Detection of a deadly virus in cow’s milk
Earlier this year, two Stanford researchers and their colleagues in Kenya made a startling discovery: A sample of fresh milk, brought to an urban center to be sold, harbored the deadly virus that causes Rift Valley fever.
Rift Valley fever, a disease that causes severe symptoms in certain types of livestock, is one of the World Health Organization’s nine priority diseases so designated for its potent pathogenic abilities – in animals and in humans. While most people who contract the disease have only mild illness, about 10% die from it or develop severe symptoms, including hemorrhagic fever, eye disease and brain inflammation.
People can contract the disease after handling diseased livestock such as goats, sheep or cattle, or after being bitten by a virus-infected mosquito, said pediatric infectious disease expert Desiree LaBeaud, who with Keli Gerken, DVM, a postdoctoral fellow at Stanford, led the study that found the virus in the milk of cattle.
Although the study does not confirm that people can contract the virus through milk, scientists have long suspected that drinking raw milk puts people at risk for the disease. Rift Valley fever is generally thought to pose the greatest risk to people living in rural areas, especially those who work with livestock.
However, a recent study by Gerken, LaBeaud and other colleagues showed that a small percentage of people in two large urban centers in Kenya had antibodies against the virus that causes Rift Valley fever, suggesting that foods of animal origin could transmit the virus.
The team’s discovery of the virus in the milk of cattle confirms this possibility. It could also help health officials shape public health policy, providing an opportunity to improve early detection of the virus through more consistent monitoring of animal milk.
“This is huge news for Kenya as Rift Valley fever has huge economic and public health consequences,” said LaBeaud, who has worked extensively in the fight against diseases transmitted by insects in Kenya.
In 2021, Gerken, a trained veterinarian, and LaBeaud developed a viral testing program with seed grant funding from Stanford Global Health, collecting and testing blood samples from animals at slaughterhouses in Kenya.
Gerken and his collaborators at the Kenya Institute of Medical Research in Kisumu trained slaughterhouse workers to take blood samples, then created animal origin maps to better track the geographical roots of the virus. Although this system has not yet detected the virus in animal blood samples, it still works in Kenya to flag the presence of the virus, and this was the basis of the new milk screening system.
In fall 2021, the team partnered with milk vendors to test milk imported from rural areas and sold in urban markets. For a time, no milk samples appeared to harbor the virus, but in April 2022, Gerken received a text message from Christabel Winter, the director of the lab in Kenya where these milk samples were being tested. One sample came back positive.
“I remember an instant feeling of validation,” Gerken said. While she and LaBeaud had suspected the virus could be found in milk, this was cold, hard evidence that it indeed could, and it underscored the value of their partnership with local milk vendors. “I’m really glad they agreed to work with us,” she said.
They have now found evidence of the virus in milk twice.
The finding shows that people in urban areas are at higher risk of contracting Rift Valley fever than previously thought – and it underscores the need for greater surveillance, said Bryson Ndenga of the Institute. Kenya Medical Research Center in Kisumu, a longtime LaBeaud and Gerken collaborator. .
Gerken said she hopes the discovery can help the Kenyan government develop more systemic preventative measures to stop the spread of the virus.
“There will always be someone upstream of the urban markets, milking the animal, who will be at risk,” Gerken said. “The more cases we find, the more we need to understand why outbreaks spread and advocate preventive measures to protect people who handle animals.”
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