Search For Vaccine

Press, Volume C, Issue 29436, 11 February 1961, Page 8

Search For Vaccine

As well as directing the hydatids research programme, Mr Gemmell is carrying out research into the physical, factors affecting the tapeworm eggs under varying conditions and to see whether a vaccine to protect either the dog or the sheep is practicable. In the laboratory, Mr Gemmell has shown that T. hydatigena eggs will survive at 45deg. F. under moist conditions for more than a year and the majority of them will survive for more than three months. Under desiccation, or complete dryness, they die almost immediately. A definite relationship has been shown between temperature and the time of survival. At high temperatures, such as lOOdeg F„ they will live for less than a week. At 90deg they will live for a fortnight and about three to four weeks at 80deg. Many of the eggs are immature when they are shed in the dog's faeces and mature on the pasture. The practical application of these findings is that during the winter there is a steady build-up of eggs on the pasture when it is short grazed by the sheep and the cooler temperatures extend the survival rate of the eggs. By the spring, when the lambs begin grazing, there is the highest concentration of eggs present to infect them.

The search for a vaccine which would confer immunity from hydatids on sheep or dogs is a much longer term investigation and one in which the odds for success are very small. In all types of internal parasites so far investigated by scientists only one successful vaccine has been produced. This was for the lung worm. Overseas information suggests that the most promising method would be to look for a live vaccine but even if it is found possible to produce a vaccine it may not be economically practicable. The embryo which hatches from the egg in the gut of the sheep has to first penetrate the

intestine lining and enter the bloodstream to be carried to the liver and subsequently to other organs and there is some evidence from research in Kenya that there may be some primary resistance of the gut lining to penetration. Why tapeworms with similar life cycles to E. granulosus and T. hydatigena and identical secondary hosts—the dog —do not infect sheep is possibly because of some agent which prevents the embryo penetrating the intestine, as eggs will hatch in the digestive juices of animals that are not subsequently infected. If this agent can be found, the next step would be to try and transfer its action to the animal capable of being infected in the form of a vaccine. If a vaccine could produce anti-bodies in the bloodstream, which would prevent the embryo penetrating the gut lining, it would die and have co I further effect on the host. Technique A technique has been developed by the unit as a modification of one used at Liverpool on research into beef measles—elso caused by a tapeworm—and consists of taking eggs from the dog and subjecting them to varying periods in a solution of digestive juices in the laboratory at body temperature. A suspension of the hatched embryos is then injected into foreign sites in the dog and the sheep—tissues other than the gut. Eggs of the tapeworm, T. pisiformis, which infects the rabbit, do not penetrate the intestine of the sheep; the dog —with one rare exception—does not get hydatids and is only host to the tapeworm and not the cyst forming stage; and the rabbit is unaffected by T hydatigena which causes such heavy damage to lamb and sheep livers. These factors shown in the survey may hold the key to the discovery of a new cure or means of prevention.