Rhesus monkey (Image source)
A new study published this week in Nature Microbiology brings good news for research toward the persistence of Ebola virus in asymptomatic individuals.
Ebola virus (EBOV) is the virus that causes Ebola (now referred to as Ebola virus disease, or EVD), a viral hemorrhagic fever that has a very high associated death rate (up to 90%). EVD symptoms include fever, severe headache, muscle ache, weakness, diarrhea, fatigue, vomiting, abdominal pain, and unexplained bleeding. The virus is endemic to central Africa, but outbreaks are rare. Earlier this month, an outbreak of Ebola in the Democratic Republic of Congo was declared over by the World Health Organization (WHO); eight people were known to be infected, and four died. This outbreak is the first since the conclusion of the 2014-2016 multi-country epidemic in West Africa, with total cases numbering more than 28,000, and total deaths in excess of 11,000.
EBOV has been known to persist in the seminal fluid of survivors for up to 18 months after the onset of symptoms, as well in tissues of the eye and brain. Viral transmission has been shown as a result of male-to-female sexual activity by EVD survivors, and has even been shown to cause new cases of EBD. As such, the WHO discourages male EVD survivors from sexual activity for up to 12 months after the onset of symptoms, or until their semen has tested negative twice in consecutive monthly tests.
This new study, by Zeng et al., is performed with rhesus monkeys. Though non-human primates (NHPs) have previously been used as experimental animal models for EVD, the rhesus monkeys used in this study are the first NHPs to display detectable, persisting EBOV genomic RNA in individuals that survived experimental EBOV infection.
The researchers found that a small percentage of these rhesus monkey EBOV survivors had detectable EBOV genomic RNA in their testes (1.32%), brains (1.25%), and most prevalently, their eyes (11.54%). Notably, EBOV genomic RNA was absent in these individuals from tissues known to be affected in acute EVD infections (liver, lymph, spleen). Researchers were also able to prove that in these tissues of these rhesus monkeys, viral replication was still taking place up until the point where the individuals were euthanized.
The 2013-2016 EVD outbreak in Liberia, Guinea, and Sierra Leone brought new information on EVD sequelae, or long-term symptoms from previous infections or illness. It is important to fully comprehend the persistence of EBOV in human survivors, and its impact on sequelae, as well as the morbidity and mortality risks for EVD survivors. It is especially important to fully recognize any risks of post-recovery EVD flare-ups or transmission. For these reasons, the discovery of the rhesus monkey as an animal model will greatly facilitate research into these questions with a living organism.