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Vaccination
The goal of vaccination is to stimulate the animal's immune system by giving a killed or weakened form of an organism. The animal then will be protected from becoming ill when exposed to the real disease. The two major parts of the immune system are humoral immunity (B-lymphocytes that produce antibodies) and cell-mediated immunity (T-lymphocytes that directly or indirectly destroy infected cells). Vaccination is, therefore, critical in the prevention and control of disease. For the most part, veterinary vaccines are safe and efficacious and have saved many animal lives. An important point to remember is that vaccination is a medical procedure that should include the same considerations and reasoning skills required when deciding to use any medical treatment or surgical procedure. The goal should be to vaccinate more total animals in the populations and to vaccinate individual animals less frequently and only for disease for which there is a risk of exposure.
Risk Assessment
The potential risk of exposure to various diseases should be determined for each individual patient. Vaccination recommendations will vary depending on the environment of the dog. Consider the differences of exposure between a poodle living on the 20th floor of a high-rise who never goes outside versus the racing greyhound and the suburban golden retriever in a fenced yard versus the rural bird dog who roams many acres.
Duration of Immunity
If humans generally receive only childhood vaccinations then why do dogs have annual revaccinations throughout their entire lives? This is definitely an area of intense research and controversy. Most vaccines produced before 1995 were evaluated only for efficacy for several weeks after the last vaccination was given, and the recommendations for annual revaccination were arbitrary. The exception, of course, is rabies vaccine that must provide a one- or three-year duration of immunity under strict guidelines from the USDA. Since 1995, new vaccines for diseases that have not previously had an approved product must be shown to produce the duration of immunity advertised on the label. To really determine the efficacy of vaccines, vaccinated and unvaccinated animals need to be kept for at least one and upwards of many years and then challenged with exposure to the real disease. Ideally, most of the unvaccinated animals that are challenged should get sick and/or die and most of the vaccinated animals should not get sick. The difficulty and expense (in dollars and animal lives) of such studies are great, so it is understandable why results from such studies are limited. Appropriate vaccination with distemper and parvovirus vaccines probably affords long-term immunity (years) whereas vaccination with leptospirosis and kennel cough vaccines may last less than one year.
Core Versus Non-Core Vaccines
Core vaccines are those which should be given to every dog to provide protection against diseases that are serious, common or a risk to humans. These include parvovirus, distemper, adenovirus (infectious hepatitis) and rabies. Non-core vaccines are those given on the basis of risk assessment. These vaccines include coronavirus, parainfluenza, leptospirosis, bordetella, Lyme disease and giardia.
Maternal Antibodies
Newborns can respond immunologically at birth, but the response is slow and inferior compared to older animals. Protection for newborns is provided by antibodies that are passed in the colostrum from the dam to the puppies during first 24 to 72 hours after birth. Eight-two percent to 98 percent of the maternal antibodies come from the colostrum, while only two percent to 18 percent are transferred in utero. The amount of antibody the pups receive depends on the antibody titer of the dam and how much colostrum each pup receives. Maternal antibodies can interfere with the ability for puppies to respond to vaccination by inactivating the vaccine just as if it were the real disease. Vaccination, therefore, starts at six to eight weeks of age when the levels of maternal antibodies are waning. There is a critical period of susceptibility to disease for puppies when there are not enough maternal antibodies to protect them from it but too many to allow for active immunization. Exposure to disease during this time may result in sick puppies even though they may have received one or more vaccinations. The most common example is exposure to parvovirus.
Types of Vaccines
The two most common types of vaccines are modified live (attenuated) and non-infectious, or killed, vaccines. There are newer types of vaccines becoming available that include recombinant vector, recombinant protein and purified subunit vaccines. The infectious agent in modified live vaccine (MLV) has been modified so that it will produce an immune response but not cause disease as it replicates in the body. The advantages of MLVs are that they provide rapid protection of a long duration; are less allergenic; produce cell-mediated, humoral and local immunity; overcome maternal antibody more quickly; and can often protect with one dose. Disadvantages of MLVs include the risk of contamination with other infectious agents, the possibility of producing disease in immunocompromised patients, the possibility of reversion to virulence and the need to be handled more carefully to maintain effectiveness. Killed vaccines have the advantages of being safer in immunocompromised patients, not reverting to virulence and being more stable in storage. The disadvantages, however, are that killed products are more likely to cause allergic reactions because they contain more foreign protein, require the addition of adjuvants (non-specific immunostimulants), require that two doses be given initially, require more frequent revaccination, and don't stimulate much cell-mediated immunity or local immunity. In regard to the newer types of MLVs, the recombinant vector vaccines take the genetic code for key immunogenic proteins and insert it into a non-pathogenic organism such as pox virus, herpes virus, bovine papillomavirus or Simian virus 40. The goal is to stimulate immunity without the potential of causing disease. Examples of this vaccine are Merial's Recombitek CDV for distemper and Recombitek-R for rabies protection. The newer types of non-infectious vaccines are a purified subunit vaccine containing just the specific proteins to stimulate a good immune response (Pfizer's Leukocell-2 vaccine for cats) and the recombinant protein vaccine where the desired gene is cloned into an organism that produces a desired protein. The protein is then harvested and purified (Merial's Recombitek Lyme vaccine).
Vaccine Failures
Not all vaccines will effectively immunize; this can be due to host, vaccine or human factors. Host factors include whether the animal is incubating a disease, is debilitated or malnourished, or has a high or low temperature, existing maternal antibodies or some form of immunodeficiency. Vaccine factors that may cause failure to immunize include improper storage or handling, strain differences between the vaccine and the actual infectious agent, excessive attenuation and reversion to virulence. Human factors that cause vaccine failure are improper vaccination protocol; vaccine interference; improper mixing, route of administration, or disinfectants; and the concurrent use of some antimicrobials.
Note: In the next issue of The Pet Tribune, Dr. Anderson will detail adverse reactions to vaccinations and the current status of several canine vaccinations, among other important topics.
-Dr. Susan B. Anderson, Diplomate, is a member of the American Board of Veterinary Practitioners. Certified in Canine and Feline Practice, Anderson practices and teaches at the University of Florida College of Veterinary Medicine in Gainesville, Fla.
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