This study has found an animal level seroprevalence of 8.5% and a flock level seroprevalence of 9.8%.
However, caution would be advised in extrapolating these figures to the national sheep population as the sample is biased due to the opportunistic nature of the sampling protocol. Secondly, a greater proportion than the national average of the land area within the Sligo Regional Veterinary Laboratory (RVL) catchment area would be classified as upland with rough grazing, and therefore would be more likely to have a tick burden, than areas with a greater proportion of improved grassland. Further there was an uneven distribution of submissions from the counties within the catchment area. It has been shown in a previous report that a significant determinant in submitting a carcass to an RVL is distance from the submitting farm to the RVL and this study emphasises this further with 47% of submissions coming from Co. Sligo. The relatively small sample size and the number of positive animals may also raise concerns relating to study power. Despite these shortcomings, this study provides interesting information on the epidemiology of louping ill in Irish sheep flocks in the northwest of Ireland.
The greatest proportion of sero-positive animals was in the adult (greater than 24 month old) category. This may in part reflect historical seroconversion where an animal developed immunity following infection, and may not be necessarily associated with active infection at the time of sampling.
If an adult sheep were introduced to the farmland, it would have had no prior immunity to the condition and would be more susceptible. Of the 17 cases encountered in the present study, three of these were in recently purchased sheep. However, it cannot be determined if these affected sheep had been on a tick-infested land parcel earlier in their lives. Traditionally, a seasonal pattern of occurrence has been observed with louping ill. Louping ill and listeriosis were diagnosed at post mortem in six of the 17 positive cases.
The elevation of the land grazed by sheep in flocks where louping ill was diagnosed was greater than that of the flocks where no sero-positive animals were found. This is as expected as the tick required for the transmission of louping ill is found on rough grazing land which tends to be at higher elevations.
Despite the awareness of louping ill as an endemic disease in Ireland for many years, this is the first published report detailing its occurrence in the north west of Ireland.