Biological control using sterilizing viruses: host suppression and competition between viruses in non-spatial models

Summary 1.  Research is currently underway to develop genetically engineered viruses that can sterilize pest animals. The technique, known as viral-vectored immunocontraception (VVIC), promises to control mammalian pests such as the European rabbit, the house mouse and domestic cats. 2.  Using host–parasite models we explored the degree of control of the host population that can be attained when hosts that recover from infection become permanently infertile. The models assume some demographic compensation for reduced fertility in the host population, and are tailored to address issues raised by the use of the myxoma virus as an agent to sterilize rabbits. A ‘pay-off’ function is developed, which defines the degree to which host density is suppressed by a sterilizing agent. 3.  The results show that sterilizing viruses can reduce host abundance, and that hosts with low birth rates and moderate mortality rates are the best targets for VVIC. High transmissibility increases the pay-off from VVIC, but because virulent parasites kill most of the hosts that they infect, the pay-off is highest if benign parasites are used as the vector of contraceptive antigens. We argue that appropriate pay-off functions should be developed as a basis for research and development on genetically modified organisms. 4.  The host–parasite models are extended to include a competing strain of virus that does not sterilize the host. We analysed these models using a general approach to the analysis of competition, which has not often been applied to epidemiological models. The extended model shows that host sterilization per se does not affect the competitive ability of viruses, a result that applies to a broad class of models in which the per capita growth rates of competing parasites are linear functions of limiting competitive factors.