Performance and humaneness of chloropicrin, phosphine and carbon monoxide as rabbit-warren fumigants

Abstract

Concerns about the humaneness and efficacy of chloropicrin (CLPN) and phosphine (PH3) as warren fumigants for the control of European rabbits (Oryctolagus cuniculus) prompted this investigation into the field performance and humaneness of carbon monoxide (CO) as an alternative fumigant. Comparative trials were performed in a naturally formed 1.3-m3 warren fitted with gas, temperature and humidity sensors as well as infrared cameras and microphones. Trials used concentrations of 5% and 6% CO introduced at 400 L min−1, and standard field practices for CLPN and PH3. Rabbits exposed to CLPN displayed signs of intense irritation and extreme distress during a lethal toxicosis lasting a mean of 82.5 min, supporting previous conclusions that CLPN causes suffering. Phosphine gas killed 10 of 12 rabbits, in a mean of 225.3 min. This relatively poor performance was attributed to the low rate of gas production and passive diffusion through the warren, reflected in highly variable warren concentrations between trials. Phosphine caused greater agitation on the onset of first symptoms than did CO; however, the behavioural symptoms after collapse were broadly similar. Carbon monoxide dispersed evenly in the warren, caused no immediate irritation or distress and produced a gradual stupor before unconsciousness and death. Concentrations of 6% CO killed rabbits almost three times faster than 5% CO, suggesting that relatively small increments of concentration and exposure time are crucial in obtaining a consistently rapid death. A concentration of 6% CO caused death in 8 of 10 rabbits in a mean of 28.3 min; some 2.9 and 8 times faster than CLPN and PH3 respectively. The time from collapse until death did not appear to be appreciably different for PH3 and 6% CO. The present study demonstrated that CO has potential as a rapid-acting, humane and effective fumigant for rabbit control.