Response of Puppies to Vaccination with Canine Distemper and Canine Parvovirus
Vaccination of puppies is the cornerstone of every small animal clinic. Therefore, it is important that the veterinarian have a thorough knowledge of all aspects of vaccination, its advantages and also its pitfalls. Canine parvovirus (CPV-2) and canine distemper (CDV) viruses are two important, potentially fatal diseases of dogs, which can be prevented by vaccination. In this lecture I will present information that we have researched regarding vaccination against these two important diseases.
Vaccination with effective vaccines does not always guarantee successful immunization. Why do we have these failures? First we must remember that we are dealing with live attenuated vaccines, that if effective will give the pup both humoral and cellular immunity. These are the most effective and are generally recommended over killed vaccines. In order for the pup to be immunized by live attenuated vaccines, it is essential that the relatively small amount of virus administered to the animal replicate and spread to target organs. Being attenuated does not allow the vaccine virus to cause disease.
Interference by maternal antibodies is regarded as a major cause of vaccination failure in young dogs. In dogs, IgG antibodies are transferred from the dam to her offspring through the colostrum. After suckling, pups' antibody titers average about 75% of that of the dam, and approximately 95% of this maternally derived antibody is absorbed from the colostrum. The absorption of IgG colostral antibodies takes place in the first 24 hours after birth and is facilitated by pinocytosis across the mucosa of the pups' intestine. The maternally derived, passive antibodies decline exponentially and average 8.4 days. Therefore the greater the initial concentration of IgG in the colostrum, affords the pups a longer period of "protection" against both pathogenic virus or vaccine virus.
What happens when we vaccinate pups? This of course depends on their maternal IgG concentration to the virus vaccine been presented to the pup. If the IgG level is low the virus will be able to replicate and stimulate the immune system to produce first IgM and then IgG antibodies. Where the antibody concentration is high, the virus will be neutralized by the pups maternal humoral antibodies, will not replicate and no antibody response will occur. When deciding when to vaccinate, our problem begins in that we cannot assess the initial maternal antibody titer of the pup. Historically, Baker and his colleagues from Cornell considered this problem in 1959, when dealing with vaccination against CDV. Baker proposed assessing the antibody titer of the dam just before parturition, and by use of a normograph predicting the best time for vaccination of the pup. The idea was never actualized due to the lack of a readily available test for CDV IgG at the time.
Before discussing vaccination programs I want to clarify two important concepts: The first concept is the difference between vaccination and immunization. Vaccination is the act of administering vaccine. Not every vaccination results in immunization. It is only after the immune system has been stimulated to produce antibodies that we can speak of immunization. The second concept is regarding "boostering" of antibody titers by subsequent vaccinations. As mentioned, susceptible pups respond to initial vaccination by live attenuated vaccines by the development of IgM and then later IgG specific antibodies. Any subsequent vaccination by live attenuated vaccine close to the time of the initial vaccination, does not booster or increase the antibody titer. Booster vaccination only applies to killed or inactivated vaccines.
Now how do we draw up a vaccination program for pups? There are many vaccination programs available, and also a lot of confusion and desperation in trying to chose the best plan. No one vaccination program can be considered perfect, and every veterinarian must develop his/her program in relation to the status of the clientele and conditions of pups.
One of the immunization strategies used to overcome the problem of "vaccination failure" due to maternal antibodies is based on administration of multiple vaccinations during the early age of a puppy from about 6 weeks to 4 months of age. By vaccinating frequently, we are bound at some stage to reach the point when the maternal antibody titer of the puppy has dropped to a low enough level so as to allow successful immunization. This approach is costly to the client in terms of the number of vaccination and leaves the veterinarian in doubt as to the success of immunization until about 4 months of age, when most pups have lost their passive immunity. Furthermore, one of the most important concepts of prophylaxis of pups during the period of vaccination is isolation. Before confirmation of immunization success, pups should be isolated from other pups of unknown immunization status in order to reduce the risk of infection. Under the multiple vaccination program, pups should be isolated and deprived of social interaction until about 4 months of age. Socialization of pups at this young age is important for development of social skills, and its deprivation at this critical age can have permanent deleterious social effects. This period should be shortened as much as possible.
An alternative approach to multiple vaccinations would be to determine the level of immunity after administration of a limited number of vaccinations. The development of a standardized in-clinic rapid dot-ELISA procedure for CPV and CDV IgG and IgM antibody assay could allow the veterinarian to gauge the real-time response of pups, thereby accurately assessing the current immunization status of the pup after vaccination.
Several serologic assays are available to diagnose disease or assess immunity; however, the only practical assay suitable for the veterinary practitioner is the ELISA test. The ELISA technique is being used with increased frequency in veterinary medicine for the assay of antibodies to a growing number of pathogens. The method can be standardized and is rapid, easily available, and reliable and requires no specialized laboratory equipment. A dot-ELISA test was developed and verified for the purpose of assaying and semi-quantifying canine CPV and CDV IgG and IgM antibodies. Using this kit we demonstrated in beagle dogs that it was possible to measure the titer of antibodies in the pregnant bitches just before parturition and the initial titer of antibodies in the pups shortly after birth. We then were able to follow the decline of the maternal passive antibodies during the early stages of life of the pups, and then finally measure the efficacy of vaccination for both CPV and CDV. The validity of the technique was tested in parallel with the standard hemagglutination inhibition test.
A follow-up study was carried out under clinic conditions on 40 pups from household environments. The pups were tested for CPV and CDV IgG antibody titers in order to assess their immunization status following one of two vaccination protocols depending on the age of the pup at first presentation to the clinic: Pups of 6 to 8 weeks of age received a monovalent CPV vaccine followed by two polyvalent vaccines, whereas pups older than 8 weeks of age received two polyvalent vaccines. The rate of vaccination success for the two protocols was judged two weeks after the last vaccination using the dot-ELISA test and found to be 78 and 71%, respectively. The results demonstrated that it was possible to evaluate the immunization status of puppies using a rapid in-clinic ELISA test.
The aim of this lecture is not to suggest a specific vaccination protocol or to recommend a particular vaccine manufacturer, but rather to demonstrate a standardized method for determining immunization success in the clinic setting. Each veterinarian should develop a vaccination program specific for his or her needs. Furthermore, this method may be used for selected pups such as Rottweilers, which are particularly susceptible to CPV.
The technique of measuring vaccination response has the advantage that it can be used to test the optimal scheduling, choice of manufacturer and batch of vaccines. In this regard it must be borne in mind that not all vaccines have similar efficacy, and not all batches of an efficacious vaccine are equivalent. Significant differences have been found to exist among various commercial canine vaccines containing CPV-2 in their ability to induce antibody and protective immunity to CPV-2 in pups when maternally derived antibody is present. The differences in effectiveness among vaccines is likely attributable to immunogenicity of the virus strain in the vaccine and the titer of the vaccinal virus. This may be related to high tissue culture passages resulting in vaccine viruses losing their immunogencitity and as a result their effectiveness. The possibility of decline in antigenicity for a given vaccine over time and or passages may not always be checked by the vaccine manufacturer and reported to the veterinarians. Furthermore, there is the element of vaccine handling between the time of manufacture and delivery to the veterinary clinic, which may account for devitalization of live attenuated vaccines. All those elements relating to the choice of a vaccine can be tested using the dot-ELISA test kit.
In conclusion, the ELISA test for CPV and CDV antibodies offers the veterinarian the opportunity of vaccinating pups and concurrently assessing immunization success or failure, therefore allowing a more reliable and efficient vaccination program. In this way vaccination programs can be "tailor-made" for each individual pup without using generalized vaccination schedules that may result in vaccination failure and pup mortality. The method was found to be especially useful in the clinic, and is suggested as a simple rapid clinic-based test for the assessment of the immune status of pups.
Finally, I want to discuss a unique circumstance requiring special procedures for vaccination.