Now a research team led by a biomedical scientist at the University of California, Riverside has determined that the yellow fever virus, a hemorrhagic fever virus, replicates primarily in the liver. Therefore, other organ failures that often follow in people with the disease are due to secondary effects.
When the virus targets the liver, it replicates rapidly causing significant damage to liver cells. In the process, inflammatory cytokines - proteins secreted by cells especially of the immune system - are made in massive amounts, which soon gain access to the blood stream. These cytokines are most likely responsible for the damage to distant organs, the research team's findings suggest.
The research team also identified a clinical parameter that could greatly help in managing yellow fever cases.
"Yellow fever causes severe loss of lymphocytes," said Ilhem Messaoudi, an associate professor of biomedical sciences in the UC Riverside School of Medicine, who led the research project. "This process, called lymphopenia, occurs before any measurable changes in liver enzymes can be detected - that is, about a day or so before we see changes in the liver. It could provide an earlier clinical outcome measure of subsequent disease severity, giving doctors a good prognostic tool for offering more aggressive supportive care for these patients."
Study results appear today in PLOS Neglected Tropical Diseases.
The research, performed on rhesus macaques (currently, the best model for studying human yellow fever infection) at Oregon National Primate Research Center, is the first study on yellow fever in non-human primates in more than 20 years.
"Yellow fever is truly a neglected tropical disease," Messaoudi said. "Even though it continues to cause fatality, it remains understudied. While it is true there is a highly effective vaccine, it remains extremely challenging to get comprehensive vaccine coverage in sub-Saharan Africa and Latin America. Moreover, the vaccine works well if you are between one and 55 years old. It is not safe for babies or the elderly, who could develop yellow fever from the vaccine."
According to Messaoudi, a good understanding of the pathophysiology of yellow fever in a robust animal model gives researchers also a preclinical animal model for testing new inactivated vaccines, which would be much safer for infants and the elderly.
She explained that yellow fever is characterized by two phases. During the acute phase, the patient has fever and myalgia - not exactly disease-specific symptoms. These symptoms last about 48 hours. While most people are able to resolve them, some enter remission in which the fever disappears for about 24 hours only to be followed by the toxic phase in which viral loads greatly increase. Yellowing of the skin and the whites of the eyes (which is what gives the disease its name) is a consequence of liver failure.
"About 30 percent of unvaccinated people who contract the disease go through the toxic phase, which can be fatal," Messaoudi said.
Because they were interested in identifying which genes in white blood cells were turned on by yellow fever in the animal model, the researchers performed an analysis of gene expression early during the infection - prior to any clinical symptoms, indeed before any indication that the infected animal was sick. They compared changes in gene expression due to the wild type virus infection to changes induced by vaccination, making theirs the first study to examine changes in gene expression induced by wild type yellow fever infection. They found that by Day 3, yellow fever induced changes in the expression profiles of nearly 800 genes.
"This is a large amount of changes in gene expression," Messaoudi said. "In just 72 hours after infection, half of the genes were up-regulated, or turned on, while the rest were down-regulated. Also, about two-thirds of genes related to some function of the immune system were suppressed by the infection. Those that were up-regulated were highly pro-inflammatory cytokines, which likely cause organ damage."
Messaoudi explained that vaccination of the animals induced expression of 46 genes, most of which were up-regulated and in the pathways of antiviral immunity. In other words, the vaccine was inducing the up-regulation of many genes that were good for making antibodies for protection later on, in contrast to the wild type virus that was inducing changes that were suppressing immunity and causing cell death.
"The question is: Can we use gene expression as a diagnostic in the clinic?" Messaoudi said. "Could we run a quick analysis on patients' white blood cells and determine which infected person is at high or low risk? Supportive care would follow for all patients, but it would change the dynamics of how aggressive the treatment needs to be."
Yellow fever does not infect any species besides primates. Outbreaks in primates have led to outbreaks in humans. The virus is endemic or intermittently epidemic in 45 countries (32 in Africa and 13 in South America). Yellow fever is a very rare cause of illness in U.S. travelers. According to the Center for Disease Control, "there is no specific treatment for yellow fever; care is based on symptoms. Steps to prevent yellow fever virus infection include using insect repellent, wearing protective clothing, and getting vaccinated."
Next, Messaoudi and her colleagues plan to study gene expression for animals that survive infection. They also plan to identify viral proteins responsible for suppressing the immune system, which could lead to antiviral therapies.
INFORMATION:
The research was supported by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health. Messaoudi was joined in the study by Flora Engelmann (first author of the research paper) and Thomas Girke at UC Riverside; Laurence Josset at Hospices Civils de Lyon, France; Byung Park and Alex Barron at Oregon Health and Science University; and Jesse Dewane, Erika Hammarlund, Anne Lewis, Michael K. Axthelm and Mark K. Slifka at Oregon National Primate Research Center.
All rhesus macaques used in the study were handled in strict accordance with the recommendations described in the Guide for the Care and Use of Laboratory Animals of the National Institute of Health, the Office of Animal Welfare and the United States Department of Agriculture. All animal work was approved by the Oregon National Primate Research Center Institutional Animal Care and Use Committee.
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