High Serum Folate Values in Lambs Experimentally Infected with Anaplasma phagocytophilum

Folates are involved in nucleic acid, protein and amino acid synthesis. In human serum, it has been estimated that 60% to 70% of folates are bound to proteins (Wagner 1985). Folate binding proteins (FBPs) are therefore crucial to the assimilation, distribution and retention of the vitamin folic acid and have been identified in various cells, extracellular fluids and tissues from humans and several animal species. FBPs have different functions based on their biochemical properties and can be divided into 3 classes: high-affinity folate binding proteins (HFBP), membrane-associated folate-binding proteins, which function in the transport of folate compounds across cell membranes, and cytoplasmic-binding proteins with a high affinity for specific reduced-folate compounds (Henderson 1990). Tick-borne fever (TBF) caused by the rickettsia Anaplasma phagocytophilum (formerly Ehrlichia phagocytophila) is a common disease in domestic ruminants on Ixodes ricinus infested pastures in Norway (Stuen 1997, Stuen & Bergström 2001). During the acute infection, A. phagocytophilum are found in cytoplasmatic inclusions in blood leucocytes, mainly neutrophils. Up to 95% of the neutrophils can be infected. The rickettsemia is followed by a severe neutropenia lasting for 1-2 weeks (Foggie 1951, Woldehiwet & Scott 1993). The reason for this neutropenia is not known, but may be due to destruction and removing of infected cells (Hudson 1950, Woldehiwet 1983, Rikihisa 1991). Neutropenia in the peripheral blood of lambs is seldom observed except for lambs infected with A. phagocytophilum. Recently, during a trace element deficiency study in Norway, some lambs grazing on I. ricinus infested pastures had both a severe neutropenia (<0.7x109 cells/l) and increased serum folate values up to 901.2 nmol/l (Kleppa, unpublished result). In contrast, the normal range of serum folate concentration in sheep has been estimated to vary from 3.2 to 6.6 nmol/l (Branda 1981). Unfortunately, EDTA-blood was not available for A. phagocytophilum examination. The aim of the present study was therefore to evaluate the variation in serum folate concentration in lambs experimentally infected with A. phagocytophilum. Eight lambs, 4 to 5 months old, of the Dala and Rygja breeds were used. All lambs were kept indoors from birth and throughout the whole experimental period. The lambs were fed hay, silage and concentrates. Four lambs were inoculated intravenously on day 0 with 1 ml of a whole blood dimethyl sulphoxide stabilate of an ovine A. phagocytophilum variant. Sequencing result of the 16S rDNA gene of this variant was found identical with the DNA sequence of Acta vet. scand. 2003, 44, 199-202.