Since we do not want dogs to have to endure an episode of disease to develop their own protection, we turn to the creation of active immunity by vaccination. This is the deliberate triggering of the immune response by the giving of a small, weak dose of the disease in order to stimulate the production of antibodies.
Essentially modern vaccines fall into four main classes:
Attenuated living vaccines
Fortunately bacteria and virus can be modified to reduce their ability to cause disease, but at the same time maintain their ability to stimulate the formation of active antibodies. This is achieved by a process called attenuation and is usually brought about by growing the organism in an unnatural host, e.g. in eggs, a different species of animal or in tissue culture. The amount of attenuation depends on how long the germ is maintained in the “Artificial” system. The trick is to get the right balance to achieve maximum antibody production in the animal given vaccine and yet not cause disease. The big advantage of live vaccines is that usually only one dose is required in adult dogs to stimulate immunity. Dogs injected with the ideal live vaccine will not shed organisms to the dogs – indeed that is the case with modern Canine Distemper vaccines. If this is not so, then organisms shed from vaccinated dogs may “Infect” in–contact animals and if this happens, on several occasions there is always a risk that the organism’s ability to cause disease may return. Such vaccines are better not used where a good alternative dead vaccine, or a live vaccines that is not shed, is available.
These are made from “Germs” which have been killed by heat or chemical agents such as formalin. Although the organisms are incapable of multiplying in the vaccinated animal they, nevertheless, stimulate the formation of antibodies. However, two doses are normally required and an additive, or adjuvant as it is called, may be needed to enhance the effect. By and large, the immunity produced is not so long–lasting but obviously the safety factors in vaccines of this type are greater.
The lethal effect of some organisms is brought about by the fact that they produce poisonous substances called toxins. The body reacts to these by producing specific antibodies called anti–toxins, which are capable of neutralizing toxin. This can be done artificially by injecting a toxoid which is a detoxified toxin. Toxoids are generally made by inactivating toxins, either by heat or by chemical means and on injection they stimulate anti–toxin to be formed in the body. Two doses are normally required to stimulate immunity and booster doses may be needed over one, two or three years. Probably the most familiar example is tetanus toxoid.
It is possible to make mixed vaccines provided that care is taken to ensure the compatibility of the antigens. Mixed vaccines may contain, for example, two live antigens, several dead antigens or even a combination of live and dead antigens. Such vaccines make it possible to establish an effective degree of protection against diseases with a minimal number of injections.
Protection created by vaccines is generally not as long–lasting as natural immunity, so boosters are needed periodically, different intervals being advised for the different diseases. Dogs which are kept in isolated conditions, exercised only on their owner’s land and never taken to shows or training classes, are more in need of booster protection than those dogs which live in towns and mix frequently with other dogs and hence get a degree of natural “Boosting” from the low level of infection present in the environment.
Methods of Administration
Vaccines are generally given by subcutaneous or intra muscular injection, but in certain circumstances, where there is a need to stimulate local protection, they are given by other routes. For example the vaccine used to protect dogs against Kennel Cough caused by the bacterium Bordetella bronchiseptica is given by the intranasal route. Administration by this route stimulates the production of “Local antibodies” in the upper part of the respiratory tract within only a few days just where they are needed. This is followed later by the production of antibodies in the blood stream.
When to Vaccinate
Obviously the aim is to stimulate the production of active antibodies, by vaccination, as early as possible in a puppy’s life. Unfortunately, this is not easily achieved, since not only does maternally–derived antibody protect against disease, but it also prevents a proper response to vaccination. There is, incidentally, an “Immunity gap” during which puppies will not be protected by maternal antibody and have yet to produce their own active antibodies in response to vaccination. Much effort has gone into devising vaccination regimes to keep this time of vulnerability to a minimum. Canine Parvovirus disease or vaccine produces a higher level of antibody than distemper. A bitch which has a high level of CPV antibody should pass on a lot to her puppies, so they may have to wait a long time before they are ready for vaccination, perhaps up to 20 weeks. If the bitch has not much antibody to pass on, the pups may be ready for their own vaccination at six weeks. This wide variation causes a considerable dilemma and is the reason why several doses of vaccine are often recommended. Veterinary surgeons need to calculate the optimum time for vaccination in the light of local disease conditions and the history of the kennel in which the puppy was born, possibly in conjunction with blood sampling. Finally, in this connection it has to be remembered that there will always be a proportion of dogs (and humans) whose bodies fail to make any response to vaccine given to them. A multiplicity of safe effective vaccines are available these days to protect dogs against the five major infectious diseases from which they suffer.