Vaccination is an important tool in veterinary medicine, preventing disease and reducing virus circulation for many important
viral pathogens. In small animal medicine, vaccines for a multitude of agents are available, and the decisions regarding what
vaccines to use and how often to use them can be difficult to determine. This discussion will cover the various general types
of vaccines available, and summary of recommended guidelines for selection.
Vaccines are designed to enhance the specific immune response to a particular pathogen in order to prevent disease upon exposure
and natural infection. This is accomplished through inoculation with all or part of an organism. The development of a vaccine
must take into consideration the type of immunity needed, the role of memory cells, safety, cost, stability, and ease of administration.
Vaccine types can be divided into two basic categories: noninfectious and infectious.
These vaccines include killed whole virus, or inoculum containing one or more virus peptides or proteins. The former is produced
by inactivation of virus propagated in cell culture or eggs through chemical modification or other means. The latter, also
referred to as subunit vaccines, may be produced in a variety of ways, including viral protein extraction and purification,
recombinant production of virus protein, or peptide synthesis. In all cases, noninfectious vaccines require an adjuvant to
enhance the immune response, since there is no virus replication following inoculation. This lack of replication also means
that, by necessity the antigen load in noninfectious vaccines is higher than that of infectious vaccines.
Noninfectious vaccines have certain advantages. There is no chance of reversion to virulence since the virus is "dead". These
vaccines are generally stable, and safe; thus they can be used in debilitated or pregnant animals with minimal risk. However,
because there is no active replication, the response stimulated is primarily humoral, with little if any cell-mediated component.
The duration of immunity tends to be shorter than that of infectious vaccines, and multiple boosters are often required. Because
of the addition of adjuvant, and the relatively high antigenic mass, adverse reactions may be more common, including hypersensitivity
reactions and granuloma formation. Examples in small animal medicine include most rabies vaccines, and most bacterins.
Infectious, or live vaccines contain viable, replicating organisms. These organisms have been modified in some way that reduces
or eliminates their disease-producing capabilities while maintaining their antigenicity. The majority of infectious vaccines
are attenuated whole viruses, containing the relevant pathogen which has undergone multiple mutations designed to abolish
its pathogenicity. Newer vaccines use recombinant technology, including vector viruses or DNA plasmids that express immunogenic
viral proteins. Recombinant vaccines are currently available, and most use a canarypoxvirus as the vector for various virus
proteins such as the coat proteins of canine distemper virus and rabies virus.
Infectious vaccines have a lower antigenic mass, and require no adjuvant. Because of their replicative ability, these vaccines
stimulate both humoral and cell-mediated immunity, and generally fewer boosters are required to induce and maintain protection.
There is some, albeit low risk for reversion to virulence. Contamination of live vaccines with adventitious agents has rarely
occurred leading to serious consequences. Generally, infectious vaccines are of lower stability and require maintenance of
the cold chain. Many are freeze-dried, and once rehydrated, have a short period of viability. Because the agents are infectious,
there is some risk for immunosuppressed animals, and significant risk for the fetus in pregnant animals.
In considering which vaccines to use, one must consider not only the epidemiology and threat of a particular pathogen, but
also the chances of the individual animal to exposure, and the properties of the vaccines available, including safety, efficacy
and cost. Many vaccines are multivalent, necessitating only a single inoculation to induce immunity to multiple agents. While
no consensus exists, guidelines are available from various professional organizations, including the AVMA, ACVIM, and AAFP.