A hantavirus outbreak in California’s Yosemite National Park is raising concerns among public health officials and has also presented researchers with a medical riddle.
According to a 27 August announcement from the US National Park Service (NPS), the outbreak includes two deaths last month and a third, non-lethal, case of the virus. A probable fourth case is also reported.
The link between all the cases is that they involve individuals who stayed in a group of tent cabins at the park this summer. The NPS announced that they have tried to contact approximately 1700 individuals who lodged in the cabins between mid-June to mid-August in hopes that any other cases can be identified.
Hantavirus is characterized by flu-like symptoms which can appear as late as five to six weeks after exposure. It’s caused by an RNA virus that’s only transmissible through airborne particles of rodent saliva, droppings, or urine.
Just 587 cases of hantavirus have been confirmed in the US from its first identification in 1993 to 2011. ‘Sin nombre’, the most common strain (pictured), can result in a severe respiratory condition; roughly 38% of cases are fatal. Deer mice are the primary hosts for the virus, and high elevation sites, where the mice are more prevalent, can be hot spots.
“Most cases can be traced to the same house – usually we see groups of two where people were cleaning a cabin together, for example,” says Brian Hjelle, a pathologist at the University of New Mexico in Albuquerque. “But it is unusual to have four cases coming from the same geographic location.”
That’s what has experts puzzled. Hantavirus has only infected humans twice before in Yosemite, each time at higher elevation cabins and only as isolated cases.
In this instance, all four visitors stayed in different “signature cabins” – wood structures covered by a canvas tent – in a lodging area called Curry Village. After the first death, park officials say they detected the virus in deer mouse droppings in the area.
“We don’t really know the basis behind this particular cluster of cases – it may reflect the changing geographical distribution of the deer mouse, which typically favors higher elevations than Yosemite Valley,” Charles Chiu, an infectious disease researcher at the University of California in San Francisco, told Nature. “That’s mostly concerning because it’s such a densely populated area.” However, given the lack of human to human transmission and the rarity of the disease, Chiu doubts the odds of a large outbreak.
At Yosemite, NPS public health officers are in the process of screening local deer mice to determine if the mouse population or the prevalence of the virus has increased — or both. About 15 to 20% of the local deer mouse population normally carries the disease, says NPS ranger Kari Cobb.
Prompted by a case in 2010, park officials changed cleaning protocols at Yosemite this past April. That’s one reason why the outbreak comes as a surprise.
Studies have proposed a link between previous outbreaks in the southwestern US and high rainfall due to El Nino climate patterns. More precipitation means more grasses and insects for deer mice to eat and larger populations. “California does show weather responses to El Nino too but not to the same extent as New Mexico,” cautions Greg Glass, an epidemiologist at Johns Hopkins University in Baltimore, Maryland . Winter and spring weather conditions can influence mouse populations and viral prevalence, but either one could be at work here. “Even meaningful speculation will require some serious study,” says Hjelle.
Hjelle also notes that the Sierra Nevada mountain range often sees higher genetic variation in ‘Sin nombre’ hantavirus due to reassortment of viral genes. Since the ‘Sin nombre’ viral genome has not been fully sequenced, Chiu is exploring a potential study with the California Department of of Public Health of whether viral mutations might explain this summer’s unusual case cluster.