R, W. Fisher. 239
Further Remarks on Methods.
In the preparation of animal vaccine lymph, one of the many difficulties met with, in a tropical country, is the maintenance of a potent strain of seed lymph. If vaccinia is carried on from calf to calf, after a time the strain weakens in potency and the vesiculation obtained, after from six to eight moves on the calf, becomes very poor and many calves have to be rejected as unfit for collection. In the old days, in Europe, efforts were made to renew the stock by obtaining a fresh strain of vaccinia from natural outbreaks of the disease among cattle. This was an uncertain method, as the occurrence of natural vaccinia among cattle is a rare occurrence and hard to discover. Retro-vaccination, or the passage of the virus through children and back from the child to the calf, was found fairly efficient in maintaining the activity of vaccinia. The objections to this are obvious. Parents object to the collection of lymph from their children's arms, especially in a country, like India, where most of the population is uneducated and very suspicious of modern scientific procedures. The passage of calf lymph through buffaloes and back to calves, was found to keep the activity of the virus at a fairly high level, and this method is in use in many of the Vaccine Institutes of India. At Hendon, Local Government Board Lymph Establishment, the use of rabbits, for this purpose, was demonstrated to me by Dr. Blaxall and introduced in Belgaum. I find it an excellent method and, since rabbits have been used, no difficulty has been found in maintaining a seed lymph of high potency. The method in use is as follows:—
Some of the best vesicles, found on one or two calves at a morning's collection, are chosen and collected for use as seed lymph. The pulp is treated in the routine manner, being chloroformed and examined bacteriologically. This seed is used for vaccinating a further batch of calves, and again seed is collected from a calf or two which shows the best vesicles. After four or five moves on the calf, the seed is passed through a pair of rabbits. An area on the rabbit's side is carefully shaved and a non-pigmented area is selected for vaccination. Grey rabbits, so-called Belgian hares, are found the most suitable animals. The skin having been well washed and dried, the lymph is spread over the area selected and gently scratched with a piece of wood, such as a match. If incisions are made, with a lancet or scalpel, the results are very poor on the rabbit, but if the skin is well shaved, a few gentle linear scratches are sufficient. After 72 hours, vesicles will have formed and are removed by scraping over the surface of the skin with a scalpel.
