Vaccination against Canine parvovirus 2 (CPV-2) is essential for the prophylaxis of this infection, but the choice of the time to give the vaccination is critical with regards to its efficacy. The main factors to take into consideration are maternally derived antibodies (MDA) whose titre in the puppy is directly proportional to the antibody titre in the mother. The puppy’s maternally derived immunity starts to be acquired, albeit to a limited extent, already in the antenatal period, through the placenta. In this period, the foetus receives small amounts of antibodies, about 5-10% of the maternal antibodies.
The puppy’s passive immunity is completed in the first 48 hours of life when, with the assumption of the colostrum, its antibody titre can reach 70-80% of the maternal titre. The colostrum must be assumed within the first 48 hours of life because the puppy’s intestinal mucosa is permeable to the passage of antibodies in this period. The MDA are, however, a sort of two-faced Janus: they provide protection to the puppy, but are also able to block any vaccines administered. Thus, if a vaccination is performed when the young animal still has specific MDA, the antigen will be blocked and stimulation of the immune system will be prevented.
In order to obtain a good post-vaccine immune response the puppy must not have MDA or the level of these must be below that which could cause interference with the vaccination. It is very difficult to determine when to vaccinate a puppy unless its antibody titre is known. The difficulty arises from the fact that each bitch has her own immunological history (different antibody titres) and, even within the same litter, each pup can have its own colostral immunity profile.
With good approximation, the ideal time to vaccinate a puppy should be between the 50th and 60th day of life. However, this is not always the case and, in particular, is not so for all pathogens. Furthermore, it is worth recalling that the disappearance of MDA in the post-natal period is linear as a function of time, in that the antibodies have a half-life of 9-10 days. The half-life of MDA is related to body growth (dilution of the total volume of blood) and to the catabolism of proteins.
Graph 1
Graph 1 shows the decrease in antibody titre in a puppy whose MDA intake had been, for example, equivalent to a titre of 1:640. It can be noted that the antibodies do not disappear before 10-11 weeks of life.
Varying the antibody titres at birth (according to the titre in the mother) will, obviously, also change how long the antibodies persist in the puppy. In conclusion, each puppy has its own MDA profile.
At this point it is worth remembering the relationships between:
- the titre of MDA and protection from infection;
- the titre of MDA and response to vaccination.
With respect to the antibody titres, the response of dogs to infection is as follows:
- <1:20 → infection with disease;
- 1:20 - 1:80 → subclinical infection;
- >1:80 → complete protection.
Again with respect to antibody titres, the response of dogs to vaccination is as follows:
- <1:20 → good response
- >1:20 → lack of response (interference).
Putting together this information, it can be readily deduced that there is a particular period in the puppy’s life (immunological gap) in which the antibody titres are below the protective threshold (<1:80) but still to high to allow an active response to vaccination. For this reason, a puppy vaccinated in this particular period could subsequently develop the disease since it was not immunized.
Graph 2
Graph 2 shows the same example as in graph 1, but integrated with the information given above. It can be seen that the “immunological gap” (shown in red) extends from the 5thto 8th weeks of the puppy’s life, although it could obviously be anticipated or delayed as a function of the initial antibody titre.
It can be readily appreciated from the foregoing how complex it is to identify the best moment to vaccinate a puppy without knowing its antibody titres. The information also provides a rational explanation for the many episodes of disease observed in “correctly” vaccinated puppies.