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Title: DIAGNOSIS OF AMITRAZ RESISTANCE IN BOOPHILUS MICROPLUS IN NEW CALEDONIA WITH THE MODIFIED LARVAL PACKET TEST

Author
item DUCORNEZ, S - IAC, PAITA, NC FRANCE
item BARRE, N - IAC, PAITA, NC FRANCE
item Miller, Robert
item GARINE-WICHATITSKY, M DE - IAC, PAITA, NC FRANCE

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2005
Publication Date: 6/30/2005
Citation: Ducornez, S., Barre, N., Miller, R., Garine-Wichatitsky, M. 2005. Diagnosis of amitraz resistance in Boophilus microplus in New Caledonia with the modified Larval Packet Test. Veterinary Parasitology. 130:285-292.

Interpretive Summary: In New Caledonia, France the southern cattle tick represents the most important pathological constraint to livestock production in the country. In order to control these ticks, the government provides monitoring services and pesticides to its ranchers. Amitraz is a pesticide that has been used in New Caledonia for approximately 5 years and some treatment failures have occurred. However, it has not been possible to test ticks for resistance to amitraz because no technique existed that could measure amitraz resistance in ticks. In 2002 scientists at the USDA, ARS laboratories in Kerrville and Edinburg TX developed a technique that would detect amitraz resistance in ticks. This research was conducted in New Caledonia to determine if the USDA technique was usable in this country and to determine if resistance to amitraz was present in New Caledonian tick populations. This study found that the USDA technique was usable in New Caledonia. Tests on susceptible laboratory strains in New Caledonia produced predictable and repeatable results like those obtained in the USA. The testing of many populations of field-collected ticks from New Caledonia showed that many were susceptible to amitraz. However, 2 populations tested were found to be resistant to amitraz. This research benefited New Caledonia by isolating populations of amitraz resistant ticks which could then be eradicated before amitraz resistance spread throughout the country. This strategy has the potential to save the New Caledonian tick control program money since amitraz is an inexpensive product relative to the alternatives available on the market. Additionally, proper treatment of cattle leads to lower tick burdens and healthier cattle. These factors work together to increase the profits of producers. If New Caledonia is able to prevent the spread of amitraz resistance through detection and eradication of resistant tick populations, then other scientists can use the New Caledonian experience to delay or prevent the spread of amitraz resistance in other counties. This is especially true of the USA-Mexico interaction. The southern cattle tick is in Mexico and many populations are developing resistance to amitraz. It is important that we work together to find and prevent the spread of amitraz resistance or else we will loose an important weapon in the control of these ticks.

Technical Abstract: B. microplus represents the most important pathological constraint to livestock production in New Caledonia. The infestation of cattle by ticks is controlled by chemical applications on a regular basis. Only two acaricides are currently used in the country: deltamethrin in 46% of the cattle production facilities and amitraz in the premises where resistance to deltamethrin has been diagnosed. The modified LPT (Larval Packet Test) has been used to conduct a survey for amitraz resistance in New Caledonia. In 2003, ticks from 29 farms including farms using deltamethrin (n=8) or amitraz (n=21) were collected. Fourty-five modified LPTs concerning 18 different tick populations produced valuable results (probit analysis). Sixteen populations were defined susceptible to amitraz and two populations were considered amitraz-resistant (Gadji and Néty). For these two populations, the LC90 estimates were higher than the values observed for the susceptible populations. The slopes generated from the susceptible populations were relatively high with a mean value (SE) of 3.8 (+/-0.4). A decrease in the slope of the regression line was observed for the Gadji population (1.9 (+/-0.1)), but it was not the case for the Néty population. Furthermore at 0.03% amitraz, for the susceptible population, the percentage of mortality observed was 100% in the 3 replicates (or sometimes between 98 and 100%). For the Gadji and Néty populations, at the same concentration, a non-negligible percentage of larvae were still alive. This is the first description in New Caledonia of populations of B. microplus resistant to amitraz using the modified LPT. A thorough survey of tick susceptibility to amitraz in cattle farms of the country should be conducted to assess the presence of amitraz-resistant populations in other areas. The emergence of amitraz resistance after a relatively short time of utilisation of this molecule has some important consequences on the strategy and organisation of tick control in New Caledonia and this paper discuss some of the urgent actions which should be undertaken.