Basically a newborn puppy is not naturally immune to diseases however, it does have some antibody protection which is derived from its mother's blood via the placenta. The next level of immunity is from antibodies derived from the mothers first milk. Now where the problem comes in is that these maternal antibodies can actually block the effectiveness of a commercial vaccine and since it is different for each puppy in a litter no one can accurately predict when the maternal antibodies will reach a low enough level for the vaccine to work. This results in 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 puppy 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 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 of age, 40% of the puppies were able to respond to the vaccine. The number increased to 60% by 16 weeks of age, and by 18 weeks, 95% of the puppies were protected by the vaccine.
To add to this is what is called the Herd Effect or Herd Immunity which occurs when the vaccination of a portion of the population (or herd) provides protection to unprotected individuals. Herd immunity theory proposes that, in diseases passed from individual to individual, it is more difficult to maintain a chain of infection when large numbers of a population are immune i.e. puppy classes ( but only if the local residents who are not taking class are also not using the area to walk and potty their dogs.) The higher the proportion of individuals who are immune, the lower the likelihood that a susceptible animal will come into contact with an infectious animal. However all it takes is 1 virus shedding animal to infect another so places such as vet offices are actually high risk.
In a nut shell
- Puppies are given a series of booster shots (3) starting at 7 weeks to 18 weeks to cover this window of susceptibility and then again at 1 year of age to ensure adequate antibody levels. Multiple doses of vaccines are given not because MLV vaccines require multiple doses to immunize, instead, they are given multiple times to ensure they are given when the passive antibody to that specific vaccine virus has declined to a level that won’t neutralize (inactivate) the vaccine virus.
- Each puppy's response is different so the only likely safe period is after the third vaccination when given after 16 to 18 weeks of age.
- It is necessary to wait at least 3 to 4 weeks to readminister the vaccine.
- Ideally, there should be no more than three doses of core vaccines administered to a given pup during the early neonatal period.
- A fully vaccinated animal will not pass on the disease, however an unvaccinated healthy looking adult dog can be a carrier.
- Transmission can occur both by direct contact as well as indirect contact through contaminated soil, shoes, clothing, used crates and other materials.
- Viruses such as parvo are particularly long-lived in the environment, lasting anywhere from 1 to 7 months -- commonly surviving 5-7 months in an outside environment. Regular old bleach is still 100% effective against Parvovirus. The dilution for bleach is one part bleach to 30 parts water however due to the large amounts of virus particles shed in the feces of an infected dog (shedding lasts two weeks or more after exposure) and the longevity of the virus, complete eradication of the virus is often impossible.
- Precautions need to be taken to lessen the risk of coming into contact with a diseased or carrier animal yet not so restrictive that it hinders the socialization and training that is so critical to ensuring the puppies proper cognitive, temperamental and behavioural development.