First recognized Ebola outbreaks

The index case at Yambuku, in the Bumba Zone of northern Zaire, fell ill on 1 September 1976, five days after an injection of chloroquine for presumed malaria at the outpatient clinic of the Yambuku Mission Hospital. Within a week others who had received injections there were sick. Almost every later patient had either been injected at the hospital or had close contact with a case. Women aged 15 to 29 had the highest incidence, which investigators tied to injections at the prenatal and outpatient clinics. After about four weeks the hospital closed; 11 of its 17 staff had died.

Between 1 September and 24 October there were 318 cases and 280 deaths, with only 38 serologically confirmed survivors. Most cases lay within 70 km of Yambuku, though a few patients sought care in Bumba, Abumombazi and Kinshasa, where some secondary and tertiary cases followed. House-to-house searches of about 550 villages found cases in 55. The illness usually lasted about a week: vague symptoms, then worsening sore throat, rash, intractable abdominal pain and bleeding from many sites, mainly the gut.

The Sudanese outbreak began earlier, in June, and ran to November. Of its 284 cases, 67 were in Nzara, where it seems to have started among workers at a cotton factory, and 213 in Maridi, where a large, busy hospital amplified it. Spread required close contact with an acute case and usually came from nursing a patient. The death rate was 53 percent. In the factory's cloth room, 37 percent of workers appeared to have been infected.

Laboratories isolated the agent from the blood of 8 of 10 suspected cases in Zaire, using Vero cells. It resembled Marburg virus in shape but was immunologically distinct, and it was named Ebola virus, after the Ebola River near the affected village. Because the trip from Nzara to Bumba took four days or less, the Zaire investigators thought it quite possible that a traveler from Sudan had passed the virus to a hospital needle. WHO now counts the two as separate diseases, caused by Sudan virus in Nzara and Ebola virus in Yambuku. Licensed vaccines and treatments exist only for Ebola virus disease.