The purpose of the sentinel animals was to see if after being placed, they contracted the virus. This process would allow Johnson to try to figure out how they caught it.
The monkeys and guinea pigs were sentinel animals, like canaries in a coal mine: they would be placed in cages inside and near Kitum Cave in the hope that some of them would break with Marburg virus. There are no instruments that can detect a virus. The best way to find a virus in the wild, at the present time, is to place a sentinel animal at the suspected location of the virus and hope the animal gets sick. Johnson figured that if any of his monkeys or guinea pigs crashed, he would be able to isolate the virus from the sick animals and would perhaps be able to discover how the animals had caught it.
At the end of the investigation, none of the animals had become sick. This posed a problem for Johnson because he didn't like killing animals, and the experiment itself, required him to euthanize the animals when it was concluded.
None of the sentinel monkeys became sick. They remained healthy and bored, having sat in their cages in the cave for weeks. The experiment
required that they be sacrificed at the end of the time so that the researchers could take tissue samples and observe their bodies for any signs of infection. At this point, the hard part of primate research began to torment Gene Johnson. He could not being himself to euthanize the monkeys. He couldn't stand the idea of killing them and couldn't go into the cave to finish the job. He waited outside in the forest while another member of the team put on a space suit and went inside and gave the monkeys massive shots of sedative, which put them to sleep forever.
