Study tracks lasting cognitive and functional effects after tick-borne encephalitis

Tick-borne encephalitis is often described as an acute infection: a sudden illness marked by fever, headache, neurological inflammation and, in the most serious cases, altered consciousness or focal neurological damage. But a new prospective study from Norway suggests that the disease may leave a subtler imprint long after patients have left hospital. One year after confirmed tick-borne encephalitis, patients generally reported cognitive abilities within the normal range, yet they still experienced more everyday memory problems and post-encephalitis symptoms than demographically comparable controls. The findings challenge the idea that recovery can be judged solely by whether a patient survives the acute infection or appears neurologically well during a routine examination.

The study, published in the Journal of Neurology, followed 109 adults aged 16 years or older who had been hospitalized with laboratory-confirmed or otherwise verified tick-borne encephalitis in Norway. The participants were drawn from a larger prospective multicenter research program and were assessed using standardized questionnaires three months and 12 months after admission. Researchers also recruited 80 controls from the patients’ families, friendship groups and workplaces. Because these controls came from the social networks of the patients, they were intended to resemble them in age and background more closely than a random population sample might. The approach provided a practical comparison group, although it also means the controls may not represent the wider Norwegian population.

The investigators focused on three aspects of recovery that can be difficult to capture in a short clinical consultation: executive function, everyday memory and overall symptom burden. Executive function refers to the mental processes used to organize behavior, maintain attention, shift between tasks, monitor performance and control impulses. These abilities are essential for working, managing finances, planning meals, driving and coping with unexpected demands. The researchers measured them with the 75-item Behavior Rating Inventory of Executive Function–Adult version, or BRIEF-A. Its nine domains are combined into a Global Executive Composite, expressed as a standardized T-score with a population mean of 50 and a standard deviation of 10. Scores of 65 or higher were defined as clinically significant executive difficulties.

Memory was assessed with the Everyday Memory Questionnaire-Revised, a 13-item instrument designed to capture lapses in ordinary life rather than performance on a laboratory test. Participants rated how often they forgot appointments, lost track of conversations, misplaced objects or experienced similar problems. The total score ranges from zero to 52, with higher values indicating more frequent difficulties. A score of 19 or above was treated as clinically relevant impairment, corresponding to approximately 1.5 standard deviations above the mean for the control group. The third tool, the Rivermead Post-Concussion Symptoms Questionnaire, measures 16 physical, cognitive and psychological symptoms experienced over the previous 24 hours. Its scores range from zero to 64, and higher scores indicate a heavier symptom burden. Although originally developed for brain injury, the scale can capture complaints such as fatigue, dizziness, irritability, poor concentration and headaches that may also follow encephalitis.

The central question was whether patients improved between the three-month and one-year assessments. On the measures selected as primary outcomes—the BRIEF-A Global Executive Composite, the everyday memory score and the Rivermead symptom score—the researchers found no significant change from three to 12 months. This does not mean that every patient remained unchanged, nor that no individual improved or deteriorated. It means that, across the group, average questionnaire scores did not show a statistically reliable shift during that nine-month interval. The result raises the possibility that many patient-reported consequences of tick-borne encephalitis become established early in recovery, rather than steadily resolving throughout the first year.

At 12 months, the average executive-function scores remained within the normal range. That result is important because it suggests that most patients did not, as a group, report major global impairment in the broad organizational and self-regulatory abilities measured by the BRIEF-A. Yet normal average scores can conceal meaningful problems in individual patients, and a person may struggle with a specific task without crossing a clinical threshold on a composite scale. The study therefore examined individual BRIEF-A subscales, including working memory, planning and organization, task monitoring, emotional control and the ability to shift between activities. These detailed domains can reveal patterns that disappear when many abilities are compressed into a single overall score.

The clearest difference between patients and controls emerged in everyday memory and general post-encephalitis symptoms. Patients had higher EMQ-R and RPQ scores at the 12-month assessment, indicating more frequent memory lapses and a greater burden of physical, cognitive or psychological complaints. The differences persisted in analyses designed to account for potentially influential factors, including age, sex, medical comorbidities, anxiety and depression. This adjustment is technically important: fatigue, poor concentration and forgetfulness can be amplified by mood symptoms or unrelated health conditions. The findings do not prove that every reported symptom was directly caused by the virus, but they strengthen the case that the infection can be followed by persistent difficulties that patients notice in daily life.

Tick-borne encephalitis is caused by viruses transmitted primarily through the bite of infected ticks, with occasional transmission through contaminated unpasteurized dairy products in some regions. The virus can initially produce a febrile illness and then invade the central nervous system. Inflammation of the meninges causes symptoms such as headache, vomiting, neck stiffness and sensitivity to light or sound, while inflammation of the brain itself can lead to confusion, altered consciousness, seizures and focal neurological signs. The Norwegian researchers classified acute disease as mild, moderate or severe according to meningeal symptoms, changes in consciousness, focal findings and multifocal neurological involvement. They also recorded imaging abnormalities from acute-phase magnetic resonance imaging or computed tomography when scans had been performed for clinical reasons.

The study’s design gives its conclusions more weight than a single retrospective survey, but it also defines their limits. Patients were enrolled during acute hospitalization and followed prospectively, reducing the risk that people would have to reconstruct their symptoms from memory years later. The investigators used validated questionnaires, applied predefined thresholds and performed adjusted multivariate analyses, including MANOVA and MANCOVA models that considered several outcomes at once. They also examined anxiety and depression with the Hospital Anxiety and Depression Scale and assessed the robustness of their findings through sensitivity analyses. However, only 109 of 153 enrolled patients completed the 12-month follow-up, and the three- and 12-month samples were not identical for every participant. Some patients were assessed at only one time point, while the control group did not undergo clinical neurological investigations. Controls recruited through patients’ social networks may also have been unusually similar to, or different from, the patients in ways that questionnaires cannot fully capture.

The practical message is that recovery from tick-borne encephalitis may require more than a conventional neurological examination. A patient who has regained strength, normal speech and an apparently intact memory in conversation may still forget instructions, lose track of tasks or experience persistent symptoms that interfere with work and family life. The Norwegian results support the use of structured follow-up that asks directly about everyday memory, concentration, fatigue and post-infectious symptoms, particularly when patients report that they are “better” but not back to their previous level of functioning. The study does not establish a universal long-term prognosis, identify a single biological mechanism or show that rehabilitation will reverse the problems. It does, however, provide a clear signal: after the acute danger has passed, the effects of tick-borne encephalitis may remain visible in the ordinary mental workload of everyday life.