Methods

Abscess data were collected from 22 painted turtle (Chrysemys picta) populations in central Minnesota. Turtles were captured from May through August 2001 and 2002, May 2003, and August 2004 using basking traps, baited hoop traps, and by hand. Front claw length and position of the cloaca relative to the rear edge of the carapace were used to classify each captured turtle as male or female (Gibbons and Lovich 1990). Turtles with no discernable secondary sex characteristics and a carapace length (CL) < 9–10 cm were considered juveniles (Ernst et al. 1994). Turtles received a permanent, unique identification code drilled into the marginal carapace scutes so they could be identified if recaptured. Every captured turtle that possessed an aural abscess was noted. Contingency table analysis was used to determine if there was a nonrandom association between the presence of an abscess and the type of trap in which a turtle was captured.

Results

Turtles with abscesses occurred in 9 of the 22 sampled populations. Nineteen out of 2703 painted turtles marked between 2001 and 2004 possessed aural abscesses (0.7 %). The incidence of abscesses varied among the 9 populations that had abscessed turtles from 0.5% to 9.1% (Table 1). All abscessed turtles appeared otherwise healthy and none showed signs of lethargy or other illness. Nine turtles had an abscess on the right side of the head, 8 turtles had one on the left side, and in 2 animals the side was not recorded. All but one of the turtles captured with abscesses were adults. The smallest turtle, a juvenile, had a CL of 94 mm. The remaining turtles had CL ranging from 97 to 168 mm. Two individuals with abscesses were captured multiple times. The first was a male captured 3 times in 2002 in Lake Maria (Wright County). The second was an adult female first captured in 2002 in Beaver Lake (Stearns County) and subsequently recaptured in 2004 with no apparent change in the abscess. Turtles with abscesses were captured in every month trapping was conducted.

Table 1. Incidence of aural abscesses in painted turtles captured in central Minnesota lakes from 2001 to 2004.

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Turtles with abscesses were caught in basking traps at a slightly higher rate than in hoop traps, compared to turtles without abscesses. The basking trap/hoop trap catch ratio was 7.7:1 for turtles with abscesses and 4.7:1 for turtles without abscesses, although this difference was not statistically significant (Pearson chi-square contingency table analysis, χ² = 0.631 p = 0.4271).

Discussion

The incidence of aural abscesses in painted turtles was variable among populations in central Minnesota. The only other account of abscesses in painted turtles appears to be Christiansen et al. (2005) reporting 1 abscess in 212 captured turtles (0.47%) along the Mississippi River in Iowa. The prevalence of aural abscesses in other emydid turtle species is quite variable: 26 of 2477 (1.0%) in Florida box turtles (Terrepene c. bauri) in Florida (Dodd and Griffey 2004); 5 of 19 (26.3%) in eastern box turtles (Terrepene c. carolina) in Virginia (Holladay et al. 2001); 19 of 90 (21.1%) in eastern box turtles in New York (Tangredi and Evans 1997); and 16 of 460 (3.47%) in ornate box turtles (Terrepene ornata) in Iowa (Christiansen et al. 2005). The prevalence of abscesses in Minnesotan painted turtle populations was within the range of published accounts, although some populations have much higher rates than others, such as Lake Sylvia in Stearns County, where 9.1% of 33 captured turtles possessed abscesses.

Turtles bioaccumulate several pollutants (Helwig and Hora 1983), and organochlorine compounds have been implicated in the occurrence of aural abscesses in box turtles (Tangredi and Evans 1997; Holladay et al. 2001). Organochlorine compounds are thought to interfere with vitamin A metabolism, a shortage of which can predispose turtles to aural abscesses (Murray 1996; Brown et al. 2004). Because the link between abscesses and organochlorine compounds is far from conclusive (Willer at al. 2003), it is important to identify average abscess rates in wild turtle populations so that findings on contaminants can be put into proper context. Knowledge of the average rate of aural abscesses in turtles will allow researchers to identify populations with unusually high abscess rates that may require further investigation. Populations like the one in Lake Sylvia in this study would be a candidate for such further investigation, which would involve submitting turtle liver biopsies and water samples for organochlorine analyses.

Painted turtles increase body temperature in response to bacterial infection (Monagas and Gatten 1983). Because basking is an important means of thermoregulation, one would expect more frequent or longer basking in turtles with abscesses compared with healthy turtles. Frequent basking would make turtles more likely to be captured in basking traps as compared to hoop traps. The failure to find significant differences in capture rates of turtles with abscesses in basking traps compared to turtles without abscesses is surprising given the different behaviors that influence the efficiency of each trap style. However, it is possible that turtles with abscesses bask for longer periods of time rather than more frequently, which would not be reflected in increased capture rates. Further work is needed to determine how painted turtles alter behavioral or basking patterns in response to bacterial infection.