Hasle et al. Acta Veterinaria Scandinavica 2010, 52:55 http://www.actavetscand.com/content/52/1/55 Page 6 of 9 Farm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 IFAT IgG positive Noticed Notified Figure 3 Comparison of three sources of information for the occurrence of babesiosis on the farms in this study. IFAT IgG positive: At least one cow positive for IFAT Babesia divergens IgG. Noticed: Farmers’ statement that redwater occurs in cows on the farm or is detected when adult cattle are imported to the farm. Notified: Clinical cases registered on the diary cow health cards, compiled by the Norwegian Dairy Herd Recording System from 1996-2008. Because cows are parasitised by hundreds of ticks in the course of a season, and a single bite from an infected tick is sufficient for transmission of Babesia, [41] cows are likely to contract B. divergens if it is pre- sent in their pasturing areas. The screening of cows for B. divergens infection would therefore be expected to be a sensitive method for detecting the presence of the parasite in a locality, if testing is performed at a time of the year when Babesia-antibodies are at the highest. Serum samples that we collected on Jomfruland in May were not directly comparable to those that we collected in October and November, as the May samples could either contain persistent antibodies from the previous year, or there might be early infections from the same year. The mean temperature April 1st-15th was 5.3°C, and no temperatures of below 0°C were recorded (The Norwegian Meteorological Institute), which means that tick questing may well have occurred during this period. With an incubation time of 1-3 weeks [3], seroconver- sions during May 2004 would be expected to occur. As we tested only once for each locality we did not demon- strate the seasonal and yearly variation of antibodies described by l’Hostis et al. [38]. Further studies are needed to decide which month would be optimal for detecting the presence of B. divergens in a locality along the Southern Norwegian coast. However, ticks are still parasitizing the cows in October and November and these months would therefore be expected to be a good choice for detecting B. divergens antibodies. The sensitivity of serologic testing for detecting B. divergens will depend on the cut-off level that is set for a positive score on the test. At a cut-off level of 1:40 (+ +) the sensitivity and specificity of an individual anti- body test are reported to be 100% and 97%, respectively [32]. Setting the cut-off value at this level would, there- fore, likely result in the detection of a few false positives due to non-specific cross reactivity. This problem is illu- strated by our results on Farm 24, where only one cow was found to be positive at the detection level of 1:40 (++), and there were no positive tests at more stringent detection levels. This result could represent either a false positive or a low titre in a cow that was infected a long time ago. Because the aim of this study was to be able to detect the present occurrence of B. divergens at a particular locality, a high sensitivity for detecting the pathogen on a given farm is desirable, and the number of cows tested is crucial. By testing a median of twelve cows per locality, we were able to achieve a much higher sensitivity for detecting B. divergens on a given farm than farmers’ observations and the existing NDHRS can provide. At all the farms where samples with 1/40(+++) were detected there were also samples positive at 1/160, indicating that these are real positives. Therefore, by setting a cut-off level of 1:40 (+++) for defining a case of seropositivity for B. divergens, anti- body testing should result in a specificity of nearly 100%, unless cross-reacting Babesia spp. are occurring and, consequently, the risk of falsely concluding that B. divergens occurs on a farm will be small. The related species B. capreoli cause babesiosis in roe deer and red deer [42], and roe deer may also be infected by the newly discovered Babesia sp. EU1 [43]. These parasites cannot be serologically distinguished from B. divergens. They cannot give clinical infection in cattle, but there is a possibility that a subclinical infection may cause sero- conversion [44], although Schmid et al. [45] found no seropositive cows in an area in which ticks positive for