An animal who is immune to a specific pathogen has natural substances in his system called antibodies that attack and destroy that pathogen before it can cause disease.
When a dog becomes ill with an infectious disease, his immune system makes antibodies against that particular pathogen. These antibodies protect the dog against reinfection. The dog has now acquired active immunity. Active immunity is self-perpetuating; the dog continues to make antibodies long after the disease has gone away. Any time the dog is exposed to that particular pathogen, his immune system will produce more antibodies. The duration of active immunity varies, depending on the pathogen and the dog. Following natural exposure, active immunity often persists for life. In general, immunity to viruses lasts longer than immunity to bacteria.
Active immunity also can be induced by vaccination. The dog is exposed to heat-killed pathogens, live or attenuated (antigens that have been treated åto make them less infectious) pathogens rendered incapable of causing dis- ease, or toxins and pathogen products that will also stimulate a response by the dog’s immune system. As with natural exposure, vaccination stimulates the production of antibodies that are specific for the particular pathogen in the vaccine. However, unlike natural exposure, the duration of protection may be limited. Accordingly, to maintain high levels of protection, booster vaccines are recommended. How frequently a dog will need boosters depends on the antigen used, number of exposures to the pathogens, the dog’s own immune response, and the type of vaccination used. Vaccination schedules need to be customized for each individual dog.
Vaccinations may not be successful in all dogs. Rundown, malnourished, debilitated dogs may not be capable of responding to a disease challenge by developing antibodies or building immunity. Such dogs should not be vaccinated at that time, but should be vaccinated when they’re in better health. Immunosuppressive drugs, such as cortisone and chemotherapy agents, depress the immune system and also prevent the body from making antibodies.
Another type of immunity is called passive. Passive immunity is passed from one animal to another. The classic example is the antibodies newborn pups absorb from the colostrum of their mother. Puppies are best able to absorb antibodies from their mother’s milk during the first 24 hours of life. The immunity persists only as long as the antibodies remain in the puppies’ circulation. The duration of immunity depends on the concentration of antibodies in maternal milk when the pups were born. Dams vaccinated just before they were bred have the highest antibody levels and are capable of protecting pup- pies for up to 16 weeks. However, some veterinarians believe this additional booster is unnecessary.
Puppies younger than 3 weeks old may be incapable of developing antibodies in response to vaccination because of physical immaturity or interference by passive maternally acquired antibodies. Maternal antibodies can bind the antigen in the vaccine and keep it from stimulating the immune system. These passive antibodies disappear at between 6 and 16 weeks of age. Therefore, when vaccinating very young puppies, the vaccine must be given more frequently to ensure that the vaccine will stimulate immunity as soon as maternal antibody levels decline and can no longer interfere with the vaccine.
Another source of passive immunity can occur with a transfusion of blood products with antibodies into a dog with a serious infection or immune problem. This is not done frequently, but can be a life-saver for some dogs.