey population, yet the four infected caretakers remained without recognized symptoms; the text leaves unexplained why the virus behaved so differently in people. It proposes that a very small genetic change, perhaps altering one of the virus particle’s seven proteins, might have transformed its effects in humans. That explanation remains conjectural, but it frames the contrast as a biological uncertainty rather than proof that the strain was harmless.

Philip Russell later understood the episode as more frightening in retrospect: the evidence from the monkeys suggested that small changes might enable rapid respiratory transmission among humans. The possibility was not an outcome of the 1990 outbreak, but a warning about what the virus might become. The episode therefore ended without human illness while leaving its central question open: whether the difference between a devastating monkey disease and a mild or symptomless human infection could persist if the virus changed.