Factors influencing the fecal egg and oocyst counts of parasites of wild European rabbits Oryctolagus cuniculus (L.) in Southern Western Australia.

Abundance of intestinal parasites was monitored by fecal egg and oocyst counts for samples of wild rabbits Oryctolagus cuniculus with different levels of imposed female sterility from 12 populations in southwestern Australia. Differences in egg counts of Trichostrongylus retortaeformis between seasons and age groups were dependent on the sex of the host. Pregnancy may have been responsible for these differences because egg counts were consistently higher in intact females than in females surgically sterilized by tubal ligation. Egg counts for Passalurus ambiguus were influenced by season and host age but there were no differences between sexes or between intact and sterilized female rabbits. No differences were detected in the oocyst counts of the 8 species of Eimeria between male and female rabbits or between intact and sterilized females. Seasonal differences were detected in oocyst counts of Eimeria flavescens and Eimeria stiedai. The overwhelming determinant of coccidian oocyst counts was host age, with 6 species being much more abundant in rabbits up to 4 mo of age. There was a suggestion that egg counts of T. retortaeformis and oocyst counts of several species of Eimeria were reduced in populations where rabbit numbers had been depressed for at least 2 yr, but there was no evidence that short-term variations in rabbit numbers had a measurable effect on parasite abundance.

Evaluation of the association of parasitism with mortality of wild European rabbits Oryctolagus cuniculus (L.) in southwestern Australia.

Abundances of the parasitic nematodes Trichostrongylus retortaeformis and Passalurus ambiguus, and 8 Eimeria species were estimated by fecal egg and oocyst output in 12 discrete free-ranging populations of wild rabbits (Oryctolagus cuniculus) in southwestern Australia. Comparisons of parasite egg and oocyst counts were made between those rabbits known to have survived at least 2 mo after fecal samples were collected and those rabbits that did not survive. There were significant negative relationships between parasite egg and oocyst counts and survival when all age groups and collection periods were pooled for several species of coccidia and for T. retortaeformis. However, when the same comparisons were made within rabbit age groups and within collection periods, there were very few significant differences even where sample sizes were quite large. The differences indicated by the pooled analysis for coccidia were most likely due to an uneven host age distribution with respect to survival, combined with an uneven distribution of the oocyst counts with rabbit age. The result for T. retortaeformis was similarly affected but by a seasonal pattern. Parasitism by nematodes and coccidia did not appear to be an important mortality factor in these rabbit populations, at least at the range of host densities we examined. This suggests that other factors must have been responsible for the observed pattern of density-dependent regulation in these rabbits.

Fertility control of overabundant species; Can it work for feral rabbits?

Demographic changes were monitored in free-ranging European rabbit (Oryctolagus cuniculus) populations in Australia following the surgical imposition of four levels of female sterility (0%, 40%, 60%, 80%). Rabbit productivity decreased with increasing sterility level, but a greater proportion of offspring were recruited into populations with high levels of sterility. Adult rabbits, particularly sterile females, also survived better in the high sterility populations. Thus we were able to experimentally demonstrate that two density-dependent processes were operating on our rabbit populations: one was acting on juvenile survival, the other on the survival of infertile adult rabbits. However, these compensatory mechanisms were insufficient to overcome the effects of sterility in the high sterility populations and the seasonal abundance of rabbits decreased. Female sterility levels of 60%-80% would benefit rabbit control programs by reducing the peaks of rabbit abundance. This effect is likely to be enhanced if fertility control could be integrated with other pest control strategies.

Fluoroacetate content of some species of the toxic Australian plant genus, Gastrolobium, and its environmental persistence.

Gas chromatography confirmed the relatively high concentrations of fluoroacetate found in toxic Gastrolobiums, a genus of indigenous Australian plants. Fluoroacetate concentration in these plants ranged from 0.1 to 3875 micrograms/g (ppm) dry weight, with young leaves and flowers containing the highest concentrations. However, there was considerable intrastand variation between individual plants of at least two species with coefficients of variation ranging from 94% to 129%. Despite the high concentrations of fluoroacetate in many species, only one of nine soil samples collected from beneath these plants contained fluoroacetate. None of the 16 water samples collected from nearby streams and catchment dams contained fluoroacetate. This suggests that fluoroacetate does not persist in this environment. Fluoroacetate was also found in the genus Nemcia, and very low levels of fluoroacetate (ng/g) were detected in the foodstuffs, tea and guar gum. The latter indicates that other plant species may produce biologically insignificant amounts of fluoroacetate.

Physical versus chemical defence mechanisms in toxic Gastrolobium.

The degree to which physical defence mechanisms are present in toxic species of Gastrolobium was compared with the known fluoroacetate (the toxic principle) concentrations of these plants using both histological leaf sections prepared from fresh leaves (4 species), and a variety of visual external traits measured from herbarium specimens (28 species). There was a strong negative correlation between the presence of physical deterrents (e.g. area of fibres, number and length of spines) and the fluoroacetate concentration of each species. This suggests that, with respect to their leaves, individual species have established a compromise between producing physical grazing deterrents and the adoption of chemically mediated antiherbivore strategies.

The effect of sub-lethal doses of bromadiolone on the breeding performance of house mice (Mus domesticus).

Both laboratory and field strains of Mus were exposed to wheat containing 0.0001% bromadiolone under laboratory and outdoor conditions, respectively. While both strains readily consumed the poisoned wheat, ad libitum sub-lethal doses of this anticoagulant equating to between 20% and 70% of the acute LD50 per feed had little apparent effect on the breeding performance of these mice. The implications of these findings are discussed with respect to the use of anticoagulants as pesticides.

Metabolism of fluoroacetate in the skink (Tiliqua rugosa) and the rat (Rattus norvegicus).

Administration of 100 mg sodium fluoroacetate (compound 1080) per kilogram body weight to T. rugosa resulted in a 3.4-fold increase in plasma citrate levels 48 h after dosing while administration of 3 mg sodium fluoroacetate per kilogram body weight to R. norvegicus produced a fivefold increase in plasma citrate levels within 4 h. Administration of 300 mg sodium fluoroacetate per kilogram body weight reduced the oxygen consumption of the skink by between 2.5 and 11% while in the rat, 2 mg sodium fluoroacetate per kilogram body weight reduced oxygen consumption by between 28 and 57%. Aconitate hydratase activity in extracts of liver acetone powders from T. rugosa was less inhibited by (-)erythrofluorocitrate (Ki: 0.065 mM) than that in extracts derived from R. norvegicus (Ki: 0.026 mM). The rate of defluorination of fluoroacetate in erythrocytes and in extracts of liver acetone powders of T. rugosa was 8- and 4.5-fold greater, respectively, than that found in similar preparations from R. norvegicus. A rapid rate of defluorination together with a low reliance on aerobic respiration favoured detoxification of fluoroacetate in T. rugosa rather than its conversion into fluorocitrate. Though defluorination in this species helped to minimize the immediate effects of fluoroacetate on aerobic respiration, it resulted in rapid depletion of liver glutathione levels.

Significance of sulfhydryl compounds in the manifestation of fluoroacetate toxicity to the rat, brush-tailed possum, woylie and western grey kangaroo.

Levels of citrate in kidneys and livers of rats with normal glutathione levels increased 6.8 and 1.7-fold respectively 2 h after dosing with 1.5 mg of compound 1080 (= 95% sodium fluoroacetate) per kilogram body weight. In animals with liver glutathione levels 15% of normal, increases in plasma and liver citrate levels after dosing with fluoroacetate were significantly greater than those of control animals. Cysteamine and N-acetylcysteine, like glutathione, partially protected aconitate hydratase from fluorocitrate inhibition in rat liver preparations but were unable to replace glutathione as a substrate for the defluorination of fluoroacetate in vitro. N-Acetylcysteine did not diminish plasma citrate levels of glutathione-deficient rats dosed with fluoroacetate, while cysteamine inhibited the rate of in vivo defluorination in glutathione-deficient brush-tailed possums. It is suggested that non-physiological sulfhydryl compounds are ineffective antidotes to fluoroacetate intoxication in vivo. The in vivo defluorination patterns of four mammal species with differing sensitivities to fluoroacetate did not indicate a direct relationship between tolerance and rate of defluorination and it is also suggested that a high level of activity of the glutathione-S-transferase responsible for the defluorination of fluoroacetate is not the major mechanism for circumventing fluoroacetate toxicity in resistant mammals.