The worm is typically found in pork as well as wild game such as bear and mountain lion. Trichinellosis cannot be spread through human contact.
"People who are consuming walrus need to be aware that there is a possibility of being exposed to this parasite," said Yuri Springer, a co-author of the report and an Epidemic Intelligence Service officer for the Alaska Division of Public Health.
"They need to be aware of the types of symptoms they would be experiencing should they be infected, and they need to know that if they are infected, they can seek medical care to alleviate those symptoms," he said.
All 10 people affected in the Alaska outbreaks have recovered.
If a human or animal eats meat infected with Trichinella cysts, the CDC says, their stomach acid dissolves the hard covering of the cysts, releasing the worms, which then pass into the small intestine.
The worms mate and lay eggs that develop into immature worms, which travel through the arteries and into the muscles. There, they curl up and return to their original cyst formation, and the life cycle continues.
The first symptoms of trichinellosis can include abdominal issues such as nausea, diarrhea, vomiting and stomach pain. Further symptoms may include headaches, fevers, chills, cough, facial swelling, aching joints and muscle pain, caused by inflammation in the muscles from the worms burrowing through.
Tourists are not at risk of getting the infection from eating walrus, Springer said, because consumption takes place solely in coastal villages.
Under the Marine Mammal Protection Act, only Alaska Natives are allowed to hunt and consume walrus, polar bears and several sea species, according to the report. It is not commercially available or served at restaurants.
"But we definitely have folks coming up from time to time to hunt bear. That's part of recreational hunting," Springer said. "And I believe those persons may potentially be at risk if they consume meat from the harvested bear. So, yeah, those folks should definitely be aware of the risk of exposure."
Anyone in the lower 48 who hunts and consumes wild game -- including bear, boar and mountain lion -- is susceptible, Springer said.
Trichinellosis and its impact
Trichinellosis used to be more common in the US, mostly caused by the ingestion of undercooked pork, according to the CDC.
The number of cases began to decline in the mid-20th century, after better sanitation management was implemented in the pork industry.
Infection has become increasingly rare, with an average of 15 cases per year reported in the US between 2008 and 2012, according to the CDC.
Globally, more than 65 000 cases and 42 deaths were reported across 41 countries between 1986 and 2009, one study
reports. Pork was the major source of infection, but wild game was also frequently reported.
In 1975, there was an outbreak of trichinellosis in Alaska when 29 people ate walrus meat and most became ill, according to a study
. Between that time and the latest illnesses, outbreaks occurred only in 1992 and 2002. Forty-four percent of these cases were connected to walrus consumption, and the rest were either related to bear or seal consumption, or had unidentified causes.
Springer said officials are not entirely sure what has caused the decline since 1975 or the rise of the new outbreaks, but there are a few possibilities. These include the timing and location of walrus hunting, changes in the parasitic burden in walrus populations, methods used to store and handle walrus meat, and reporting practices among ill persons.
The first recent outbreak occurred in 2016, when five coastal villagers who had consumed walrus meat that was raw or pan-fried to "medium" rare complained of symptoms, the report said.
Blood tests indicated that these cases had eosinophilia cells in their blood, a disease-fighting blood cell that is a common sign of parasitic infection. They were treated with anti-parasite medication and informed of the dangers of consuming undercooked or raw meat.
In 2017, five more villagers were diagnosed, and similar protocols were followed.
Dr. Daniel Eiras, assistant professor of infectious disease at NYU Langone Medical Center, who was not a part of the new CDC study, said he is skeptical of the use of an antibody blood test in diagnosing the disease.
"Antibody testing for trichinella, for many of these worm infections, is not great," Eiras said. "There are a lot of false negatives, so you can't really rely on antibody testing."
But the symptoms that were reported, as well as the cases' similarity to previous ones, are related to the infection, he said.
Spreading the word
A public service announcement has been distributed throughout the areas of Alaska impacted by the outbreak, as well as other coastal villages that could be affected in the future, Springer said.
The CDC report stresses the importance of being culturally sensitive when addressing the issue in areas where subsistence hunting is prevalent. This enables "members of the target population to make informed decisions that integrate their traditional practices with their awareness and tolerance of risks," the report said.
Springer said he and his colleagues have let affected populations know about the ways in which they can reduce their risk of infection. They have informed them that the parasite can be killed only by thoroughly cooking the meat, not by freezing, drying or smoking it.
"Once we explained those things to the patients, as well as other members of the community, they were very understanding and receptive to the information," he said.