Image created by Dr. Michael J. Miller
A team of biomolecular engineers, pathologists, and internal medicine specialists at the University of Texas Medical Branch, working with a colleague from the University of Houston, has developed a quick test for tick-borne spotted fever rickettsioses (SFRs).
In their paper published in the open-access journal PLOS ONE, the group describes how they isolated an enzyme that was present in all groups of Rickettsia and used it as a marker for developing a test that could be done quickly to diagnose tick-borne SFRs.
Tick-borne SFRs are a family of diseases, all caused by the Rickettsia bacteria. As the name suggests, they are transmitted to humans from infected ticks. The researchers note that the number of people infected is rising due to encroachment on lands where ticks live, and a warming planet, which is increasing the places where ticks can live.
People infected with a tick-borne SFR can experience a range of symptoms, running from mild to severe. Treatment for SFRs includes antibiotics, and early diagnosis is considered to be crucial for a good outcome.
SFRs are currently diagnosed by looking for telltale antibodies in the bloodstream using indirect immunofluorescence—unfortunately, such antibodies do not typically appear for seven to 10 days, which allows the bacteria to proliferate, making treatment more difficult.
Other tests are available but tend to require specialized equipment. In this new effort, the research team has developed a test that can be used before symptoms arise.
The test was developed after the researchers identified an enzyme that could be used as a marker for people infected with SFRs. The test is similar to others that use paper chromatography—in this test, nanoparticle reporters were used along with specific antigens that work against the enzyme the team had identified. The result was read using a standard lateral-flow fluorescence reader.
The researchers evaluated their test on guinea pig and mouse models and found it able to detect an SFR with 100% specificity in 95% of the animals tested. In testing using human blood samples, they found the test could be used to diagnose SFRs up to two days prior to the onset of symptoms. They note that more testing needs to be done to validate their results, but suggest their approach could lead to point-of-care testing for future patients, allowing for better outcomes.
Reference: Richard Willson et al, Development of a rapid antigen-based lateral flow assay for tick-borne spotted fever rickettsioses, PLOS ONE (2025). DOI: 10.1371/journal.pone.0312819
Abstract
Tick-borne spotted fever rickettsioses (SFRs) continue to cause severe illness and death in otherwise-healthy individuals due to lack of a timely and reliable diagnostic laboratory test. We recently identified a diagnostic biomarker for SFRs, the putative N-acetylmuramoyl-l-alanine amidase RC0497. Here, we developed a prototype laboratory test that targets RC0497 for diagnosis of SFRs. The concentrations of RC0497 in sera of Rickettsia rickettsii-infected guinea pigs and R. conorii-infected mice were determined by stable isotope dilution–parallel reaction monitoring mass spectrometry (SID-PRM-MS), ranging from 0.1 to 1.1 ng/ml. Using europium chelate nanoparticle reporters, we developed a lateral flow assay (LFA) and evaluated the test with a panel of serum samples of mock and experimentally infected animals. Interestingly, 21 of 22 (95.5%) serum samples from R. rickettsii-infected guinea pigs and R. conorii-infected mice yielded positive results with a ratio of test line / control line greater than the cutoff value determined for non-infected animals. All uninfected samples were in agreement with the intended results, suggesting that the initially assessed specificity of the test is 100%, among these samples. Mice infected with a lethal dose of R. conorii and treated with doxycycline on day 3 post-infection (p.i.), upon RC0497 detection by LFA, displayed significantly decreased rickettsial loads, comparable to the sublethal infection group on day 5 p.i.. A panel of human serum samples spiked with various concentrations of recombinant RC0497 were analyzed by LFA, suggesting that the limit of detection of the LFA was 0.64 ng/mL. These findings suggest that the timely detection of RC0497 by a europium LFA offers guidance for treatment, leading to a significant improvement in infection outcomes. This work, for the first time, shows significant promise for a rapid and easy-to-use platform offering a timely diagnostic assay for severe SFRs.