SEASONAL DYNAMICS OF THE KAROO PARALYSIS TICK (IXODES-RUBICUNDUS) - ACOMPARATIVE-STUDY ON MERINO AND DORPER SHEEP

Karoo paralysis in South Africa is induced in livestock by feeding fem ale Ixodes rubicundus ticks when infestation densities on hosts exceed certain critical levels. It has been shown previously that Angora goa ts are at a higher risk of being paralysed than Merino sheep, and such differences have been related to differences in feeding behaviour and spatial distribution of the two small-stock breeds. We hypothesized t hat differences in infestation densities with Karoo paralysis ticks wo uld also occur between Merino and Dorper sheep breeds. A study was con ducted under natural conditions in the south-western Free State, to co mpare infestation burdens of the two sheep breeds and also to investig ate seasonal patterns and annual variations in terms of rainfall and t emperature. Ten animals of each breed ran free in an area with a known history of Karoo paralysis and were examined on an approximately fort nightly basis, from March 1992 to December 1995, to determine tick abu ndance. Differences between the two breeds were significant (P < 0,05) during 1992 and 1993, but not during 1994. During the first two years , peak abundance of ticks was reached earlier in Dorper than in Merino sheep, and it also reached higher levels in Dorper than in Merino she ep ((x) over bar = 17,9 and 7,3, respectively). In 1993, two Dorper, b ut no Merino sheep, were paralysed. Dorper sheep are clearly at a high er risk of being paralysed than are Merino sheep, and as such, they ca n serve as indicators of adult tick activity and hence of the time to commence prophylactic treatment. Differences between the two breeds ar e probably related to differences in grazing patterns. Marked Variatio n in abundance and the time of onset of peak activity of I. rubicundus occurred over the years. Tick numbers were high in 1993 and 1995, but very low in 1992. In 1993, peak activity occurred earlier (April) tha n during the other years (June or July). These differences are related to differences in prevailing environmental conditions that influence tick activity in a complex manner. Heightened humidity and lower tempe ratures during the early stages of seasonal activity of the tick (Apri l or May), normally result in peak abundance of ticks on hosts at that time.