[smartpuppiepets]

Window of susceptibility

The age at which young animals can effectively be immunized is proportional to the amount of antibody protection they received from their mother. High levels of maternal antibodies present in a young animal's bloodstream will block the effectiveness of a vaccine. When the maternal antibodies drop to a low enough level in the animal, immunity (protection from disease) can be produced through vaccination.

The antibodies from the mother generally circulate in the newborn's blood for a number of weeks. There is a period of time from several days to several weeks in which the maternal antibodies are too low to provide protection against the disease, but too high to allow a vaccine to work. This period is called the window of susceptibility. This is the time when despite being vaccinated, a young animal can still contract the disease.

The length and timing of the window of susceptibility is different in every litter, and even between individuals in a litter. A study of a cross section of different puppies showed that the age at which they were able to respond to a vaccine and develop protection (become immunized) covered a wide period of time. At six weeks of age, 25% of the puppies could be immunized. At 9 weeks, 40% of the puppies were able to respond to the vaccine and were protected. The number increased to 60% by 16 weeks, and by 18 weeks, 95% of the puppies could be immunized.

As you can see, it is really impossible for us to determine, when in the presence of passive immunity, an individual animal should be vaccinated. There are just too many variables. Even if we did blood tests on them, each animal in the litter would probably have a different titer. Some will have absorbed more antibodies, the antibodies may have broken down more quickly in others, or some may have used a portion of their antibodies if they encountered harmful bacteria or viruses. Additionally, a young animal may have a protective titer (level of antibodies) for one disease but not enough for another.

Progress is being made. Some of the newer vaccines can stimulate active immunity in the young animal even when maternal antibodies are present. As vaccines improve, we will hopefully be better able to protect puppies throughout their early life.