Severe, long-term fatigue, post-exertional malaise, pain and sleep problems are characteristic signs of the disease.
The causes of the condition are not known with certainty, although it has been established that the onset in most cases follows a viral or bacterial infection.
"Our study now shows that objective measurements are available that show physiological differences in the body's reaction to viruses between ME patients and healthy controls," said
The extensive spread of the SARS-CoV-2 coronavirus during the COVID-19 pandemic gave researchers a unique opportunity to study what happens in people with ME/CFS during a mild virus infection and compare this with what happens in healthy controls.
The research team, in collaboration with the Bragee Clinic in Stockholm, initiated a study early in the pandemic, involving 95 patients who had been diagnosed with ME/CFS and 110 healthy controls. They provided blood and saliva samples on four occasions during one year.
The researchers analysed samples for antibodies against SARS-CoV-2 and latent viruses and found a special fingerprint of antibodies against common herpes viruses in saliva.
One of these viruses was the Epstein-Barr virus (EBV), which has infected nearly everybody. Most people experience a mild infection during childhood.
People who are infected with EBV in their teenage years can develop glandular fever, also known as "kissing disease".
The virus then remains in a latent condition in the body.
The EBV virus may proliferate in situations in which the immune system is impaired, causing fatigue, autoimmune responses, and increased risk of lymphoma if allowed to continue, the researchers said.
Around half of the participants were infected with SARS-CoV-2 during the first wave of the pandemic and developed mild COVID-19, they said. In more than one-third of cases, the infection had been asymptomatic, so the person had not been aware of the infection.
After the SARS-CoV-2 infection had passed, however, the researchers detected specific antibodies in the saliva that suggested that three latent viruses had been strongly reactivated, one of them being EBV.
The reactivation was seen both in patients with ME/CFS and in the control group but was significantly stronger in the ME/CFS group. This can have negative consequences, one of which is that the immune system attacks certain tissues, such as nerve tissue, in the body, the researchers said.
Previous studies have also shown that the mitochondria, which produce energy in the cells, are affected, which suppresses the energy metabolism of people with ME/CFS, they said.
"Another important result from the study is that we see differences in antibodies against the reactivated viruses only in the saliva, not in the blood. This means that we should use saliva samples when investigating antibodies against latent viruses in the future," said Rosen.
He noted that there is a great deal of overlap between the symptoms of ME/CFS and those of long COVID, which is experienced by around one-third of patients who contract COVID-19.
Exhaustion after light exercise, brain fog and unrefreshing sleep are common symptoms, while impaired lung capacity and abnormal senses of smell and taste are more specific for long COVID.
The findings can contribute to developing immunological tests to diagnose ME/CFS, and possibly also long COVID, the researchers added.
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