This type of vaccination is the most modern development in the fight against Brucellosis of cattle and is the only type of vaccination which gives promise of being useful to the farmer owning an infected herd.
It has been developed over the last ten years, first by Drs. Cotton and Buck and later by a large body of U.S. veterinarians. In their hands it has given excellent results.
The plan calls for the vaccination of calves between the ages of four to eight months but preferably at the fifth month of age with a large dose of living Brucella abortus microbes of the Strain 19.
This strain is one of reduced virulence but is not entirely avirulent. Thus, although it causes animals which are vaccinated with it to develop marked positive agglutination reactions, it will permanently infect animals only in exceptional circumstances as far as is known at present.
Calves vaccinated with Strain 19, when five months of age, are stated to give negative reactions in the majority of cases by the time they reach breeding age, though a small percentage of such animals may continue to react to the test for considerably longer periods.
Mature animals will react positively to test for a long period—(four to five years) but very little is known of the behaviour of Strain 19 in adults and its use in such animals cannot be recommended until much more work has been carried out.
In tests in the United States of America over 80% of calves vaccinated with Strain 19 appear to have been protected against subsequent virulent infection. Actually, although a great number of calves have been vaccinated in America and very few instances of abortion due to Brucella abortus have been encountered in vaccinated calves when they reach maturity, it cannot be said that all vaccinated calves have been exposed to infection with the germ. However, it has given such good results in the U.S.A. that the Bureau of Animal Industry has adopted it as second method of attack on Brucellosis of Cattle in sanctioning its use in conjunction with the test and slaughter method as the official methods of control of Brucellosis of cattle. It should be noted that the United States authorities have not sanctioned Strain 19 vaccination as a sole method of control of Brucellosis, but as an adjunct to the agglutination test.
What Does Strain 19 Do?
Used as a vaccine on calves it first produces, as stated, a marked positive agglutination response in the blood of such calves but this gradually subsides as the calves mature and by the time they have reached breeding age the majority again give negative reactions to the test.
Following vaccination, heifer calves, in the majority of cases, are resistant to infection by virulent Brucella abortus at least during their first period of pregnancy. Information available from the very large field experience being conducted in the U.S.A. suggests that vaccinated animals are also resistant during subsequent pregnancies, but work reported by other authors suggests that the resistance may weaken considerably during the second pregnancy, at least under some circumstances.
Too much reliance cannot be placed on the U.S.A. field experiment, large though the number of calves vaccinated is, because we do not know enough of the conditions in the herds in which the work was carried out. On the other hand work done with Strain 19 in Canada suggests that credit may have been given to Strain 19 for preventing infection of calves in herds in which Brucellosis was not spreading at all, and this may have obscured the true value of this Strain as a vaccine.
However, the evidence suggests strongly that Strain 19 will prove a considerable help in controlling Brucellosis. This Strain was brought to Australia from America by the author, in 1938, and experiments have been under way for some time to attempt to evaluate it under our conditions. These experiments were delayed first by failure to secure sufficient land close to the Veterinary Research Station. Glenfield, on which to run experimental cattle, and later by military duties undertaken by the author. They are now advanced, however, and all vaccinated calves will be exposed to virulent Brucella abortus this year. Results of this first exposure are awaited with great interest.
Attempts have also been made to secure owners who are willing to permit their herds to be used in field experiments with the vaccine, and to accept the restrictions imposed on the sale and movement of cattle necessary to the test. Unfortunately, it has not been possible to find such men until recently but it is hoped that more owners will prove public-spirited so that evaluation of Strain 19 may proceed without undue delay.
What Can We Expect Strain 19 To Do In Our Herds?
Firstly, let make it quite clear that it will be unlikely that Strain 19 vaccination of calves will prove an efficient method of control of Brucellosis by itself unless under exceptional circumstances.
No vaccine yet developed has proved 100 per cent. effective In controlling any disease and chronic diseases such as Brucellosis are usually not so readily controlled by vaccines as are acute diseases.
It is hoped, on the evidence available at present, that Strain 19 used as a vaccine on calves, will confer some degree of resistance on the majority of calves vaccinated with it and so reduce the spread of infection through herds in which it is used. Thus, by adopting a long range policy, a herd owner may be able to reduce the degree of infection in his herd to a point where lie will be able to apply the "test and slaughter" plan without submitting himself to considerable financial loss.
Further, once he has thus eradicated the disease, by continuing with calfhood vaccination he will have a disease-free resistant herd instead of a disease-free susceptible herd as under our present system.
Again, for those progressive farmers who have undertaken eradication of the disease from their herds and have persisted to obtain a disease-free herd, Strain 19 calfhood vaccination holds out the hope that by its use they too may develop a resistant herd. If these men espouse Strain 19 they should realise that they will probably have to cull a small percentage of vaccinated calves because they still give a positive reaction to the agglutination test at time of mating. It is felt, however, that in the light of our present knowledge it would probablY be worth this expense to develop a resistant herd. This will occur gradually and will depend on the number of calves vaccinated each year and reared to replace animals in the herd culled for various reasons.
Let us sound a note of warning in concluding our remarks on calfhood Brucellosis vaccination. We know that a vaccine capable of controlling Brucellosis has been a dream of the farmer and veterinarian alike for many years past, in fact, ever since the disease was proved to be of bacterial origin. Now that it appears that a vaccine has been developed that is capable, if used intelligently, of helping to control any disease and that good hygiene, herd management, intelligent culling, the correct use of the vaccine combined with the use of the agglutination test, will all be required for the adequate control of the disease. The farmer who pins his faith solely to a vaccine will undoubtedly be disappointed in an unknown proportion and it has yet to be proved that Strain 19 alone was responsible for eradicating the disease from a single herd.