Cross-Reactivity Opens Diagnostic Opportunity

The C6 peptide Borrelia miyamotoi test may help identify this tick-borne infection. Koetsveld and colleagues examined C6 reactivity in sera from both mice infected with Borrelia miyamotoi and from 46 patients with PCR-positive Borrelia miyamotoi disease (BMD). Their results support the use of the C6 peptide test used for Lyme disease as an indication of Borrelia miyamotoi infection.

They found, “Cross-reactivity against the C6-peptide was confirmed in 21 out of 24 mice experimentally infected with B. miyamotoi.” And a C6 antibody response was present in 39 of 46 patients.

This cross-reactivity is not a flaw—it’s a diagnostic opportunity. The C6 ELISA is widely available and routinely used in Lyme disease evaluation. If it also detects B. miyamotoi infection, this provides a readily accessible screening tool for an organism that currently has no FDA-approved testing.

High Sensitivity of C6 Test for B. miyamotoi

Interestingly, another recent study “showed that in a set of 43 sera from 24 patients with [Borrelia miyamotoi disease] from the United States the C6 ELISA was also positive in the vast majority (>90%) of convalescent phase serum samples.”

This high sensitivity—greater than 90% in convalescent sera—makes the C6 peptide test a valuable Borrelia miyamotoi test indicator, particularly when no other diagnostic options are available.

The C6 ELISA detects antibodies against a specific peptide sequence found in VlsE, a surface protein of Borrelia burgdorferi. The fact that B. miyamotoi antibodies also react with this peptide suggests antigenic similarity between the two organisms, despite their genetic and clinical differences.

Diagnostic Pattern: Positive C6 with Negative Western Blot

Koetsveld warns, however, that since “BMD [Borrelia miyamotoi disease] and Lyme borreliosis are found in the same geographical locations, caution should be used when relying solely on C6-reactivity testing.”

When both diseases occur in the same regions and the C6 test detects both, how do clinicians differentiate between them?

The answer lies in the Western blot pattern.

When to Pursue Further Borrelia miyamotoi Testing

Koetsveld and colleagues recommend further testing if the Western blot is negative.

“We propose that a positive C6 EIA with negative immunoblot, especially in patients with fever several weeks after a tick bite, warrants further testing for B. miyamotoi.”

This diagnostic pattern—positive C6 ELISA with negative Western blot—becomes a clinical clue for B. miyamotoi infection.

In standard Lyme disease evaluation, a positive C6 is typically followed by Western blot confirmation. If the Western blot is negative, the case is considered “not Lyme disease” and often dismissed. But this study suggests that pattern actually indicates possible B. miyamotoi infection.

Testing for the glycerophosphodiester phosphodiesterase (GlpQ) gene may be helpful as a Borrelia miyamotoi test option for detecting infection. GlpQ is specific to B. miyamotoi and does not cross-react with B. burgdorferi, making it a confirmatory test when B. miyamotoi is suspected.

Timeline of C6 Reactivity in B. miyamotoi Infection

“Our results demonstrate that cross-reactive antibodies against the C6 peptide regularly occur in patients with [Borrelia miyamotoi disease],” writes Koetsveld.

He adds, “with as much as 90% (95% CI 68.3–98.8) of patients being C6-reactive in samples taken 10 to 19 days after onset of disease.”

This timeline is clinically important. C6 reactivity appears relatively early—within 10-19 days of symptom onset—making it useful for diagnosis during the acute phase of illness when treatment is most effective.

Concurrent Infection Complicates Diagnosis

It is also possible that patients could have seronegative Lyme disease concurrent with B. miyamotoi. In fact, 10 of the 39 patients in this study were reactive by Western blot for Lyme disease.

This raises an important question: were these patients co-infected with both organisms, or does B. miyamotoi infection produce some degree of Western blot reactivity that doesn’t meet full CDC criteria for Lyme disease?

The same tick that transmits B. burgdorferi also transmits B. miyamotoi. Co-infection is biologically plausible. A single tick bite could introduce both organisms, leading to simultaneous infections that produce overlapping serologic responses.

Diagnosing Borrelia miyamotoi remains challenging given the overlap with Lyme disease testing and the possibility of concurrent infection.

Clinical Application and Limitations

The C6 peptide test provides a readily available tool for detecting B. miyamotoi infection, but interpretation requires careful clinical judgment.

A positive C6 with positive Western blot meeting CDC criteria suggests Lyme disease. A positive C6 with negative or indeterminate Western blot—particularly in a patient with fever, recent tick bite, and symptoms lasting several weeks—should prompt consideration of B. miyamotoi and potentially GlpQ gene testing for confirmation.

However, clinicians should not rely solely on C6 reactivity to make treatment decisions. Clinical presentation, exposure history, laboratory findings (such as leukopenia and thrombocytopenia, which are common in B. miyamotoi but rare in Lyme disease), and response to empiric therapy all contribute to diagnosis.

Frequently Asked Questions

Can the C6 test detect Borrelia miyamotoi?
Yes. Studies show that over 90% of patients with Borrelia miyamotoi disease are C6-reactive in convalescent phase sera, making it a useful screening indicator. C6 reactivity appears within 10-19 days of symptom onset.

When should clinicians suspect B. miyamotoi?
A positive C6 EIA with a negative Western blot, especially in patients with fever several weeks after a tick bite, warrants further Borrelia miyamotoi testing. This pattern suggests B. miyamotoi rather than Lyme disease.

What is the GlpQ test?
The glycerophosphodiester phosphodiesterase (GlpQ) gene test is a specific Borrelia miyamotoi test that can help confirm infection. Unlike C6, which cross-reacts with Lyme disease, GlpQ is specific to B. miyamotoi.

Can patients have both Lyme disease and B. miyamotoi?
Yes. In this study, 10 of 39 patients were reactive by Western blot for Lyme disease, suggesting concurrent infection is possible. The same tick transmits both organisms, making co-infection biologically plausible.

Should a positive C6 with negative Western blot be dismissed?
No. This pattern is often dismissed as “not Lyme disease,” but may actually indicate B. miyamotoi infection. Further testing with GlpQ gene PCR should be considered, particularly when clinical presentation includes fever and systemic symptoms.

Clinical Takeaway

The C6 peptide test, widely used for Lyme disease screening, demonstrates unexpected diagnostic utility for Borrelia miyamotoi infection through cross-reactive antibody responses. Koetsveld’s study examined C6 reactivity in mice and 46 PCR-confirmed B. miyamotoi patients. Results showed 21 of 24 infected mice and 39 of 46 patients developed C6 antibody responses. Another U.S. study confirmed this finding, reporting over 90% C6 positivity in convalescent sera from B. miyamotoi patients. This high sensitivity—comparable to or exceeding that for Lyme disease—creates an accessible screening tool for an organism that currently has no FDA-approved testing. The C6 ELISA is widely available, routinely ordered, and familiar to clinicians. If it also detects B. miyamotoi, this provides immediate diagnostic capability without waiting for specialized testing development. However, interpretation requires careful clinical judgment. Since B. miyamotoi disease and Lyme borreliosis occur in the same geographic regions and both produce C6 reactivity, differentiation depends on Western blot patterns. Standard Lyme disease produces positive C6 followed by positive Western blot meeting CDC criteria. B. miyamotoi produces a different pattern: positive C6 with negative or indeterminate Western blot. This diagnostic signature—positive C6 ELISA with negative immunoblot, particularly in patients with fever several weeks after tick bite—warrants further testing for B. miyamotoi through GlpQ gene detection. The timeline matters clinically. C6 reactivity appears within 10-19 days of symptom onset, with 90% sensitivity during this window. This early antibody response makes C6 useful for diagnosis during acute illness when treatment is most effective. Concurrent infection complicates interpretation. Ten of 39 patients in this study showed Western blot reactivity for Lyme disease. Were these co-infections with both organisms, or does B. miyamotoi produce partial Western blot reactivity that doesn’t meet full CDC criteria? The same tick transmits both pathogens, making simultaneous infection biologically plausible. The clinical application is clear but requires nuance. A positive C6 with positive Western blot suggests Lyme disease. A positive C6 with negative Western blot—especially with fever, recent tick exposure, leukopenia, and thrombocytopenia—should prompt B. miyamotoi consideration and GlpQ testing. Currently, this diagnostic pattern is often dismissed as “not Lyme disease” without further investigation. Recognizing it as a potential B. miyamotoi signature could prevent missed diagnoses and delayed treatment.