Control of fertility in the red fox (Vulpes vulpes): effect of a single oral dose of cabergoline in early pregnancy

Abstract

Single oral doses of 100 microg kg(-1) cabergoline (CAB) were previously found to affect the reproductive success of silver fox (Vulpes vulpes) vixens from Day 28 post insemination (PI) onwards by causing abortions and postnatal mortality. The present trial investigated the effect of a single oral dose of 100 microg kg(-1) CAB given to farmed silver fox vixens at 7, 14, 21 or 28 days PI. Six vixens were dosed with CAB at each day PI and abortions, total post-partum litter loss and the combined effects of both were compared with a group of vixens that received a placebo and a control group of vixens that received no treatment. No significant difference existed between the placebo and control groups in the incidence of abortion, postnatal litter loss and litter size at 3 weeks post partum. Overall, a significantly higher frequency of abortions were detected in vixens dosed with CAB (6/24) compared with the combined placebo and control (0/41) groups (P<0.01). Cabergoline administration was significantly associated with complete litter loss (P<0.01) and the combined effects of abortion and litter loss (P<0.001). Oral CAB significantly reduced the reproductive success of vixens when given at Day 21 and Day 28 PI, but not at Day 7 and Day 14 PI.

Stability of cabergoline in fox baits in laboratory and field conditions

“…In this stability study, we used a nominal dose of 800 µg per bait, assuming that this would be the highest amount required in a control program. This was based on the bodyweight of vixens (mean and maximum weights are 5 and 8 kg, respectively: Coman 1988) and the dose rate of 100 µg kg -1 which was the effective antifertility dose in farmed vixens (Marks et al , 2002. Emesis limited the effectiveness of higher doses.…”

“…There is uncertainty about the lowest effective dose, especially as the response varies with the timing during conception, reproduction and post-natal care (Marks et al , 2002. In a field study in Victoria, Australia, in August-September 1994, baits were loaded with 170 µg of cabergoline (Marks et al 1996).…”

“…However, as foxes may have consumed more than one bait, the effective dose cannot be determined from these data. In farmed foxes, although Marks et al (2002) found that doses less than 100 µg kg -1 were considerably less effective, Lengwinat et al (2001) reported that doses in excess of 15 µg kg -1 were likely to prevent pregnancies from proceeding to term. Wild foxes may be more sensitive to the effects of cabergoline as they are more prone than captive-raised foxes to aborting litters , possibly due to greater levels of stress, which is known to contribute to reproductive failure in foxes (Hartley et al 1994).…”

“…Moreover, if rabies were to establish in Australia, foxes in urban areas could represent a reservoir for this disease, making the spread of rabies difficult to limit (Marks and Bloomfield 1999). Additional techniques for the control of foxes, particularly those in urban areas, are needed, and fertility control, using agents such as cabergoline, may be effective (Marks et al 1996(Marks et al , 2002Pech et al 1997;McLeod and Saunders 2001).…”

“…It acts by reducing the secretion of prolactin, which in foxes has an important luteotropic effect and is essential for the maintenance of pregnancy. A single oral dose of cabergoline 0.1 mg (100 µg) kg -1 can cause abortions and postnatal cub mortality from at least Day 21 of the 52-day pregnancy (Marks et al , 2002.…”