The effect of lamb age to a natural Anaplasma phagocytophilum infection

Tick-borne fever (TBF) is caused by the bacterium Anaplasma phagocytophilum that is transmitted by the tick Ixodes ricinus, which is a major challenge in sheep farming along the coast of south-western Norway. Few efficient and sustainable preventive measures are available, but older lambs seem to be more susceptible than younger lambs to an A. phagocytophilum infection. A field experiment was carried out in 2008 and 2009 on two sheep farms with the breed Norwegian White Sheep (NWS). Three treatment groups of lambs on each farm and year were established: 1; lambs ≥3 weeks old when turned out to pasture and born in early spring, 2; lambs ≤1 week old when turned out to pasture and born in late spring, 3; lambs ≥3 weeks old when turned out to pasture and born in late spring. The study included a total of 337 lambs distributed to treatment group 1, 2 and 3 with 116, 111 and 110 lambs respectively. Recordings of tick-counts, rectal temperature, clinical symptoms and mortality together with weight, blood serology and blood smears were used to analyze the effect of age of lambs to a natural A. phagocytohpilum infection. Gompertz weight curve parameters were estimated for all lambs and were used to compare weight gain in lambs between the treatment groups. There were observations of tick-bites, clinical disease (including fever) and mortality, but no significant effect of lamb age to a natural A. phagocytophilum infection was detected. However, lambs infected with A. phagocytophilum in group 2 had higher (P < 0.05) maximum spring growth rate (358 g/day) than infected lambs in group 1 (334 g/day) and group 3 (310 g/day). Further, lambs not infected with A. phagocytophilum showed no weight differences (P < 0.05) between treatment group 2, compared to groups 1 and 3. Pasturing of ≤1 week old lambs on tick-infested pastures in tick endemic areas, can therefore be recommended to prevent weight loss due to an A. phagocytophilum infection. Note should however be taken on annual and seasonal variations in tick activity relative to lambing, different genetic variants of A. phagocytophilum involved and turnout time as these factors probably will influence the effect of pasturing young lambs.