Quick test could catch fatal tick fever before it’s too late

A prototype rapid diagnostic test could save lives by enabling early diagnosis and effective treatment for several kinds of tick-borne fever. The assay uses europium-based nanoparticle reporters to identify a biomarker for the tick-borne disease and could diagnose cases before symptoms appear. 

Tick-borne spotted fever rickettsioses (SFRs) are a group of diseases caused by Rickettsia bacteria and are transmitted to humans and other mammals through the bites of infected ticks. Cases of SFRs are rising dramatically, partially due to tick ranges and numbers expanding with climate change. 

SFRs vary in presentation, from mild to fatal, depending on the species of tick, however, early symptoms are non-specific. Mediterranean spotted fever, which is endemic to several countries, and Rocky Mountain spotted fever (RMSF) are the most severe varieties, with RMSF causing fatalities in 23% of untreated cases and 4% of treated cases. 

While Rickettsia bacteria are susceptible to several antibiotics, early diagnosis and treatment are crucial – half of RMSF-related deaths occur within nine days of the onset of symptoms. SFRs can be diagnosed by detecting antibodies to Rickettsia in blood via indirect immunofluorescence assay. However, antibody responses don’t typically appear until seven to 10 days after symptoms manifest and also occur in more than 10% of healthy individuals. Alternatively, the bacteria can be isolated and cultured in a laboratory, but this requires specialised facilities, personnel and equipment and takes time.  

En route to developing the first rapid diagnostic test for SFRs, researchers in Texas identified an enzyme that was highly conserved across the spotted fever group of Rickettsia. They then developed an antigen-based lateral flow assay using europium-based nanoparticle reporters and specific antigens against the enzyme that can be read by a commercial lateral flow fluorescence reader. 

Using guinea pig and mouse models of SFRs, the team characterised concentrations of the highly-conserved enzyme at various points during the course of the disease and evaluated their test’s performance in both infected and uninfected animals. The assay showed a specificity of 100% and a sensitivity of 95% in the animal models. Using human blood samples spiked with the enzyme, the researchers estimate that their test could diagnose SFRs one to two days prior to symptoms.  

Although acknowledging that further studies are needed to validate the test, the scientists believe that their lateral flow assay could translate to a point-of-care diagnostic tool and improve patient outcomes through earlier SFR diagnosis and treatment.