The National Park Service (NPS) has launched an investigation into a mysterious illness that has affected multiple rafters following separate trips down the Colorado River in the Grand Canyon.

Rafters reported experiencing severe symptoms after completing two-week trips that began as early as mid-May. Social media posts detailed clusters of unexplained illnesses that have left medical professionals searching for answers.

According to a Facebook group, "Grand Canyon Private Boaters," four individuals within a group of 16 became ill after a two-week trip in mid-June. Symptoms reported included fever, chills, fatigue, and pneumonia. Some experienced more severe complications, such as fluid in the lungs and loss of consciousness, with one individual requiring hospitalization.

Another rafter described a painful, "cobblestoning" condition on a previously bruised area of the shin that mimicked a broken bone. This condition did not improve and appeared to worsen, according to the report.

Speculation about the cause has circulated online, with some healthcare professionals suggesting potential links to mosquito-borne viruses such as dengue fever or chikungunya. One infectious disease epidemiologist reportedly indicated that based on the reported symptoms, mosquito bites, and the absence of gastrointestinal issues or tick bites, a viral cause is likely. Arizona is home to mosquitoes capable of transmitting these viruses.

However, these viruses are uncommon in the U.S. compared to other regions. The expert noted that if an individual who had been exposed to these viruses internationally entered the canyon and transmitted it to local mosquito populations, it could explain the cluster of illnesses, especially if patients did not report recent international travel, potentially delaying diagnosis.

One individual, as a precautionary measure, reportedly began receiving a rabies vaccine despite not exhibiting specific rabies symptoms, highlighting the uncertainty surrounding the cause.

NPS and the Arizona Department of Health Services have not immediately responded to requests for comment.