By Stan Deresinski, MD, FACP, FIDSA

Clinical Professor of Medicine, Stanford University

Dr. Deresinski reports no financial relationships relevant to this field of study.

SYNOPSIS: Ebola makes a comeback, but meets a vaccine. Lassa fever and Rift Valley fever also make their mark, while Keystone virus infects a teenager in Florida.

SOURCE: International Society for Infectious Diseases. ProMED Mail. Available at: Accessed July 8, 2018.

Lassa Fever

Between Jan. 1, 2018, and June 10, 2018, 1,999 suspected cases of Lassa fever were reported from 22 Nigerian states and 437 of these cases were laboratory-confirmed. The case fatality among confirmed cases was 24.9%. Thirty-eight healthcare workers have been infected. Among the 5,328 identified contacts who have completed 21 days of follow-up, 84 were symptomatic and 29 have been confirmed. The outbreak has been diminishing for a number of weeks.

Lassa fever is a viral hemorrhagic illness that is caused by Lassa virus, which is an arenavirus present in West Africa, particularly in the countries of Benin, Ghana, Guinea, Liberia, Mali, and Sierra Leone, in addition to Nigeria. Humans typically can become infected via exposure to aerosol or through contact with droppings of infected multimammate rats; contact may be direct or via food or items that have been contaminated with excreta from rodents. In addition, person-to-person transmission occurs in healthcare settings from exposure to body fluids of infected patients.

Approximately four-fifths of patients are asymptomatic or have quite mild symptoms. The incubation period is 2-21 days in symptomatic patients with progression to hemorrhagic fever in a small proportion. Although the overall case fatality rate is approximately 1%, it is 15% in those who require hospitalization. Ribavirin administration beginning within six days of fever onset generally is recommended, but this is based on limited and low-quality evidence.

Rift Valley Fever

In Kenya, a Rift Valley fever outbreak started with recognition of the index case identified on May 11, 2018. As of June 20, 90 cases had been reported. This was accompanied by an epizootic that also is occurring in Rwanda, which, along with Uganda, has suspected human cases.

Rift Valley fever virus is a Phlebovirus (family Bunyaviridae) that is primarily a disease of livestock in Africa, with frequent transmission into human populations. Although originally apparently confined to the Rift Valley of eastern Africa, it now is present in much of Africa, including Madagascar, as well as the Arabian peninsula. Humans acquire the infection either through contact with infected animals or through mosquito bites. Most human infections are asymptomatic, but approximately 10% of patients with symptoms develop uveitis, retinitis, and/or retinal hemorrhage. The illness may progress to hemorrhagic fever.


Ebola reemerged on April 1, 2018, this time in the Equateur province in the Democratic Republic of Congo, with, as of July 3, 53 cases (38 laboratory-confirmed and 15 probable cases). The 29 deaths represent a case fatality rate of 55%.

This outbreak provided an opportunity for further testing of a recombinant vaccine, rVSV-ZEBOV, consisting of a vesicular stomatitis virus expressing Ebola surface glycoprotein. A ring strategy was employed with vaccine administration to case contacts. The vaccine also was administered to healthcare and other frontline workers in the affected areas, as well as in areas to which spread of the outbreak was of concern.

Keystone Virus

Keystone virus, a California serogroup Orthobunyavirus, was detected by non-biased sequencing and cell culture from a 16-year-old male in Florida with fever and a non-pruritic erythematous papular eruption that had begun on his chest and spread to abdomen, arm, back, and face.1 The infection resolved within a few days. This was the first reported detection of the Keystone virus in humans, although older serological studies suggested prior exposure to it. The virus infects a variety of small vertebrates as well as mosquitoes. The findings reported here are highly suggestive of the virus causing symptomatic infection, but they do not prove it.

These represent just a small fraction of recent viral activity. Middle East Respiratory Syndrome continues to cause disease in the Middle East, Nipah has affected several individuals in the Kerala state of India, a new virus (Ntwetwe) was detected in cerebrospinal fluid of a child in Uganda with encephalitis, and more.


  1. Lednicky JA, White SK, Stephenson CJ, et al. Keystone virus isolated from a Florida teenager with rash and subjective fever: Another endemic arbovirus in the southeastern United States? Clin Infect Dis 2018; doi: 10.1093/cid/ciy485. [Epub ahead of print].