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To compare microscopy, culture and PCR for the diagnosis of anthrax in blood samples from sheep and cattle.Blood samples were stored at room temperature and at 37°C after receipt, over a period of 15–17 days. Aliquots were plated onto blood agar and blood smears were prepared. Following microscopic examination, DNA was extracted from blood smears and subjected to a multiplex PCR assay targeting the Ba813, cap and lef markers.PCR provided the most reliable means for the detection of Bacillus anthracis in deteriorating blood samples (15–17 days) and was also successful in diagnosing anthrax in blood smears that had been stored for 6 years and a blood sample which had been stored for 18 months at −20°C. While less successful than PCR, culture for B. anthracis on 7% sheep blood agar was typically more reliable (2–17 days) than the examination of blood smears (2–6 days) for encapsulated bacilli.This work demonstrated the superiority of PCR for the diagnosis of anthrax from blood smear scrapings, particularly when microscopy is unreliable.