Factors related to cattle infestation level and resistance to acaricides in Boophilus microplus tick populations in New Caledonia

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Abstract

Boophilus microplus, even in the absence of babesiosis, is a major disease of the cattle in New Caledonia where the particular farming system associates continental European breeds and a tropical climate tempered by the Pacific Ocean. In order to have a better understanding of the factors involved in cattle tick infestation, to decrease the possible wastage and use of chemicals and to increase the lifetime of the acaricides, the veterinary authorities investigated the conditions of the chemical treatments. A survey among 148 cattle farms of the whole of New Caledonia was carried out and factors that explain the development of tick resistance and cattle infestation have been determined. From this survey, three typologies for the main characteristics of the farms have been set up, the technical practices of the farmers and the tick control measures applied by the farmers, respectively. Some variables are significantly associated with the tick resistance to deltamethrin but their contribution to the explanation model is always moderate: farms in the south, with a positive resistance gradient from east to west, absence of bush fire and membership to a cattle farmers organization. The more the farmers have intensified their breeding—male castration, weaning, heifer separation, drenching, etc.—and pasture—high stock rate, mowing, extra feeding of the cows, many paddocks, etc.—techniques, the higher was the probability for the ticks in their farm to be resistant to deltamethrin. The technical details of the acaricide treatment had a low contribution to the explanation model. However, the use of a spray generated more resistance than a dip. Furthermore, there is a negative resistance gradient when the farmers increased the treatment interval average. Considering infestation, none of the variables from the three typologies were associated with the two infestation variables (1: semi-engorged tick females and 2: other ticks) at the herd level. However, the seven studied variables—the three typologies, breed, age, body condition score and breeding status—affected significantly the two infestation variables at the cow level, but their predictive ability remained very low (R2<3.5%). This result—individual effect more important than herd effect on the infestation—is confirmed by the importance of the variance of the intra-farm factors (99%) when compared with inter-farm factors (1%). Cows of Charolais breed, in poor body condition, old, pregnant or lactating, and those of the farms with irrational and high pressure control of ticks are the most infested.

Introduction

The cattle tick (Boophilus microplus) imported in New Caledonia from Australia by American troops during the last world war (Rageau and Vervent, 1959) was free of the diseases usually transmitted by this tick. The main cattle bred by farmers are Bos taurus breeds (mainly Limousin cattle) highly susceptible to ticks (Wharton et al., 1973) and therefore, we meet on this island some particular conditions for the development of an efficient couple host–parasite relationship. Tick control is organized at the territory level and the acaricide is freely delivered to cattle farmers according to the size of their herd and the type of treatment unit. This strategy is based on the initial eradication program with one treatment a month during the whole year. The main advantage of this method is that all the farmers will treat their cattle with the same acaricide chosen for the whole territory allowing for easier control and a more efficient assistance to the farmers to overcome their tick problems. On the other hand, this method is very expensive for the community and quite far from the currently recommended, environmentally safer integrated methods of tick control. Therefore animal health authorities decided to investigate the conditions of the chemical treatments to try to decrease the possible wastage and the use of chemicals.

Furthermore, the generalization of the cattle tick resistance against deltamethrin (Butox™) since the first cases of resistance diagnosed in 1992 (Beugnet and Chardonnet, 1995) led New Caledonia to investigate in further detail the process of this resistance. In particular, the knowledge of the conditions leading towards the resistance of the ticks to that acaricide in order to increase the lifetime of the next acaricides is required. The current resistance status of the New Caledonian cattle ticks against the previously used acaricide, ethion (Rhodiacide™), was also studied. At the same time, it appeared useful to have a better understanding of the factors involved in cattle infestation by this tick.

The present paper reports the methodology and results of a survey on a sample of cattle farms of the whole of New Caledonia. The objective is to identify the factors or groups of factors that could be valuably associated with tick resistance and cattle infestation.

Section snippets

Description of the survey

The survey was carried out from March to July 1998. Ten percent (n=148) of the 1500 cattle farms of New Caledonia were submitted to a detailed investigation. These 148 farms were randomly chosen among the registered farms of the territory and visited one time only. With the assistance of the field veterinarians and technicians of the sector, the survey staff (usually one veterinarian and one technician) investigated the farms the day of dipping scheduled freely by the farmers, but at least 4

Typology 1: main characteristics of the farms

Six different classes were distinguished. The first class (frequency of 8.8%), composed of the “East Coast farmers”, groups together some aged farmers of this humid and quite remote coast. They are traditional farmers (using bush fires), not members of any cattle association. The second class (2.7%) represents the small very specific group of the “Government farms”. The third class (23.7%) is the “Small farmers of the West Coast”. The characteristics of this class are a pasture surface <100 ha,

Factors related with tick resistance to acaricides

The deltamethrin resistance is associated with different factors taken into account in this survey (Table 6). The geographic location of the farm is the most important of them. This result has been previously cited by different authors (Roulston et al., 1981, Jonsson et al., 2000). It is not clear why the farms in the southwest show a higher rate of resistance than in other areas. Some explanations may be proposed to explain the role of this factor (Sutherst and Comins, 1979). (1) Common

Acknowledgements

We wish to thank the Government of New Caledonia who supported this work with the agreement of the partners of the “Comité de lutte contre les tiques”. Thanks are due to the field province technicians and veterinarians that helped us in the organization of the survey. Thanks also to the IAC technicians Joel Mathieu, Jean-Claude Hurlin, René Guiglion, who assisted us in our visits of farmers and Laurent Chardonnet and Huguette Gaı̈a, who made the laboratory resistance tests, and to all the

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