Tetracyclines as a Fluorescent Marker in Bones and Teeth of Rodents

Abstract

Tetracyclines are antibiotics that chelate with calcium ions to induce a golden-yellow fluorescence in bones and teeth under UV light ( 3600-3700 A ) . Three of these compoundsdemethylchlortetracycline ( DMCT ), doxycycline monohydrate ( DC ), and tetracycline hydrochloride (TC) were tested on laboratory rats as potential markers for field studies with rodents. DMCT proved the superior compound. A single oral dose of 50 mg/kg of body weight labeled the lower jaws of white laboratory rats for up to 6 months. Uncleaned jaws frozen for 6 months showed no degradation of fluorescence. The fluorescent mark was detectable on the jaws 1 day after rats ingested DMCT bait, and was visible in the incisors (and thus detectable without dissection) between 2 and 8 weeks after ingestion. Exposure of a DMCT oat groat bait to sunlight and moisture for 7 days did not reduce marking ability. Preliminary laboratory results indicate that DMCT labels the bones and teeth of four species of wild rodents much as it does in laboratory rats. These data suggest that DMCT in bait is a promising marker for field studies of rodents. A major segment of rodent control research is oriented towards development of methods to determine (1) the percent of animals that consume field baits, (2) the movement of individuals into or out of baited areas, and (3 ) the elapsed time since bait ingestion. Sampling by liveor killtrapping may indicate population changes related to control operations, but does not reveal if survivors received a sublethal dose or even if they found the bait. Dyes added to the bait help to detect bait ingestion, but are short-lived (Nass and Hood 1969:588). Tagging animals with leg bands, ear tags, etc., are techniques for studying movement, but require many man-hours for sighting, retrapping, and record keeping. Radio telemetry is an extremely useful tool in following movement, but usually samples a relatively small segment of a population. In conjunction with these techniques, a perrnanent tracer that could be incorporated into a well-accepted bait would be a definite advantage in research on the control of field rodents. Tetracyclines are a group of antibiotics that chelate with calcium ions in bones and 1 Present address: Bureau of Sport Fisheries and Wildlife, P. O. Box 244, Hilo, Hawaii 96720. teeth. Milch et al. ( 1957 ) reported that tetracycline in tissue is indicated by a golden-yellow fluorescence under ultraviolet light. Weber and Ridgway (1962) used tetracyclines to label scales and bones of sockeye salmon (Oncor7zynchus nerka), king salmon (Oncorhynchus tshaunytschu), and rainbow trout ( Salmo gairdrzeri ) . Linhart and Kennelly ( 1967:320 ) incorporated a tetracycline into antifertility baits for coyotes ( Canis latrans) . Their data showed that coyotes treated with a single oral dose of 10 mg,/kg were marked for up to 5 months. This study demonstrates that demethylchlortetracycline shows real promise as a rodent marker. The assistance received from P. L. Hegdal, S. B. Linhart, and H. P. Tietjenn of the Denver Wildlife Research Center, is gratefully acknowledged. Special thanks are extended to J. L. Oldemeyer for statistical analysis of data. METHODS All tests were conducted with \5listar white rats ( 6-month-old males unless otherwise noted). Only adults were used because the quantity and intensity of fluorescence is greater in younger animals ( Mi]ch