“You bleed like a hemophiliac who has been in a fistfight. Your skin develops bruises and goes pulpy, and tears easily, and becomes speckled with purple hemorrhages called petechiae, and erupts in a maculopapular rash that has been likened to tapioca pudding. Your intestines may fill up completely with blood. Your eyeballs may also fill with blood. Your eyelids bleed. You vomit a black fluid. In the pre-agonal stage of the disease (the endgame) the patient leaks blood containing huge quantities of virus from the nose, mouth, anus, and eyes, and from rips in the skin. In the agonal stage, death comes from hemorrhage and shock”—Richard Preston’s “Crisis in the Hot Zone” (1992).

Several different strains of the Ebola hemorrhagic virus have emerged since the disease was first discovered in 1976. The Centers for Disease Control and Prevention (CDC) estimates the average case fatality rate falls somewhere around 50 percent, or 12,911 people, but several outbreaks have been even deadlier. In 2003, Ebola Zaire killed nine of every 10 infected people, totaling 128, who fell ill in the Republic of the Congo before the disease was contained.

The prognosis is not much better for other species affected by the virus. According to conservationists, Ebola has killed far more chimpanzees and gorillas than humans. Between 77 and 95 percent of all great apes that have contracted the disease in the wild have died from it. Wildlife experts have argued this amounts to a total loss of one-third of the animals’ population size. Ebola has now outpaced habitat encroachment and poaching as the greatest threat to the survival of chimpanzees and gorillas—crucially endangering both animals. World Wildlife Fund estimates of chimp populations range from 172,700 to 299,700; for gorillas, the organization estimates only between 100,000 and 200,000 remain in the wild.

Meanwhile, Ebola remains a potent danger not only to human populations in equatorial Africa, but also to people around the globe. In our hyperconnected world with more than 44,000 international airports, even extreme travel restrictions will not make the situation safer. In fact, public health experts warn shuttering flights to and from Ebola-stricken countries would likely make a pandemic even worse. Cutting off the supply of aid workers to affected regions would make the virus more difficult to contain.

In their search for a cure, researchers have turned to the great apes for answers. They hope studying chimpanzees, gorillas, and other non-human primate species will unlock new insights in how best to vaccinate people and animals against the Ebola virus.

The discovery of several potential vaccines—each in various stages of preclinical or clinical testing—has restored hope for both wildlife conservation and public health. New drug formulations and inoculations against Ebola could save primates from the brink of extinction while sparing both animals and humans from dying in large numbers.

In their search for a vaccine, researchers are motivated to find preventative treatments that could benefit all species—but that’s not the complete story. Studying Ebola in monkeys is important because the route of transmission to humans is often traced to the consumption of infected monkey meat and bats (“bushmeat”). And because the virus is transmitted via bodily fluids including blood, both healthcare workers working with infected people and those involved in the preparation of wild animals for food are especially at risk.

Additionally, the inclusion of primates in Ebola research, particularly rhesus macaque monkeys, has yielded important conclusions about the persistence of Ebola in reproductive organs. The reproductive systems of humans and non-human primates are much more alike than they are different. So, the discovery in 2017 that detectable amounts of the virus persisted in the eyes, brains, and testes of some laboratory macaques (after their infections abated) has led researchers to conclude human survivors may risk transmitting the Ebola virus sexually to their partners.

Because of the biological similarities between humans and monkeys, Ebola develops and progresses similarly in both species. This information has facilitated the development of vaccines that could, potentially, be administered to chimpanzees and gorillas in the wild—as well as to human beings in the clinic.

As one can imagine, the prospect of capturing a critical mass of wild primates to inject them with a preventative vaccine is rather daunting. Even more so, considering that three doses are needed to produce broad immunity. But in 2017, researchers tested a formulation that can be effective after a single vaccine dose—and it can be administered via edible bait that’s left for the animals. Additional studies are necessary to determine the effects of heat and exposure on the vaccine’s effectiveness.

A year earlier, a single-dose vaccine became the first to earn FDA approval for administration to healthcare workers in the field. It protects against Ebola Zaire, the strain that’s been up to 90 percent fatal in past outbreaks. By immunizing people who are likely to have been exposed to patients with Ebola, the study was designed to mimic smallpox containment strategies—keeping a “ring” around the virus to prevent it from spreading.

And in Feb. 2018, researchers announced their whole-virus Ebola vaccine, which has effectively protected laboratory monkeys against all known strains, has been approved for a phase 1 clinical trial in Japan. This remarkable progress that has been made in the search for a vaccine was made possible by studies with laboratory primates that may have saved the lives of countless people and endangered animals.

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