Which animals to vaccinate and when. Parvovirosis, the infection caused by Canine parvovirus 2 (CPV-2), is widely distributed throughout the world and, in unprotected animals, is associated with high rates of morbidity and mortality as a result of gastrointestinal problems in particular. Consequently, it is good practice to vaccinate all puppies and give a booster vaccination also to adult animals. Dogs whose history is not known should be considered at risk and vaccinated, planning periodic boosters also for these animals.
Vaccination against parvovirus infection is usually carried out together with other antigens starting from 8 weeks of age using a live, attenuated virus, with a booster after 3-4 weeks. However, the choice of the time to administer the vaccine is fundamental in order for the vaccine to be effective. The main factor to consider is the problem of maternally-derived antibodies (MDA) whose titre in the puppy is directly proportional to the antibody titre in the mother.
Maternal antibodies specific for parvovirus have a longer half-life than those against other pathogens and high titres of these antibodies can persist in the puppy for up to 10-12 weeks and even up to 20 weeks in some cases, depending on the antibody titre in the mother and the amount the puppy assumes with the colostrum. This prolonged half-life is responsible for the critical period of 2-4 weeks characterizing this infection, that is, a window period of vulnerability during which the MDA are present at low enough levels to leave the puppy highly receptive to the disease, but at high enough levels to prevent the animal from raising an effective response to vaccination.
In fact, despite the marked efficacy of the attenuated vaccines currently available on the market, there continue to be episodes of disease in regularly vaccinated puppies. The problem is fundamentally due to interference from MDA, which are able to prevent immunization of the puppy. This creates notable problems, above all in environments in which the risk of infection is particularly high (kennels).
A reasonably valid system for overcoming the problem of interference by MDA on immunization is the use of so-called “high titre” vaccines, capable of overwhelming the effect of MDA thus reducing and, in some cases, eliminating the “immunological gap”. A vaccine dose equivalent to 107 TCID50, that is, 100-1000 times higher than the dose of “traditional” vaccines, can immunize puppies with an antibody titre between 1:40 and 1:160.
Even though it is still experimental, another official method, which is useful above all in highly infective environments, is based on the administration of the vaccine by the intranasal route. By this route, a vaccine dose of 105 TCID50 was able to immunize 100% and 72% of puppies with MDA titres of 1:40 and 1:80, respectively. This method of vaccination can significantly shorten the critical immunological period and may be effective in kennels with circulating infection.
Nevertheless, it is difficult to establish the best age at which to vaccinate a dog against parvovirus, precisely because the persistence and titres of MDA can vary considerably in relation to the various factors described previously.
In particular epidemic conditions it is essential to carry out serological studies on puppies to collect sufficient information to establish precisely when to vaccinate the animals.
THE VACCINATION
As for distemper, if the puppy is at high risk, the first series of vaccines can be started at 6 weeks of age, administering further doses at 9-11 and 12-14 weeks. In accordance with the most recent indications from international scientific bodies, the last vaccination in the first year of life should be given at 16 weeks of age.
The minimum duration of immunity (DOI) provided by vaccination with attenuated CPV-2 is 7 years. Consequently, after the first series of vaccinations and first annual booster, repeat vaccinations every 3 years are considered protective. In the case of the use of a dead vaccine, annual repeat vaccinations are recommended, unless this type of vaccine has not been used as a booster after an initial series of vaccinations using an attenuated vaccine: in this case repeated vaccinations every 3 years are considered protective.
Unlike distemper virus, which has been demonstrated to have immunosuppressive activity, numerous studies have clearly shown that the component of Canine parvovirus present in the vaccines used does not have any immunosuppressive activity, but can cause myocardiopathy in puppies under 4 weeks old. Furthermore, this vaccine has been shown to have different efficacy depending on the breed of dog vaccinated: some clinical experience has indicated that Dobermanns, Pinschers and Rottweilers, breeds which are genetically related, are more sensitive to infection by parvovirus and do not respond, or respond only partially, to repeated administrations of the specific vaccine.
EFFICACY OF THE VACCINES
Another important issue is the efficacy of the commercially available vaccines with regards to the currently circulating variants. In fact, it is known that almost all vaccines are raised against the CPV-2 variant that has not been present in the canine population for several years now. According to the opinion of some researchers and some companies that produce vaccines, it is not necessary to prepare new vaccines with CPV-2a, 2b or 2c strains, since the vaccines on the market are still effective against the variants. However, some research has clearly shown that puppies vaccinated with CPV-2 have very high titres of neutralising antibodies against the homologous virus (vaccine), but significantly lower titres of antibodies against the antigenic variants. Furthermore, there are numerous reports of foci of disease caused by parvovirus variants (above all CPV-2c) in dogs regularly vaccinated with formulations containing the original CPV-2 type.
These data indicate that the “real” immunity induced by traditional vaccines against CPV variants could be very low even if, immediately after the vaccination, the antibody levels may be able to confer some protection.
After much dispute (scientific and commercial) it is comforting to know that in the USA and in other countries, including Italy, vaccines prepared with the CPV-2b antigenic variant have been on the market for some time now.