Hasle et al. Acta Veterinaria Scandinavica 2010, 52:55 http://www.actavetscand.com/content/52/1/55 Page 5 of 9 Figure 2 Fraction of cows positive for Babesia divergens IFAT IgG antibodies at a titre of 1:40 (+++) or higher in 24 different farms along the southern Norwegian coast, arranged form west to east. Error bars: 95% confidence intervals. the 16 cows that had been imported from inland local- ities one year before the study, indicating that there is a substantial risk of babesiosis in their present locality. The presence of B. divergens was confirmed by IFAT in a total of 17 of the 24 farms we tested. Farmers had observed redwater in only ten of the farms where B. divergens was detected, and only four of these cases of redwater had been recorded by the NDHRS (Figure 3). All of the cows on one of the farms in the study were B. divergens-antibody positive, though the owner had never seen any cases of redwater. We detected B. diver- gens antibodies in 17 of the 25 cows that we tested on Jomfruland, where Radzijevskaja [27] found no infected ticks. The PCR pilot study gave no positive results. Discussion In Norway and Sweden the only cattle Babesia reported is B. divergens [35,36]. With regard to this and the strong reaction to the antigen used we assume that the seropositive animals were/had been infected with the species Babesia divergens. Our results demonstrate that testing of cattle for seropositivity to B. divergens is a far better method for mapping the distribution of this pathogen than using indirect methods, such as inter- viewing farmers or relying on the NDHRS. When it pre- sents clinically, redwater is easily recognizable by farmers and veterinarians, and because prompt treat- ment is usually required to prevent deleterious effects of the disease, veterinarians often treat the disease without performing any laboratory tests. There are few data available on the attack rate of bovine B. divergens infec- tions. Our data indicate that there are many subclinical cases of B. divergens infection, which is in agreement with previous studies on outbreaks [7,37] and in stable infected herds [38]. An extensive study of B. divergens seroprevalence was conducted in Northern Ireland, showing an overall seroprevalence of 31,8%[39], i.e., close to the overall seroprevalence in our limited mate- rial. A second study carried out in Northern Ireland [40] found consistent estimates when comparing results from a farm survey, a veterinary practise survey and ser- oprevalence data, with an estimated clinical incidence of 0,26% per year. The number of cases in the Agder coun- ties, according to the NHDRS, is 18.4 cases per year in a population of ca. 10400 dairy cows (Statistics Norway, http://www.ssb.no/emner/10/04/10/jt1999/tab-2001-04- 03-07.html, Jordbrukstelling 1999), which would give an incidence of 0.18% per year. Our results indicate an incomplete registration of cases of redwater in the NHDRS, possibly because veterinarians are not always consulted e. g. during the dry period, in mild cases of redwater, or that the farmers fail to observe redwater while the cows are out at pasture. The farms that we included in our study were not randomly selected, but were chosen because the pastures were in wooded areas, and were situated near the coast in the distribu- tion area of I. ricinus in Norway. They would therefore be expected to have more babesiosis than average farms in the same counties.