AN IN VITRO LARVAL IMMERSION MICROASSAY FOR IDENTIFYING AND CHARACTERIZING CANDIDATE ACARICIDES

Authors: WHITE, W. HUNTER - ELANCO, GREENFIELD, IN; PLUMMER, PHILIP - ELANCO, GREENFIELD, IN; KEMPER, CONNIE - ELANCL, GREENFIELD, IN; Miller, Robert; Davey, Ronald; KEMP, DAVID - CSIRO, BRISBANE AUSTRALIA; HUGHES, SUZANNE - QDPIF, BRISBANE AUSTRALIA; SMITH II, CHARLES - ELANCO, GREENFIELD, IN; GUTIERREZ, JESUS - ELANCO, GREENFIELD, IN

Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2004
Publication Date: 11/15/2004
Citation: White, W.H., Plummer, P.R., Kemper, C.J., Miller, R.J., Davey, R.B., Kemp, D.H., Hughes, S., Smith II, C.K., Gutierrez, J.A. 2004. An in vitro larval immersion microassay for identifying and characterizing candidate acaricides. Journal of Medical Entomology. 41(6):1034-1042.

Interpretive Summary: There are millions of unevaluated chemicals that may have potential for the control of pest tick species. The current discovery and evaluation process is time consuming and expensive. Any development that can shorten this process will allow for more chemicals to be evaluated increasing the potential for the discovery of highly valuable products to control ticks. This research has developed new technology that allows for the rapid evaluation of many chemicals with many different species of pest ticks. This technology also has the potential to become almost completely automated through the use of robotics. This will allow bioassays to be preformed on a 24 hour basis with very little human effort. The control of ticks is critical for the prevention of many human and animal diseases. Lyme disease, Rocky mountain spotted fever, and ehrlichosis are all diseases transmitted by ticks to humans within the United States. In addition, the maintenance of the cattle fever tick eradication program is based on the use of efficacious pesticides to prevent the importation of ticks on cattle from Mexico and to eradicate populations of ticks that become established within the United States.

Technical Abstract: Acaricide resistance in ixodid ticks such as Boophilus microplus (Acari: Ixodidae) presents a growing threat to the agricultural animal industry worldwide. New acaricides that are not subject to cross resistance with existing acaricide classes need to be identified and developed. Candidate drug identification often requires the screening of compound libraries for molecules with desired biological properties. While a variety of whole organism in vitro bioassays have been described for evaluating acaricide susceptibility of ticks, the Food and Agriculture Organization (FAO) larval packet test (LPT) is the approved standard technique. However, none of these bioassays, including the LPT, offers the combination of sensitivity and speed required to expediently screen larger numbers of experimental compounds for acaricidal activity. We have optimized a larval immersion micro assay (LIM) that offers superior sensitivity, the flexibility to accommodate multiple formulations, and a robust capability for rapidly screening many compounds with a minimal requirement of test article for evaluation. Dose response studies were conducted for representative members from the organophosphate, pyrethroid, pyrazole, carbamate, macrocyclic lactone and formamidine chemistries against Amblyomma americanum. Time response experiments revealed that permethrin was the most rapid acting acaricide, while fipronil had the slowest speed of kill against A. americanum in the LIM. Comparison of drug susceptibility profiles between multiple ixodid ticks suggests that A. americanum is an effective model for predicting compound potency against Boophilus spp. in this bioassay. The LIM is suitable for the identification and characterization of active molecules from small and medium sized compound or natural product libraries, and can be a useful tool to prioritize molecules for further in vivo testing in animal models.