Diseases caused by filterable viruses occur widely in the animal and vegetable kingdoms, causing serious economic loss.
Unlike bacteria, filterable viruses required living cells in which to multiply, and for many years following their discovery the only means of propagation known was by inoculation of susceptible animals.
Subsequently it was found that portions of living tissue taken from normal animals could be maintained alive in special fluids, held at body temperature, and the cells would even multiply. It was a short step from here to the inoculation of the tissue cultures with filterable viruses which attacked the living cells and multiplied within them. Embryonic tissue was employed usually, and much was chick embryo tissue taken from eggs which had been incubated for some days.
In 1911 Rous and Murphy inoculated infective Rous sarcoma material into the contents of developing eggs which had been incubated for 7-8 days. Tumours developed in various tissues injured by the passage of the needle, the most frequent result being a tumour at the point where the chorioallantoic membrane had been pierced.
Little advantage was taken of this discovery until 1931, when Woodruff and Goodpasture grew fowl pox virus on the chorioallantoic membrane.
In 1933 Burnet described a modification of the technique, in which the chorioallantois was made to drop, causing an artificial airsac and providing a large area of susceptible membrane over which the virus could spread. Since this time the method of Burnet has been most widely adopted, and it has been found that a large range of viruses will grow in this situation, and most produce characteristic lesions.
A great deal of valuable information has been obtained by the study of viruses under these conditions, and it has been found possible to study the interaction of virus and anti-body in quantitative relationships. Thus it is possible, by means of serum-virus neutralisation tests, to detect in the case of some diseases whether an animal possesses any immunity to the disease in question. Besides this, the method makes available large amounts of virus in a bacteria-free condition, which can be guaranteed pure.
Practical advantage has been taken of this source of supply in the cases of Fowl Pox and Infectious Laryngotracheitis and of Equine Encephalomyelitis for vaccination purposes.
Strange as it may seem, although the action of Equine Encephalomyelitis results in the death of the embryo of the inoculated egg within 24 hours, the virus content of the embryo is very much greater than brain tissue from infected horses or guinea-pigs. This fact has made possible the production in U.S.A. of a satisfactory vaccine, prepared by formalinising chick embryo virus, as the virus content of horse or guinea-pig brain was too low to stimulate the production of a satisfactory degree of immunity in the vaccinated subjects.
Thus it will be seen that the method has many uses, some of which are:
1. It provides a means of obtaining quantities of bacteria-free virus—containing material which can be used for experimental work or immunisation procedures.
2. It represents a suitable method for virus titration.
3. The specific inactivating properties of immune serum can be demonstrated, by which means animals recovered from the disease can be detected. This is useful as a diagnostic aid, and also in research.