The National Park Service (NPS) has launched an investigation into a mysterious illness affecting multiple rafters who took separate Grand Canyon trips in recent weeks.

Several people took to social media last week to report a cluster of unexplained illnesses that they say have left doctors searching for answers. The rafters said they embarked on two-week trips beginning as early as mid-May, and developed conditions after returning home. 

Paddling Magazine reported that park officials are now leading the investigation as rafters report experiencing severe and painful symptoms. 

Speculation about the potential cause has spread online, with some healthcare professionals and others weighing in on possible explanations, including mosquito-borne illnesses such as dengue fever and chikungunya.

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According to a Facebook group “Grand Canyon Private Boaters,” four people became sick after a group of 16 people embarked on a two-week rafting trip in mid-June.

In a July 1 post, user Steven King described a range of symptoms among members of the group, including fever, chills, fatigue and pneumonia. He said some individuals experienced more severe complications, including fluid in the lungs and loss of consciousness. 

“One person briefly lost consciousness in their doctor’s office then was subsequently hospitalized,” King said. 

King also described another person who developed what he called “cobblestoning” — an infection or cellulitis-like condition — on a bruised area of the shin that “feels like a broken bone.”

“X-ray doesn’t show a break, and it doesn’t seem to be healing at all but rather getting worse,” he said. 

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He said additional precautions were taken by some individuals as officials have yet to determine the cause of the alleged illnesses.

“One person started receiving the rabies vaccine in an abundance of caution even though they are not really showing symptoms of rabies specifically,” the user said.

King reported that the group did not encounter any ticks during the trip, but reported seeing mosquitoes on several nights while in the canyon.

Separately, Matthew Wappett — who said he went to the emergency room with a swollen knee and “bone-crushing” joint pain after a two-week trip in mid-May — also reported experiencing mosquito bites during the rafting trip. 

“We slept out every night and did get some mosquito bites,” Wappett said in a July 2 post. 

An infectious disease epidemiologist reportedly told Facebook user Dan Squire that the illnesses could potentially be linked to mosquito-borne viruses such as dengue fever or chikungunya. 

“Based on the symptoms and the reports of mosquito bites (together with the lack of GI symptoms and no reports of tick bites) it’s sounding viral – and symptoms are a good match for both Dengue and Chikungunya. (Hanta should also be ruled out),” Squire wrote, citing his friend.

Squire said the expert noted that dengue and chikungunya are uncommon in the U.S. compared with other parts of the world, but Arizona is home to mosquitoes capable of transmitting both viruses.

“If someone who had been exposed to these viruses (like during international travel) went into the canyon while still carrying the virus, they could have transmitted to local mosquito populations,” Squire wrote. 

The healthcare professional also suggested doctors could have difficulty identifying the cause if patients do not report recent international travel, potentially leading to delays in testing for certain viruses.

“People reporting symptoms like this, but without recent international travel, are highly unlikely to be tested for these viruses in AZ,” Squire added.

The NPS and the Arizona Department of Health Services did not immediately respond to Fox News Digital’s request for comment.