Development and validation of a novel detection method for Rickettsia rickettsii using a loop-mediated isothermal amplification assay

Introduction

Rickettsia rickettsii is an obligate, intracellular pathogen and the causative agent of Rocky Mountain spotted fever (RMSF). RMSF is an important zoonotic disease due to its high fatal outcome in humans. The difficulty of clinical diagnosis due to the low sensitivity and specificity of current diagnostic methods are a principal setback. We reported the development of a new method for the detection of R. rickettsii in human and tick DNA samples using loop-mediated isothermal amplification (LAMP), as well as the validation of the LAMP test for R. rickettsii in field samples of infected ticks and humans, determining the diagnostic sensitivity and specificity, as well as the reproducibility of the test.

Methods

This technique uses hydroxy naphthol blue (HNB) as an indicator of the formation of magnesium pyrophosphate, a marker for the presence of DNA. Here, we used a putative R. rickettsii gene as a target for three pairs of primers that specifically amplify R. rickettsii DNA by hairpin-based isothermal amplification technique (LAMP).

Results and discussion

The sensitivity of the assay was ~1.6-3 pg, which is 10 times more sensitive than PCR. To determine the diagnostics specificity and sensitivity, 103 human DNA samples and 30 tick DNA samples were evaluated. For the human samples, a sensitivity for HNB of 93%, a specificity of 70% and a k of 0.53 were obtained. For electrophoresis the sensitivity was 97% with a specificity of 58% and a k of 0.42. For tick samples, a sensitivity of 80% was obtained, a specificity of 93% for HNB and for electrophoresis the sensitivity and specificity were 87%. The k for both was 0.73. The degree of concordance between HNB and electrophoresis was 0.82 for humans and for ticks, it was 0.87. The result is obtained in shorter time, compared to a PCR protocol, and is visually interpreted by the color change. Therefore, this method could be a reliable tool for the early diagnosis of rickettsiosis.

Development and standardization of a Loop-mediated isothermal amplification (LAMP) test for the detection of Babesia bigemina

Bovine babesiosis is a tick-borne disease caused by protozoan parasites of the genus Babesia. Babesia bigemina is one of the most prevalent and economically important parasite species that infects cattle because of its impact on the meat and milk production industry. Effective disease control strategies should include detection of reservoir animals and early and specific pathogen detection using rapid, economical, sensitive, and specific detection techniques. The loop-mediated isothermal amplification technique (LAMP) is a one-step molecular reaction that amplifies DNA sequences with high sensitivity and specificity under isothermal conditions and requires no special equipment. The results can be observed by the naked eye as color changes. The aim of this work was to develop and standardize the LAMP technique for B. bigemina detection and its visualization using hydroxynaphtol blue. For this situation, primers were designed from the conserved sequences of the B. bigemina ama-1 gene. The results showed that at 63 °C in 1 h and under standardized conditions, this technique could amplify B. bigemina DNA as indicated by the characteristic colorimetric change. Sensitivity evaluation indicated that DNA was amplified at a 0.00000001% parasitemia, and it was demonstrated that this technique specifically amplified the DNA of B. bigemina. Additionally, this technique could amplify DNA from 10 strains of B. bigemina from three different countries. It is concluded that the LAMP technique as modified in our case could specifically amplify B. bigemina DNA and shows high sensitivity, does not cross-react with related organisms, and the product is observed by 60 min of reaction time based on color changes. This report is the first LAMP report that uses sequences that are conserved between strains of the ama-1 gene, demonstrates the results by color changes using hydroxynaphtol blue. We propose LAMP as a rapid and economical alternative method for the molecular detection of B. bigemina.

Immuno-molecular prospecting for vector-borne diseases in central Mexico

Climatic changes have influenced the temporal and spatial distribution of diseases. In livestock-grazing areas, rodents are reservoirs of zoonotic pathogens; therefore, they play an important role in the transmission of diseases affecting domestic animals and humans. The objective of this study was to investigate the presence of the zoonotic agents: Anaplasma phagocytophilum, Borrelia burgdorferi, Ehrlichia canis and Rickettsia rickettsii, as well as the presence of viral RNA from the Bunyaviridae, Togaviridae and Flaviviridae families, in wild rodents from animal production units in central Mexico. The samples were obtained from wild rodents that had access and contact with animal production units. A total of 92 rodents were captured, and samples of blood, serum and organs, such as spleen, kidney, heart and liver, were obtained. The serum was used to detect antibodies against Anaplasma phagocytophilum, Borrelia burgdorferi, Ehrlichia canis and Rickettsia rickettsii by an immunofluorescence antibody test (IFAT); the blood was used for PCR analysis; and the organs were used to obtain RNA (cDNA) to perform RT-PCR. By IFAT, all samples were positive to A. phagocytophilum and E. canis, and negative to B. burgdorferi and R. rickettsii. The samples that were positive to IFAT were used to confirm the presence of pathogen by PCR analysis. The results from the PCR were as follows: 34 samples were positive to A. phagocytophilum, and 59 to E. canis. There was no amplification of genetic material from the Bunyaviridae, Flaviviridae and Togaviridae virus families from the organs that were sampled, which suggests that the samples obtained did not contain RNA specific to these families. This is the first immuno-molecular prospecting study on vector-borne diseases in central Mexico demonstrating the presence of A. phagocytophilum and E. canis in wild rodents living in cattle grazing areas.