Animal and Aquacultural Sciences
Lise Grøva defended her thesis on August 26th 2011
Ane Gro Siri Skjelfjord
Tick-borne fever in sheep - production loss and preventive measures
Abstract from the thesis - PhD thesis 2011:32
A major challenge in sheep farming during the grazing season along the coast of south-western Norway is tick-borne fever (TBF) caused by the bacteria Anaplasma phagocytophilum
, that is transmitted by the tick Ixodes ricinus
. The justification for this study is based on our limited knowledge on the effect of A. phagocytophilum
infection on indirect losses i.e. reduced weight gain and weaning weight in lambs. Further, there is a lack of efficient and sustainable preventive measures to tick-infestation and TBF which implies a need for new knowledge. Knowledge on the effect of age of lamb to a natural A. phagocytophilum
infection and genetic variation in robustness to A. phagocytophilum
infection are possible preventive measures that are investigated in this study.
A study (PAPER I) was carried out in 2007 and 2008 to examine the prevalence of A. phagocytophilum
infection and its effect on weaning weight in lambs. The study included 1208 lambs from farms in Sunndal Ram Circle in Møre and Romsdal County in Mid-Norway, where ticks were frequently observed. All lambs were blood sampled and serum was analysed by an indirect fluorescent antibody assay (IFA) to determine the antibody status (positive or negative) to A. phagocytophilum
infection. The possible effect of A. phagocytophilum
on autumn live weight and weight gain was analysed using the MIXED procedure in SAS. The overall prevalence of infection with A. phagocytophilum
was 55 %. A reduction in weaning weight of 3 % (1.34 kg, p<0.01) was estimated in lambs seropositive to an A. phagocytophilum
infection compared to seronegative lambs at an average age of 137 days. The results indicate that A. phagocytophilum
infection has a negative but low effect on average lamb weight gain. The study also supports previous findings that A. phagocytophilum
infection is widespread in areas where ticks are prevalent, even in flocks that treat lambs prophylactic with acaricides.
A field trial (PAPER II) was carried out in 2008 and 2009 on two sheep farms in tick infested areas where A. phagocytophilum
is present, to examine if there is an effect of age of lambs to a natural A. phagocytophilum
infection. Three trial groups per farm and year, including a total of 336 lambs, were established as follows: 3E; lambs ≥ three weeks old when turned out on pasture and early time of birth, 1L; lambs ≤ one week old when turned out on pasture and late time of birth, 3L; lambs ≥ three weeks old when turned out on pasture and late time of birth. Recordings of weight, tick-counts, rectal temperature, other clinical signs of disease and mortality, together with blood serology and blood smears were used to analyse the effect of age of lambs to a natural A. phagocytophilum
infection. Gompertz weight curve parameters were estimated for all lambs, and the individual lamb parameter estimates and performance traits were further analysed using the MIXED procedure in SAS. The incidence of fever, tick-bites, clinical disease and death of lambs were analysed statistically using the PROC LOGISTIC procedure in SAS. There were incidences of tick-bites, clinical disease (including fever) and mortality in all trial groups indicating no effect of lamb age to a natural A. phagocytophilum
infection. However, lambs infected in spring with A. phagocytophilum
in the 1L group had higher (P<0.05) maximum growth rate (358g/day) than infected lambs in 3E (334g/day) and 3L (310/day) groups. Pasturing of ≤ one week old lambs on tick-infested pastures, can therefore be recommended in order to reduce weight losses due to A. phagocytophilum
. Note should however be taken on annual and seasonal variations in tick activity relative to lambing, variants of A. phagocytophilum
involved and turnout time as this probably will influence the effect of pasturing young lambs.
An infection study (PAPER III) was carried out in 2008 and 2009. Five-month-old lambs of two Norwegian sheep breeds, Norwegian White (NW) sheep and Old Norse (ON) sheep, were experimentally infected with a 16S rRNA genetic variant of A. phagocytophilum
(similar to GenBank accession number M73220). The experiment was repeated for two subsequent years, 2008 and 2009, using 16 lambs of each breed annually. Ten lambs of each breed were inoculated intravenously each year with 0.4 ml A. phagocytophilum
-infected blood containing approximately 0.5x106 infected neutrophils/ml. Six lambs of each breed were used as uninfected controls. The clinical, haematological and serological responses to A. phagocytophilim
infection were compared in the two sheep breeds. The present study indicates a difference in fever response and infection rate between the two breeds after experimental infection with A. phagocytophilum
. The clinical response seems to be less in ON-lambs compared to NW lambs, but further studies are needed to conclude on the possible higher protection against A. phagocytophilum
infection in the ON-breed than other Norwegian breeds.
Estimation of heritability for survival of lambs on tick-exposed pastures (PAPER IV) was conducted using data from the Norwegian Sheep Recording System. Data on lambs of the Norwegian White (NW) sheep breed from flocks participating in ram circles (cross flock organized breeding program) with recordings in the Norwegian Sheep Recording System and registered with cases of TBF or using prophylactic treatment against ectoparasites at any one time in 2000 to 2008 where included, making a total of 126 732 lambs. Analysis of the data was conducted using a linear model in DMU software. The estimated heritability for the direct effect on lamb survival was 0.22. The estimated maternal variance in proportion of phenotypic variance of lamb survival was close to zero. The heritability of direct effects on lamb survival indicates a potential for a selection response to improve survival of lambs on tick-exposed pastures. This heritability cannot, however, be directly attributed to robustness to A. phagocytophilum
as the lambs in this study are not confirmed infected with A. phagocytophilum
Our findings show that A. phagocytophilum
does cause a significant but relatively low reduction in live weaning weight in lambs. Furthermore, the bacteria seem to be widespread in areas with ticks, but its pathogenic effects may be variable. The proposed preventive measures of turning lambs ≤ one week old on pastures in tick infected areas show a potential to reduce indirect losses to TBF in Norwegian sheep farming, bearing in mind that annual and seasonal variations in tick-infestation will influence the effect of this preventive measure. The indications of breed differences as well as an estimated heritability of 0.22 in survival of lambs expected to be exposed to tick-infestation, indicates potential for improving performance on tick-exposed pastures. It is suggested that further studies should be done to identify genetically robust animals.
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