High seroprevalence of granulocytic ehrlichiosis distinguishes sheep that were the source of an alimentary epidemic of tick-borne encephalitis

A sheep herd from which contaminated cheese was produced, causing 21 cases of alimentary tick-borne encephalitis (TBE) in human beings, was tested serologically for the presence of specific antibodies against both the TBE virus (TBEV) and Anaplasma phagocytophilum, the cause of tick-borne fever (TBF) in ruminants, and compared with three other herds variously exposed to tick bites but without any TBE history. Virus neutralisation (VN) with the TBEV strain Hypr and CV-1 cells was used in TBE tests, and indirect immunofluorescence assay (IFA) with neutrophils from goats experimentally infected with A. phagocytophilum was used for TBF testing. In 13 sheep from the incriminated herd (N = 41), VN titres ranging from 1/4 to 1/128 traced previous TBE infection and all sheep had elevated titres of A. phagocytophilum antibodies ranging from 1/80 to 1/5120 in IFA, whereas two other herds (N = 8 and 9) were seronegative for TBEV and had significantly lower levels of A. phagocytophilum antibodies, corresponding to a lesser challenge from TBF. A control herd (N = 10) that was grazed on tick-free meadows in north Norway was completely seronegative. The respective distributions of positive titres of A. phagocytophilum and TBEV antibodies in the incriminated herd were not mutually random; the animals with higher anti-A. phagocytophilum titres tended to have lower anti-TBEV titres and vice versa (Spearman correlation coefficient, –0.86, p ≤ 0.01). The authors hypothesize that the immunosuppressive effect of TBF coinfection in sheep could be a contributory cause of TBE virus contamination of milk, an aspect of TBE epidemiology that has not been considered thus far.