Post-Treatment Lyme Disease Syndrome What the Research Says About Its Causes

A subset of people treated for Lyme disease do not get well. Why that happens is one of the most contested questions in tick-borne disease research, and the disagreement is old enough to have its own vocabulary — "post-treatment Lyme disease syndrome" in one tradition, "chronic Lyme disease" in another. Federal reports, professional-society guidelines, and congressional testimony describe the same underlying phenomenon and reach meaningfully different conclusions about its cause. This article surveys that literature and lets the disagreement stand where the evidence has not resolved it. For the clinical definition of PTLDS, who experiences it, and the broader medical controversy around its recognition, see Post-Treatment Lyme Disease Syndrome.

What PTLDS Is, and What It Isn't

The most widely cited research definition comes from the Infectious Diseases Society of America. A 2024 Cureus review describes it: PTLDS "is defined by the Infectious Diseases Society of America (IDSA) as the presence of fatigue, pain, and/or cognitive complaints with the functional impact that persists for more than six months after completing treatment for Lyme disease (LD)" (Cureus 2024). The same review notes that "PTLDS symptoms are broadly defined by prolonged somatic and neurocognitive dysfunction following standard antibiotic treatment for LD" (Cureus 2024).

Estimates of how often this happens vary with how investigators measure it. The 2024 Cureus review states that "These symptoms occur in 10%-20% of patients previously diagnosed with LD caused by the bacteria Borrelia burgdorferi and appropriately treated with a course of antibiotics" (Cureus 2024). A 2022 Johns Hopkins prospective cohort study by Aucott and colleagues reports that "Variability in treatment paradigms, study design, enrollment criteria, and especially outcome classification have led to a wide range in estimated prevalence of persistent symptoms in this population (0-50%; however, 10-20% is frequently cited)" (JHU 2022). The 2020 IDSA/AAN/ACR clinical guidelines put it more cautiously still: "The prevalence of persistent symptoms following standard treatment of Lyme disease is unclear; estimates vary depending on the patient population and methods of long-term assessment" (IDSA 2020).

The Aucott cohort study provides one of the cleanest prospective estimates against a non-Lyme comparison group. The investigators report that "13·7% of participants with a history of prior LD met criteria for PTLD compared with 4·1% of those without a history of prior LD" (JHU 2022), and that prior-Lyme participants "were approximately 5·28 times as likely to meet PTLD criteria compared with those without prior LD" (JHU 2022). The authors conclude that "PTLD is neither necessarily rare nor an inconsequential public health concern" (JHU 2022).

The symptom profile has a specific character in the literature. How patients manage these symptoms day-to-day is the practical-care question downstream of this diagnostic one. The 2024 Cureus review describes PTLDS as a "constellation of subjective clinical problems: incapacitating fatigue, pain, and neurocognitive dysfunction that persist for more than six months" (Cureus 2024). The fatigue, the review adds, is ""profound, notable, debilitating, and extreme, not as a vague form of tiredness."" (Cureus 2024) These are also, the Aucott cohort cautions, symptoms with clinical background noise:

"The most frequently identified PTLD symptoms are fatigue, pain, and cognitive dysfunction (Rebman et al., 2017b; Wormser et al., 2006). These symptoms are often non-specific and are often reported in general clinical populations as well as in patients with prior LD." — JHU, 2022, pp. 6–7. Risk of Post-Treatment Ly...

The Core Dispute: Is the Bacterium Still There?

Nearly every framing of PTLDS causes comes back to one question. When someone remains ill after antibiotics, is Borrelia burgdorferi — the Lyme disease bacterium — still alive in the body, or is it gone and the symptoms driven by something the infection left behind? The 2018 HHS Tick-Borne Disease Working Group report to Congress summarized the state of the science this way:

"Persistent symptoms after treatment of Lyme disease can be severe, yet their cause(s) remains unknown and debated." — HHS, 2018, pp. 3–4. Tick-Borne Disease Workin...

The 2020 HHS Pathogenesis subcommittee report separates two findings that often get collapsed into one. First, persistence in animals is established: "It has been established that the causative organism of Lyme disease, B. burgdorferi, can persist in a number of animal models and human case studies following infection and treatment with a “standard” course of antibiotics" (HHS 2020). Second, what this means for symptomatic human patients is a separate question:

"However, it is still unclear whether human patients with ongoing symptoms associated with Lyme disease continue to have an active infection following completion of seemingly appropriate antibiotic therapy and thus the extent to which unresolved infection or clearance of Borrelial antigens contributes to persistent Lyme disease symptoms." — HHS, 2020. Report of the Pathogenesi...

Animal-model evidence for post-treatment persistence has accumulated over three decades. The 2024 Cureus review summarizes that "In mouse, dog, and non-human primate models, B. burgdorferi persistence is measured by tissue histopathology and PCR in animals treated with antibiotics" (Cureus 2024). The same review describes Hodzic et al.'s finding that "antibiotic treatment resulted in the persistence of low numbers of spirochetes in tissues of treated mice and that ticks could acquire and transmit very low levels of infectious spirochetes" (Cureus 2024) — work often cited by researchers arguing that standard treatment is not reliably curative. Yale's Linda Bockenstedt "suggest a possible mechanism in that B. burgdorferi persists in human hosts by transforming into cysts" (Cureus 2024), one of several morphological forms invoked in the persistence hypothesis.

The 2020 HHS subcommittee goes further on the animal-model data, stating that "mice never clear B. burgdorferi infection without antibiotic treatment; humans and nonhuman primates appear to harbor low-level, persistent B. burgdorferi infection as well" (HHS 2020), and that persisting bacteria "can be metabolically active, expressing certain bacterial genes and inducing gene expression changes in the infected host, despite being non-culturable following antibiotic treatment" (HHS 2020).

Testimony to a 2012 House hearing characterized the animal findings in stronger terms. A researcher described persistence shown across "100 percent of mice, rats, hamsters, guinea pigs, gerbils, dogs, and nonhuman primates" (House 2012), and argued that "There is substantial evidence of the persistence of it after treatment with antibiotics" (House 2012).

The same House hearing framed the interpretive gap the evidence sits inside of:

"The alternative view, promoted by the International Lyme and Associated Disease Society and also by numerous academic researchers in the U.S. and around the globe, says that the science is too unsettled to be definitive, and there could be one or more causes of persistent symptoms after initial treatment in an individual who has been inflicted with the agent of Lyme disease." — House, 2012. Global Challenges in Diag...

The Counterposition: Treated Infection Resolves, Symptoms May Not

A 2022 BMC Primary Care focus-group study of chronic-Lyme patients in the Netherlands described the prevailing medical view: "a large majority of medical professionals and biomedical researchers do not point to a persistent Borrelia infection as the explanation for CLD patients’ complaints, and most guidelines do not acknowledge such infections as their cause" (BMC 2022). The authors of the same study emphasized that the phenomenon is "well-known" (BMC 2022) and occurs in "approx. 5-10% of patients who have had LD" (BMC 2022), but that it "lacks a settled explanation for its pathogenesis" (BMC 2022).

The 2020 IDSA/AAN/ACR clinical guidelines draw the practical conclusion. For patients with persistent symptoms but without objective signs of ongoing infection, the guideline panel recommended against more antibiotics:

"1. For patients who have persistent or recurring nonspecific symptoms such as fatigue, pain, or cognitive impairment following recommended treatment for Lyme disease, but who lack objective evidence of reinfection or treatment failure, we recommend against additional antibiotic therapy *(strong recommendation, moderate-quality evidence).Comment: Evidence of persistent infection or treatment failure would include objective signs of disease activity, such as arthritis, meningitis, or neuropathy." — IDSA, 2020. Clinical Practice Guideli...

The same guideline document flags a methodological challenge: "prospective controlled trials, in which healthy controls have been followed for months to years alongside patients who have been treated for Lyme disease, have found that the frequency of this symptom complex is the same in controls as in treated Lyme disease patients" (IDSA 2020), a finding the guideline authors note raises the possibility of diagnostic anchoring. The Aucott 2022 prospective cohort pushes back on that interpretation with its controlled comparison against non-Lyme participants.

Autoimmunity: The Post-Infectious Hypothesis

A second family of explanations keeps the active infection on the Lyme side of the timeline and locates the ongoing symptoms in a dysregulated immune system. The 2024 Cureus review proposes that "ongoing PTLDS symptoms seem to be related to an autoimmune response to the tissue damage and inflammation caused by the viable or nonviable spirochete pathogen" (Cureus 2024). The review is explicit that "Lyme disease and PTLDS have been linked to autoimmune conditions" (Cureus 2024), citing Arvikar and colleagues' cohort of post-Lyme rheumatoid arthritis, psoriatic arthritis, and spondyloarthritis patients.

The 2020 HHS pathogenesis subcommittee describes the autoimmune pattern most clearly in the context of Lyme arthritis. "As in rheumatoid arthritis, the prototypical autoimmune joint disease, Lyme arthritis is frequently accompanied by autoimmune T- and B-cell responses to self-antigens" (HHS 2020). Supporting the autoimmune interpretation, the subcommittee notes that "nearly all patients with persistent Lyme arthritis experience resolution of arthritis when treated with immunosuppressive drugs" (HHS 2020) — a response pattern more consistent with ongoing autoimmunity than with ongoing bacterial infection.

The Cureus review sketches a molecular hypothesis: "The first mechanism proposed is the release of autoantigens from neural tissue damage during active infection. This process causes posttranslational protein modification, creating new self-epitopes that attack the nervous system" (Cureus 2024). Testimony to a 2012 Senate hearing traced this framing back decades, noting that "As early as 1990, Allen Steere, the ‘‘father’’ of Lyme disease, who was first to describe Lyme arthritis, found that even when timely diagnosed and adequately treated, about 10 percent of the patients develop ‘‘antibiotic resistant arthritis,’’ which is an autoimmune, chronic condition" (Senate 2012).

Retained Antigen: The Bacterium Is Gone But Its Parts Remain

A third hypothesis threads between the first two. The bacterium may be killed by antibiotics, but bacterial fragments left in tissue continue to provoke inflammation. The 2020 HHS subcommittee discusses B. burgdorferi peptidoglycan — a component of the bacterial cell wall — and concludes that "B. burgdorferi peptidoglycan might be a persistent antigen in Lyme arthritis" (HHS 2020). The subcommittee's framing enumerates several possibilities that do not collapse to a single answer:

"- The bacteria may be dormant or incapable of replication, yet there may be the presence of residual antigens or the periodic release of antigens, to which the host responds to produce the symptoms associated with persistent Lyme disease." — HHS, 2020. Report of the Pathogenesi...

"- The bacteria may be entrenched in areas either inaccessible to certain classes of antibiotics (for example, poorly vascularized connective tissue, intracellular compartments), or higher doses of antibiotics are needed to achieve levels that impede metabolic activity." — HHS, 2020. Report of the Pathogenesi...

"- The bacteria may become antibiotic-tolerant, requiring repeated courses of antibiotic treatment or periods of treatment alternating with periods of no treatment." — HHS, 2020. Report of the Pathogenesi...

The 2024 Cureus review connects the retained-antigen hypothesis to chronic inflammation in cartilage, noting that "While B. burgdorferi spirochetes might be destroyed by antibiotic treatment, its DNA is able to remain in cartilage tissue and bind toll-like receptors, thus contributing to chronic inflammation" (Cureus 2024). A 2012 Senate hearing referenced a 2012 Yale finding by Bockenstedt that showed "the presence of fragments of the spirochete long after the infection" (Senate 2012), and argued that "There might be a role to the pathogen, either dividing or infective or inflammatory, but the disease can be ongoing and perpetuating without the presence of the spirochete" (Senate 2012).

Immune Dysregulation and Neuroinflammation

Beyond frank autoimmunity, researchers have described broader immune dysregulation after Lyme disease. The 2024 Cureus review notes that "In PTLDS, the tissue damage and inflammation resulting from infection are thought to persist despite treatment, leading to a variety of chronic symptoms. Inflammation within the nervous system (neuroinflammation) occurs in the central and peripheral nervous systems" (Cureus 2024). Fallon and colleagues "observed reduced blood flow in discrete white matter areas of the brain in patients with PTLDS compared to healthy subjects" (Cureus 2024), with the flow reductions associated with memory and visuospatial deficits.

A specific immunologic signature has emerged as a candidate risk marker. The 2024 Cureus review describes work reported by "Aucott et al." (Cureus 2024) on chemokine C-C motif ligand 19 (CCL19) during acute infection. A 2022 HHS Diagnostics subcommittee report corroborates this direction, stating that "elevation in the chemokine CCL19 after treatment of early Lyme disease has been associated with the later development of PTLD" (HHS 2022). Genetic factors may also contribute — the same HHS report notes that "SNPs of toll-like receptors and interleukin-6 promoter have also been linked to persistent symptomatology and disease" (HHS 2022).

Who Gets PTLDS: Risk Factors

The 2024 Cureus review summarizes established risk factors: "Risk factors for PTLDS include delayed diagnosis and treatment, incomplete or short-course treatment, and the increased severity of acute LD" (Cureus 2024). The 2022 Aucott cohort identified demographic and psychosocial predictors as well, reporting that "Risk of meeting PTLD criteria was also independently increased among females and those with higher exposure to previous traumatic life events" (JHU 2022). The psychological and cognitive dimensions of living with PTLDS are the territory of the mental health consequences of chronic Lyme. The authors proposed a biological interpretation:

"It is possible that female sex and/or increased exposure to traumatic life events impact the initial biologic response to B. burgdorferi infection and increase risk for persistent symptoms (Klein and Flanagan, 2016; Seiler et al., 2020)." — JHU, 2022, pp. 6–7. Risk of Post-Treatment Ly...

The 2022 HHS Access to Care subcommittee amplified the sex-difference finding, reporting that in the Aucott cohort "women were 4.2 times more likely than males to meet PTLD rather than return-to-health criteria" (HHS 2022).

Family Resemblance: PTLDS and Other Post-Infectious Syndromes

PTLDS does not sit alone as a phenomenon. While this article focuses on Lyme-specific mechanisms, long-term outcomes and management for other tick-borne diseases are the parallel concern for non-Lyme pathogens. The 2024 Cureus review catalogs its clinical neighbors: "Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) shares PTLDS's hallmark symptoms of debilitating fatigue, cognitive difficulties, and sleep disturbances" (Cureus 2024). The Aucott 2022 cohort situated PTLDS alongside post-acute COVID-19:

"These are often the circumstances for other infection-associated conditions as well, including recently described “post-acute” or “long” COVID after SARS-CoV-2 infection (Nalbandian et al., 2021), which anecdotally shares a similar clinical phenotype to PTLD." — JHU, 2022, pp. 6–7. Risk of Post-Treatment Ly...

Whether these share a common mechanism remains open. A 2025 Frontiers in Medicine pilot study by Fallon and colleagues offers a representative synthesis of how active PTLDS researchers currently frame the mechanistic landscape: "Emerging evidence indicates there are many mechanisms that may account for persistent symptoms, including a dysregulated immune response (persistent inflammation, autoimmunity), persistent Bb infection or Bb remnants, altered microbiome, and/or altered neural circuitry" (Front 2025).

What Cannot Yet Be Tested For

One reason the dispute endures is that there is no laboratory test that settles it for an individual patient. The 2024 Cureus review states that "PTLDS is diagnosed clinically as no quantifiable methods are available from laboratory or tissue diagnostics as of 2024" (Cureus 2024), and that "no quantitative biomarker exists to indicate a treatment endpoint and/or definite transition to PTLDS" (Cureus 2024). The absence of a biomarker has clinical consequences: "The subjective nature of these symptoms, coupled with the absence of conventional diagnostic biomarkers, makes the identification of PTLDS heavily reliant on a clinical diagnosis" (Cureus 2024).

Biomarker research is ongoing. A 2024 HHS scoping review describes "RNA sequencing to analyze gene expression in the peripheral blood mononuclear cells (PBMCs) of patients with post-treatment Lyme disease, acute Lyme disease, and healthy controls" (HHS 2024) that distinguished PTLDS patients from acute-Lyme patients and controls.

The State of the Science

The federal bodies charged with surveying this literature have repeatedly described the causes of PTLDS as unresolved. The 2018 HHS Tick-Borne Disease Working Group report concluded that "it is difficult to know if they are caused by immune dysfunction, persistent infection by the bacteria or its parts, complications from coinfections, and/or a combination of these and other pathologies" (HHS 2018), and called for "Investigations are also needed into the potential roles of immunologic responses, bacterial persistence, and coinfecting pathogens" (HHS 2018).

A 2022 HHS Disease Prevention and Treatment subcommittee concluded:

"Our understanding of PTLDS is incomplete. Multiple trials have been conducted on this syndrome using multiple antibiotics and with prolonged courses. However, symptoms are not completely alleviated by antibiotics, which suggests that antimicrobials may not be the treatment of choice for this syndrome. There is a need for a better understanding of the pathogenesis of PTLDS to help design interventions to alleviate the suffering of these patients." — HHS, 2022. Disease Prevention and Tr...

The same subcommittee recommended increased federal research investment:

"Potential Action 2.3: Increase research into the causes of persistent post-treatment symptoms attributed to tick-borne diseases, including Lyme disease, and identify or develop appropriate therapeutic approaches, including non-pharmaceutical interventions, to improve treatment outcomes." — HHS, 2022. Disease Prevention and Tr...

The 2024 Cureus review's own synthesis is notable for the number of mechanisms it is willing to hold open simultaneously:

"PTLDS might be due to an autoimmune response to tissue damage and/or direct inflammation caused by Borrelia spirochetes or bacterial fragments. Immune dysfunction caused by B. burgdorferi at the innate and adaptive level may weaken the immune system, thereby promoting persistent infection. The lipoprotein antigens associated with Lyme disease are highly inflammatory, and the retained antigens have been seen in mouse models. This may lead to the characteristic symptoms of severe fatigue, musculoskeletal pain, and cognitive symptoms." — Cureus, 2024. What Makes It Tick: Explo...

The review closes, more plainly, that "The pathophysiology behind the persistent symptoms some people experience from a primary infection is still largely unknown" (Cureus 2024). What that means for any individual patient — whether symptoms reflect lingering bacteria, misdirected immunity, residual damage, retained antigen, altered neural circuitry, or some combination — is the open question the research has not yet closed.

Sources

    Not medical advice. See a healthcare provider for medical decisions. Medical Disclaimer