This is a single-center stratified (on gender and donor), block randomized, placebo-controlled, parallel group trial with 12-months follow-up of 80 chronic fatigue syndrome/encephalomyelitis (CFS/ME) participants. Participants will be randomized to treatment by preprocessed thawed donor fecal microbiota transplant or preprocessed thawed autologous fecal microbiota transplant. Primary endpoint is the efficacy of FMT at three months by the Fatigue Severity Scale. The investigators will use patient reported outcomes for primary and secondary outcome measures.

Previous studies suggest that a dysbiosis of the gut microbiota may be a key feature in CFS/ME. We hypothesize that

A: CFS/ME is caused by a dysbiosis in the gut flora causing barrier leakage of bacterial products, a low grade systemic immune activation and disturbances in the host energy metabolism.

B: Recovery of a normal gut flora by fecal microbiota transplantation (FMT) alleviates symptoms and may even induce remission of CFS/ME.

This project aims to determine if there is a true cause and effect relationship between a dysbiotic gut flora and CFS/ME by testing if treatment of the observed dysbiosis by FMT also can resolve CFS/ME symptoms. In this process, collection of blood, fecal, and urine samples before and after FMT will open the possibility to explore the relationship between the gut flora, immune response, host energy metabolism and CFS/ME using technologies of microbiomics, metabolomics and immunological characterizations for a better understanding of the pathobiology of CFS/ME.

CFS/ME participants:

General practitioners recruit participants from the local area, by posters at the doctors' offices. In addition the study has a facebook site, named "the COMEBACK study", where interested CFS/ME subjects can submit their interest to be assessed for participation. After a telephone screening of potential participants by the International Consensus Criteria for CFS/ME and CFS/ME severity rating, eligible subjects will be referred to department of physical medicine and rehabilitation UNN Harstad (FYSMED) and re-assessed. During this screening process the investigators will keep a track record of screening failures noting reason for failure. Participants will have a physical exam and necessary workup including blood, fecal and urine tests to exclude differential diagnosis according to the Norwegian National Guidelines for Assessment of CFS/ME. Participants receive information about the study and give their written consent. Subjects earlier diagnosed with CFS/ME at FYSMED will undertake the same re-assessment.

During the work up, participants will do the Fatigue Severity Scale, Hospital Anxiety and Depression scale, SF 36, Modified DePaul Questionnaire, the Rome IV criteria for irritable bowel syndrome, and the "Repeatable Battery for the Assessment of Neuropsychological Status" test (RBANS). Heart rate variability is implemented as an additional measure when approximately 40 out of 80 participants are assigned treatment.

Donors are recruited informally from the local high schools. Donors are included and screened according to the European Consensus Guidelines from 2017. The full screening will be undertaken before the first feces donation and every 4th week. The inclusion and screening will be performed at the department of physical medicine and rehabilitation UNN Harstad. The investigators will keep a track record of screening failures noting reason for failure.

Participants receive FMT at the gastroenterology outpatient clinic at University Hospital of North Norway Harstad, Norway. No antibiotics are given prior to the intervention. The participants must do a bowel lavage using Sodiumpicosulphate/Magnesiumcitrate (Picoprep, Ferring) before intervention. The treatment will be administered by enema. Active treatment will be pre-processed frozen donor feces. Placebo will be the participant's own feces processed and frozen during the study inclusion. After the intervention, the participants have no restrictions on activity level and are asked to keep an unchanged diet without introduction of any new food supplements or probiotics in the follow up period. To keep track of change in diet investigators ask participants to do a food frequency questionnaire before the FMT and at 3 and 12 months after the intervention. Use of antibiotics, food supplement and use of medications will also be recorded.

The treatment will take place in blocks of four consecutive participants per day. A data engineer at the Department of Clinical Research at the University Hospital of North Norway, Tromsø (UNN,Tromsø) creates the allocation sequence using the REDCap software. The treatment is randomized on donor and placebo in fixed blocks of 4 with 2 active (1 donor A and 1 donor B) and 2 placebo. Block allocation will be stratified on gender.

A stratification on donor and gender will be performed by assigning full blocks of male and female participants. In the two active slots in each block of four, one active slot will be used for donor A and one for donor B. The stratification of gender reflects the higher incidence of CFS/ME in women. Donor fecal microbiota transplants stored for more than two years will be disposed and replaced with transplants from a new donor. If this happens, donor fecal microbiota transplants from the new donor will be assigned in remaining slots from the donor with the expired transplants.

Allocation is done in solitude in a closed room with no transparency, only containing a freezer with the active transplants (tagged by donor batch ID) and the placebo transplants (tagged by screening number). Before allocation of treatment, an investigator places the FMT-placebos on a table in the room. The allocator can then enter the room as the researcher placing all the placebos leaves the room. The allocator will access the randomization sequence when entering participants screening number on the REDCap software using a computer in the same room. The allocator will be the only person involved in the study that can access the randomization program at the REDCap software. If a screening number is randomized to active treatment, the allocator removes the tag from the placebo and places it on a donor FMT treatment instead. All unused placebo transplants will be disposed immediately. When finished, the allocator places the allocated treatment in a box in a designated freezer. The allocator will build a key file matching the active treatment to the donor batch id by updating a key file on paper and store it in a safe not accessible to any others. In addition the allocator will write the corresponding patient screeningnumber on tags from the used donor batch and keep them as backup in the same safe. This will allow for tracking of each individual donor batch to a corresponding participant at the end of trial when all follow up is complete.

FMT PARTICIPANTS

Inclusion Criteria:

• International Consensus Criteria for CFS/ME
• 18-65 years
• Mild-severe CFS/ME
• Fatigue Severity Scale score of 5,0-7,0
• Symptom duration for 2-15 years

Exclusion Criteria:

• Kidney failure
• Congestive heart failure
• Immuno-deficiency or use of immune-suppresive drugs
• Other disease that may explain ME/CFS symptoms discovered during diagnostic work up
• Use of antibiotics the last three months,
• Use of low dose naltrexone or Isoprinosin
• Pregnancy or breastfeeding
• Serious endogenous depression
• Chronic infectious disease (HIV, hepatitis B or C etc.)
• Introduction of new food supplements, change in diet or introduction of new medications the last three months
• Assessed not be able to follow the instructions for data and sample collection
• Very severe ME/CFS (WHO class IV)
• Symptom duration of less than 24 months or more than 15 years
• History of abdominal surgery, with the exception of appendectomy, cholecystectomy, caesarean section and hysterectomy
• Previous treatment with FMT

FMT DONORS

Inclusion criteria:

• Healthy
• Age 16-30 years
• Type 3 or 4 stool by the Bristol Stool Scale

Exclusion criteria:

• Use of peroral antibiotics past 3 months
• Use of topical antibiotics past 2 months
• Tattoo or piercing past 6 months
• Former imprisonment
• History of: -chronic diarrhea
• constipation
• inflammatory bowel disease
• colorectal polyps
• colorectal cancer
• immuno-suppression
• Obesity
• Metabolic syndrome
• Atopic skin disease
• CFS/ME
• Psychiatric disorders
• Other serious autoimmune disease
• Close relatives with serious autoimmune disease
• High risk sexual behavior
• Bowel movements that does not correspond to a Bristol Stool Scale type 3 or 4
• Journeys abroad the last six months to countries high in antibiotic resistance
• Use of food supplements, pre-, -pro, -or symbiotics past one month
• Dysbiosis grade 3 or more by the GA dysbiosis test

De-identified individual participant data for all primary and secondary outcome measures will be made available

Available when findings from primary and secondary endpoints are published. Study protocol will be available upon request when we initiate the inclusion.

Data access requests will be reviewed by study investigators. Requestors will be required to sign a Data Access Agreement