Coevolution of host and virus: the pathogenesis of virulent and attenuated strains of myxoma virus in resistant and susceptible European rabbits.

Myxoma virus was introduced into the European rabbit population of Australia in 1950. Although the virus was initially highly lethal in rabbits, there was rapid selection for less virulent strains of virus and innately resistant rabbits. To investigate the basis of resistance to myxoma virus, we have compared the pathogensis of the virulent strain of myxoma virus originally released into Australia and an attenuated, naturally derived field strain of myxoma virus. This was done in laboratory rabbits, which have not been selected for resistance, and in wild rabbits that have developed significant resistance. Wild rabbits were able to recover from infection with virus that was always lethal in laboratory rabbits. Laboratory rabbits were able to control and recover from infection with attenuated virus. This virus caused a trivial disease in wild rabbits. There was little difference between laboratory and wild rabbits in titers of either virulent or attenuated virus in the skin at the inoculation site. However, resistant wild rabbits had a 10- to 100-fold lower titer of virulent virus within the lymph node draining the inoculation site and controlled virus replication in tissues distal to the draining lymph node. Replication of virus in lymphocytes or fibroblasts cultured from wild and laboratory rabbits demonstrated that resistance was not due to altered cellular permissivity for replication. Neutralizing antibodies were present in both susceptible and resistant rabbits, suggesting that these have no significant role in resistance. We hypothesise that resistance is due to an enhanced innate immune response that allows the rabbit to mount an effective cellular immune response.

Origin and transmission of AIDS. Multi-use hypodermics and the threat to the Soviet Union: discussion paper.

PIP: Health authorities in the Soviet Union have attempted to downplay the prevalence of infection with human immunodeficiency virus (HIV) in the Soviet population and to suppress the publication of scientific articles on acquired immunodeficiency syndrome (AIDS). Nonetheless, there is some evidence that the USSR may have more AIDS cases than any other country in Europe. Particularly prevalent are cases of Kaposi's sarcoma in persons under 40 years of age. It is hypothesized that the multi-use of medical hypodermic needles represents the major vehicle of HIV transmission in the Soviet Union. Also increasing at an alarming rate in the USSR is the prevalence of hepatitis B infection, and this phenomenon appears related to the low quality of sterilization of instruments in USSR medical facilities and the reuse of hypodermic needles. Not only are single-use hypodermic needles unavailable, but there is a severe shortage of nondisposable needles. In most Western countries, the availability of disposable needles and sterilization equipment has confined needle-transmitted HIV infection to the intravenous drug user population. If the further spread of HIV infection in the Soviet Union is to be prevented, it is essential that medical facilities in areas where hepatitis B is widespread adopt the use of single-use hypodermics.^ieng

Epidemics in competition.

When two or more epidemic agents are simultaneously present in a population, they may interact to increase or decrease each other's effectiveness. One form of interaction is "competition" where each agent confers immunity to the others. Such competition occurs, for example, between different strains of myxomatosis in rabbit populations. We consider some consequences of introducing competition into mathematical epidemic models. Both deterministic and stochastic simple epidemic models are examined. In either case the conclusions are similar: the faster spreading epidemic has a considerable advantage.