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Four populations of small wild British rodents were studied by capture--re-capture methods over a period of three years. Samples of blood were taken from these animals and tested for antibodies to nine viruses. Animals were removed from another 11 sites around the UK, and immunosuppressed. Samples of tissue from these animals were tested for the presence of viruses by passage in laboratory mice and serum samples from some of them were tested for antibody to the nine viruses. Indications were found of the possible influence of epizootic outbreaks of certain diseases on animal populations.

SUMMARY: Heat inactivation curves of vaccinia virus between 50' and 60' indicate that the virus is heterogeneous in its heat sensitivity. The proportion of heatresistant particles varies inversely with the temperature of exposure. The inactivation of heat-sensitive virus is temperature dependent and seems to be a first-order reaction, while the heat-resistant fraction is inactivated at a constant slow rate unrelated to temperature over the range 5Oo-6O0.The work reported here is part of a larger investigation of virus inactivation by physical agents including ultraviolet and ionizing radiations, under conditions that do not destroy antigenicity (McClean, 1945;Collier, McClean & Vallet, 1956). The heat inactivation of vaccinia virus was determined as a preliminary to testing the protective antigenicity of heat inactivated vaccines. METHODS Virus.Most of the experiments were done with the Lister Institute strain of vaccinia virus adapted to the chick embryo; two experiments were done with the WR mouse neuro-adapted strain of virus. Infected chorioallantoic membranes were extracted in dilute McIlvaine buffer by grinding with sterile powdered neutral glass, Extracts were centrifuged at l O O O g for 15 min. to clear them of debris, titrated and stored at 4". For inactivation, extracts were diluted to a titre between about lo7 and lo8 infective units/ml. Experimental procedure. One ml. samples of virus were heated in small, thin-walled test tubes in a water bath accurate to +0-2'. Samples were removed from the bath at intervals and immediately cooled in a melting ice bath. They were stored at 4" until infectivity titrations were made: this was generally within a few hours, but on a few occasions there was an interval of 2 or 8 days.Infectivity titrations were made by pock counts on the chorioallantoic membranes of 12-or 13-day chick embryos. The refinements of membrane handling technique described by Westwood, Phipps & Boulter (1957) were used. In titrations of WR strain 3-day instead of the usual 2-day incubation was used, because after 2 days the pocks were pale and poorly developed.Diluent. All extractions and dilutions were made in McIlvaine's phosphate + citrate buffer, pH 7.2, 0-004 M-phosphate.In graphs of the results each point is the arithmetic mean of at least four experiments unless otherwise stated. The results are plotted as log Vo/V against time, where Vo is the original concentration and V the concentration at any time t. This enables all the inactivation curves to be given a common

The complications of vaccination against small pox are discussed in relation to the contemplated use as vaccines of recombinant vaccinia viruses carrying the genes for "protective" antigens derived from a range of pathogens. Recombinant vaccines are potentially extremely valuable instruments in the fight against infectious diseases, but caution is needed in their deployment. In addition to the dangers associated with the pathogenicity of various strains of vaccinia virus, there may be problems related to the ecology of the poxviruses--especially orthopoxviruses. Before recombinant vaccinia virus vaccines are widely used, ecological research is urgently needed. It should cover not only the ecology of orthopoxviruses, but also possible interactions between engineered vaccinia viruses released into the environment and wild viruses which may be resident in both target and non-target species in a wide selection of habitats.

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