Quantitative real time polymerase chain reaction assays for the sensitive detection of Besnoitia besnoiti infection in cattle

Abstract

Bovine besnoitiosis, an economically important disease in cattle in some countries of Africa and Asia, is emerging in Europe. The definitive host of Besnoitia besnoiti, the causative agent of bovine besnoitiosis, is unknown and the transmission of the parasite is not completely understood. Sensitive and quantitative DNA detection methods are needed to determine whether serologically positive animals are infectious and to examine the role of vectors (e.g. haematophagous insects) in the transmission of the parasite. To this end, we established two different 5′-nuclease quantitative assays to detect B. besnoiti infection in cattle and to estimate the parasite load in samples (BbRT1 and BbRT2). These PCRs are based on the sequence of the internal transcribed spacer region 1 (ITS-1) of the ribosomal RNA gene. Tests with serial dilutions of B. besnoiti genomic DNA in a buffer containing 100 ng/μl bovine DNA revealed a detection limit of 0.01 pg genomic B. besnoiti DNA. Reliable quantification was possible in samples containing ≥1 pg B. besnoiti genomic DNA with a coefficient of variation of ≤2%. To estimate the diagnostic sensitivity of the tests, skin biopsies and scrapings from the mucous membrane of the vestibulum vaginae (vaginal scrapings) were taken from cattle with clinical signs of chronic besnoitiosis. Regardless of the real time PCR assay used, 90.7% (39/43) of these animals were positive in at least one of two samples (skin or vaginal scrapings). Antibody titers, as determined by an immunofluorescent antibody test, and the threshold cycle values of the real time PCR obtained for skin samples and vaginal scrapings, were significantly correlated. The specificity of the PCRs was confirmed using genomic DNA from related parasites, including genomic DNA of Besnoitia spp., Neospora caninum, Toxoplasma gondii, Hammondia hammondi, Hammondia heydorni, Isospora spp., Sarcocystis spp., Eimeria bovis, Cryptosporidium parvum, and Trypanosoma brucei brucei. Since the sequence of the ITS-1 region of B. besnoiti is identical with that of Besnoitia species isolated from donkeys (Besnoitia bennetti), and reindeer (Besnoitia tarandi), both real time PCRs detected also DNA of these parasites. One of the B. besnoiti real time PCRs, BbRT1, but not BbRT2, cross-reacted with Besnoitia darlingi, Besnoitia oryctofelisi, and Besnoitia neotomofelis when large amounts of genomic DNA (10 ng) were used. The other B. besnoiti real time PCR assay (BbRT2) was specific for B. besnoiti, B. bennetti and B. tarandi, but did not react when 10 ng DNA of other related parasite species from the genus Besnoitia or other genera were subjected to analysis.