Earlier this year, two Stanford researchers and their colleagues in Kenya made a surprising discovery: A sample of fresh milk, brought to an urban center for sale, harbored the deadly virus that causes Rift Valley fever.
Rift Valley fever, a disease that causes severe symptoms in some types of livestock, is one of the World Health Organization's nine priority diseases so designated for its potent disease-causing capabilities -- in animals and in humans. While most people who contract the disease experience only mild illness, about 10% either die from it or they develop severe symptoms, including hemorrhagic fever, eye disease and brain inflammation.
People can acquire the disease after handling sick livestock such as goats, sheep or cattle, or after a mosquito infected with the virus bites them, said pediatric infectious disease expert Desiree LaBeaud, MD, who, along with Keli Gerken, DVM, a postdoctoral scholar at Stanford, led the study that discovered the virus in livestock milk.
Although the study doesn't confirm that people can contract the virus through milk, scientists have long suspected that consuming raw milk exposes people to the disease. Rift Valley fever is generally thought to pose the greatest risk to people living in rural areas, particularly those working 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 for the virus that causes Rift Valley fever, suggesting that animal-sourced foods could transmit the virus.
The team's discovery of the virus in livestock milk confirms that possibility. It could also help health officials develop 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, because Rift Valley fever has enormous economic and public health consequences," said LaBeaud, who has worked extensively combatting insect-borne illness in Kenya.
In 2021, Gerken, a veterinarian by training, and LaBeaud developed a viral screening program with Stanford Global Health seed grant funding, collecting and testing blood samples from animals at slaughterhouses in Kenya.
Gerken and collaborators at the Kenya Medical Research Institute in Kisumu trained slaughterhouse workers to take blood samples, then created maps of the animals' origins to better track the virus's geographic roots. While that system has not yet detected the virus in animal blood samples, it still operates in Kenya to flag the virus's presence, and it was the basis for the new system that screened milk.
In the fall of 2021, the team partnered with milk vendors to test milk brought in from rural areas and sold at urban markets. For a while, no milk samples seemed to harbor the virus, but in April 2022, Gerken received a text message from Christabel Winter, the manager of the lab in Kenya where those milk samples were being tested. A sample had come 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 proof 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 twice found evidence of the virus in milk.
The finding shows that people in urban areas are at greater risk for Rift Valley fever than previously thought -- and it points to the need for greater surveillance, said Bryson Ndenga of the Kenya Medical Research Institute in Kisumu, a longtime collaborator of LaBeaud and Gerken.
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 understand why outbreaks spread and advocate for preventive measures to protect people handling the animals."
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