Experimental & AppliedAcarology, 19 (1995) 147-153 147

An evaluation of strategic and threshold control measures against the Karoo paralysis tick,

ixodes rubicundus (Acari: ixodidae) in South Africa

L.J. Fourie, D.J. Kok, I.G. Horak a, and J.M.Van Zyl b Department of Zoology and Entomology, University of the Orange Free State,

PO Box 339, Bloemfontein 9300, South Africa aDepartment of Veterinary Tropical Diseases, Faculty of Veterinary Science,

University of Pretoria, Onderstepoort 0110, South Africa bDepartment of Mathematical Statistics, University of the Orange Free State,

Bloemfontein 9300, South Africa

ABSTRACT

Paralysis caused by feeding female lxodes rubicundus ticks is a major problem in large areas of South Africa. As the life cycle of the tick extends over a period of 2 years, it was hypothesized that strategic treatment of sheep with an acaricide over a 2 year period, timed to kill most engorging females, should markedly lower the biotic potential of the tick. Two flocks of sheep grazing in separate paddocks known to be infested with/, rubicundus were treated either strategically or on a threshold basis (i.e. only when tick challenge exceeded a predetermined critical level in terms of paralysis) for a 2 year period. The tick burdens of untreated control sheep running with the two flocks were monitored over a 4 year period and their seasonal dynamics determined. The times at which peak infestations occurred were similar for both flocks of sheep, but significant differences in mean tick burdens between the two flocks were recorded. Tick numbers on sheep in the strategically treated flock did not decrease during the third and fourth years of the trial as was expected. Possible reasons for this were low stocking densities, especially during times of peak abundance of adults and the presence of wild hosts which maintained tick populations.

Key words: Tick control, strategic, threshold, Karoo paralysis, Ixodes rubicundus.

INTRODUCTION

Paralysis, caused by feeding female Ixodes rubicundus ticks, is a major problem in South Africa (Stampa, 1959; Spickett and Heyne, 1988). Small and large domestic

stock and wild artiodactyls may be paralysed and may suffer mortality (Spickett and Heyne, 1988; Fourie and Vrahimis, 1989; Fourie et al., 1989). The control measures practised are mainly chemical treatment (97.8%) at a frequency varying f rom one to three per season (Spickett and Heyne, 1988).

0168-8162 © 1995 Chapman & Hall

148 FOURm ET AL.

The life cycle of I. rubicundus extends over a period of 2 years (Horak et al., 1987; Fourie and Horak, 1994). Each of the developmental stages has a distinct unimodal pattern of seasonal occurrence (Fourie and Kok, 1992; Fourie et al., 1992) and survives for only one season (Fourie and Horak, 1994). Theoretically, if tick control is applied in such a manner as to kill most engorging females on domestic hosts during the activity period of the adults (April-September) and is sustained for 2 years, there should be no need to treat animals during the subse- quent 2 years. To test this hypothesis an experiment was designed in which two flocks of sheep on the same property were either treated at regular intervals or only when sheep were infested by a pre-defined number of adult ticks. Tick burdens on untreated control animals were monitored over a 4 year period and their seasonal dynamics ascertained.

MATERIAL AND METHODS

Study area The study was conducted on the farm Langberg situated approximately 25 km north of the town Jagersfontein in the south-western Free State province of South Africa. The farm comprises an area of several thousand hectares and topographically consists mainly of hilly areas (80%). The heterogeneous vegetation is classified as False Upper Karoo (Acocks, 1988). Two adjacent fenced paddocks of natural veld with a history of Karoo tick paralysis were used in the study. The paddocks were 145 and 176 ha in size, respectively.

Experimental procedures The study was conducted over a 4 year period (1991-1994). Non-gravid, tick- naive Merino ewes of similar age (12 months) were used to stock each paddock. The sheep were replaced each year but the breed and age composition remained the same. The sheep were divided into two flocks each consisting of two groups, namely an acaricide-treated and an untreated control group (n=10). The four groups were distinquished by uniquely coloured ear tags. Acaricide-treated groups were either treated according to a strategic or threshold approach (Pegram et al., 1995). In the former mentioned case the sheep were treated with an acaricide at the onset of the seasonal activity of the adult tick (April) and subsequently at 6-8 week intervals, according to the recommendations of the manufacturer. This procedure was followed throughout the activity season of adult ticks (April- September) during 1991 and 1992, respectively. The threshold flock of sheep was treated with an acaricide only when tick burdens on any one of the untreated con- trol sheep reached an infestation density of approximately 0.32 ticks/kilogramme of host body mass. This was the equivalent of approximately ten feeding ticks and is the lowest infestation density of ticks previously recorded on paralysed Merino sheep (Fourie et al., 1989). In order to ascertain the seasonal population dynamics

AN EVALUATION OF STRATEGIC AND THRESHOLD CONTROL MEASURES 149

of the ticks infesting the two flocks of sheep and to determine the timing of acari- cide application, untreated control animals belonging to both flocks were subjected to a whole body search for attached ticks. This was done at approximately 14 day intervals (April-September 1991-1994). After acaricide treatment of the thresh- old group, ten animals were also subsequently investigated at 14 day intervals for the presence of attached ticks. This was necessary in order to determine whether a second treatment was required. The threshold group was, however, only treated once during each of the years 1991 and 1992, respectively. During the 1993 and 1994 seasons both flocks of sheep, except for ten marked untreated controls, were treated with an acaricide at regular intervals in order to prevent cases of paralysis. The two flocks were still designated as either the strategic or threshold flock to prevent confusion.

Stocking rates for each paddock were in accordance with the recommendations of the Department of Technical Services and were similar for both paddocks (0.8 sheep per hectare). During 1992 and 1993 stocking rates were lowered to between 0.53 and 0.62 sheep per hectare because of a severe drought but were increased to 0.76 again during 1994. The same paddock was allocated to either the strategic or threshold flocks of sheep for the subsequent 3 years.

Statistical analysis An ANOVA on In + 1 transformed data was used to test for differences between tick counts on the untreated control animals running with the tactical and thresh- old treatment groups both within and between years. Provision was made for the effect of time through a second degree polynome. Fisher's method was used for comparisons between groups.

RESULTS

Variations in mean tick burden, within and between years, for the untreated con- trois of the threshold and strategically treated flocks of sheep are graphically pre- sented in Fig. 1. The tick burdens showed a distinct seasonal pattern. Ixodes rubicundus was first recorded on sheep during April and the infestations termi- nated during August or September. Peak infestations were present 6-8 weeks from the onset of seasonal activity and occurred during May or June (Fig. 1). The time of specific occurrence in peak infestation for both groups was similar. Rainfall dur- ing the activity period of the ticks resulted in an increase in infestation (Fig. 1).

A summary of results for the multicomparison test between groups is given in Table 1. Mean tick burdens recorded on the untreated controls for the threshold flock during the 1991 season were significantly higher than those recorded during 1992 and 1994 (Table 1). The peak infestation burdens showed a steady decline from a mean of 14.4 ticks per sheep during 1991 to only 3.9 ticks per sheep dur- ing 1994 (Fig. 1). Tick burdens recorded during 1991 on the untreated controls for

150

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FOURIE ET AL.

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Fig. 1. Mean tick burdens on untreated control sheep of the threshold and strategically treated flocks over a 4 year period (1991-1994). Vertical bars indicate rainfall during this period.

T A B L E 1

Summary of statistical comparisons between tick burdens on the untreated controls of the strategic and threshold treatment flocks between 1991 and 1994.

Description Code A B C D E F G H

Threshold 1991 A - S S S - - S S Strategic B . . . . S - - Threshold 1992 C - - S S - - Strategic D - S S - - Threshold 1993 E - - S S Strategic F - S S Threshold 1994 G - - Strategic H

(S, significant atp < 0.05.)

the strategic flock differed significantly from those recorded during 1993. During periods of peak abundance in 1993 three times more ticks were recorded on the untreated controls for the strategically treated flock than on those for the thresh- old-treated flock. Peak burdens varied markedly on the untreated controls for the strategically treated flock. The highest mean tick burden of 15.6 ticks per sheep was recorded during 1993 (Fig. 1). Differences between the tick burdens of

AN EVALUATION OF STRATEGIC AND THRESHOLD CONTROL MEASURES 151

the untreated control groups for a particular year were significant only for 1991 (Table 1). Cases of paralysis were recorded among the untreated controls of the threshold flock during 1991 and the untreated controls of the strategic flock during 1993 and 1994.

D I S C U S S I O N

The onset pattern of seasonal occurrence of I. rubicundus is similar to that recorded previously (Fourie et al., 1989; Fourie and Kok, 1992) for the Fauresmith district of the south-western Free State. The differences in the mean infestation burdens of sheep between years in the present study are also similar to those previously recorded. Since the chances of an animal becoming paralysed are related to tick burden (Fourie et al., 1989; Fourie et al., 1991) the number of paralysed sheep will also display large annual variations. The situation is further complicated in that major differences in infestation of sheep exist between paddocks on the same prop- erty. This implies that in certain paddocks acaricidal treatment to control Karoo paralysis may not be warranted during a particular year. To assist in decision mak- ing, mean tick burdens can be estimated by counting ticks on two or three body areas of the sheep and applying these to density assessment equations (Fourie and Van Zyl, 1991). This should be conducted on a weekly basis, especially during April-June, in order to prevent cases of paralysis.

The hypothesis that strategic treatment will affect the biotic potential and, hence, the population density of I. rubicundus could not be confirmed in this study. On the contrary, instead of the low burdens expected during 1993 and 1994, peak bur- dens on the strategically treated flock in 1993 were almost twice those of 1991. Several reasons can be advanced to explain the unexpected results. Firstly, wild hosts of the adult ticks, such as mountain reedbuck (Reduncafulvorufula) and cara- cal (Felis caracal) (Horak et aL, 1987), both of which frequent hilly and moun- tainous terrain, are abundant on the farm. The results indicate that the stocking density was too low to remove significant numbers of ticks and thus prevent these from attaching to and feeding on the wild hosts which would sustain or even increase the biotic potential of the ticks. Secondly, host behaviour can markedly affect tick-host contact (Randolph and Steele, 1985; Fourie and Kok, 1992). The distribution of L rubicundus is non-random and confined to hilly areas (Fourie et al., 1991). In a region with a heterogeneous topography and vegetation, various factors such as the feeding behaviour of hosts and the spatial distribution of the tick may affect tick-host sympatry and hence contact (Fourie and Kok, 1992). The implication is that questing ticks may thus feed on alternative wild hosts in suffi- cient numbers to sustain the population density. This is especially true for Merino sheep which by preference feed on the flats and only venture into hilly areas, in which the ticks occur, once winter affects the quality of grazing on the flats (Fourie and Kok, 1992).

152 F O ~ ~rAL.

The importance of stocking densities in the control of L rubicundus was real- ized many years ago. Mally (1904) correlated the occurrence of the disease with the condition of the pasture. He stated that the tick is particularly prevalent in long grass pastures and blamed understocking as one of the main causes of the prob- lem. High stocking densities, especially grazing cattle and sheep together, have been suggested to control I. rubicundus by Stampa (1959). The main objective is to graze grasses down and thereby create conditions unsuitable for tick survival. Recent studies on the ecology ofL rubicundus have indicated that the tick mainly (99%) uses grasses at a height of approximately 45 cm (range 10-97 cm) as quest- ing sites (Fourie et aL, 1991). Short questing substrates (<10 cm) depress the extent of vertical migration (Snyman et aL, 1994) and thus also affect tick-host contact.

A possible improvement to the approach adopted in this study is to use a high stocking density of grazers (sheep and cattle) as well as predominant browsers (goats) and treat these animals with an acaricide at the onset of tick activity (begin- ning of April). Since recruitment of adults from engorged nymphs occurs until May (Fourie and Horak, 1994) the stocking density should be maintained during April and May. Such an approach should limit the number of ticks attaching to wild hosts and consequently depress the biotic potential of the tick population.

ACKNOWLEDGEMENTS

Mr A.W. du Plessis of the farm Langberg is thanked for the use of his farm and sheep and for the assistance of his stockmen. The technical assistance of Messrs E. Williams and E van der Lingen is gratefully acknowledged. The research was funded by the Meat Board, the Foundation for Research Development and Hoechst Ag-Vet.

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AN EVALUATION OF STRATEGIC AND THRESHOLD CONTROL MEASURES 153

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