The immune system protects the body against “foreign invaders” such as bacteria and other microorganisms that can cause disease. Certain proteins and other molecules of these invaders are known as antigens, and the immune-system defenses of the body respond to antigens by producing antibodies (see Introduction to Infections).
Drugs can be used to affect the immune system in several ways. Specific immunotherapy is perhaps the most familiar type—it involves giving a specific antigen (such as a vaccine) to cause a specific, controlled immune system response. As a result, vaccines can provoke effective, and often very specific, longterm immunity. Nonspecific immunotherapy can cause the immune system to produce proteins and other compounds that strengthen immunity. It may also give the immune system an overall boost to help it resist infection. Nonspecific immunity includes adjuvants, which may be added to a vaccine to increase its effectiveness, and immunostimulants, which may be given to treat longterm disease in which the immune system may be suppressed.
Several types of vaccines have been developed for use in animals. Traditionally, vaccines were grouped according to whether they contained living or killed organisms. Killed organisms are not as likely to provoke a strong immune response (providing immunity) as living ones. Because of this, vaccines that use killed organisms frequently also include additional compounds, called adjuvants, intended to increase the overall effectiveness of the vaccine. A killed vaccine may contain the entire killed organism or just the portion of the organism that provokes the immune response. A Type 1 recombinant vaccine is also classified as killed.
Although vaccines that include live organisms tend to be more effective, there are some challenges involved in developing them, because the live organisms can also cause disease if not modified in some way. Attenuated vaccines are vaccines containing live organisms that have been altered so that they are less likely to cause disease. They can reproduce, which will cause the animal to mount a strong immune response. However, even if vaccines are attenuated, they can sometimes revert to virulence, causing the disease they were intended to prevent. To further ensure safety, Type 2 recombinant vaccines, or gene-deleted vaccines, were developed to safeguard against attenuated vaccines reverting to a form that can cause disease. The specific genes that cause disease in the host are found and deleted. The resulting vaccine includes live organisms that can reproduce and generate a strong immune response in the host, but can never cause disease.
Live vectored vaccines, or Type 3 recombinant vaccines, are an alternate method of inducing strong immunity with no risk of reversion to virulence. Using technology, the genes that code for a protective protein are removed from the disease-causing organisms and placed into a “vector” organism that does not cause disease. This vector reproduces in the host, producing high levels of the protective protein. The host develops a strong immune response to this protein, which then protects it from exposure to the original disease-causing organism. These vaccines are essentially free of adverse effects and are very stable.
DNA vaccines make it possible to immunize an animal by just injecting it with the DNA coding for a protein from the disease-causing organism. In addition to preventing diseases, DNA vaccine technology can also be used to treat diseases, including certain cancers.
In most countries, the production of vaccines is strictly controlled and regulated by government authorities. All vaccines are checked for safety and potency.
The simplest and most common method used to give a vaccine is with an injection in the muscle or under the skin. Intranasal vaccines are also available for specific diseases, but they can be difficult to give, especially to large animals. Vaccines can also be given in feed or drinking water, a method that is most often used in the poultry industry. Fish can be vaccinated by immersion in a solution of antigen, which is absorbed through their gills. Transdermal (absorbed through the skin) vaccines are also being developed.
Vaccines stimulate an immune response, which lasts a varying amount of time depending on the specific vaccine and disease-causing organism. This means that a specific method and schedule of subsequent vaccine administration is needed to maintain immunity. In young animals there are challenges to mounting an immune response that must be considered, including the age of the animal and overcoming the immunity passed to the newborn by the mother. In mature animals some vaccinations are usually given yearly, while vaccination every 2 to 3 years is sufficient to ensure immunity with others.
Pet owners should work with their veterinarian to determine the best vaccination schedule for their animal(s). Few vaccines are available for species of pets other than dogs, cats, and horses. Ferrets should be vaccinated against rabies and canine distemper. Psittacine birds (parrots and parakeets) should be vaccinated against avian polyomavirus.
Modern, commercially produced, government-approved vaccines are generally very safe. The most common risks with vaccines include injection-site reactions (such as pain or swelling, which usually subside within a short time), allergic responses, incomplete inactivation, disease in animals with compromised immune systems, neurologic complications, and, rarely, contamination with other live agents. The stress of vaccination may be enough to activate an infection already present in the animal. Hypersensitivity reactions—which range from mild to anaphylactic shock—can also occur. All animals should be observed for a period of time following vaccination. Discuss with your veterinarian what signs you should watch for. In rare cases, certain vaccines have been linked to development of a type of skin cancer in cats at the site where the vaccine was given.
In addition to vaccination, there are other ways of creating or increasing immunity against disease. Passive immunity involves one animal producing antibodies by active immunization, and then transferring those antibodies to a susceptible animal to confer immediate protection. The natural (and very important) form of passive immunization is the transfer of maternal antibodies to offspring across the placenta and in the colostrum (the first milk that is full of essential antibodies). Antisera may be produced in dogs against distemper and in cats against panleukopenia (also known as feline parvovirus enteritis). The effects of passive immunity are only temporary, however, as they only last for as long as the transferred antibody lasts, generally a few weeks.
In the same way, antibodies can be harvested from a portion of the blood to create immune globulin, which can then be given to another animal to provide immunity. For example, tetanus immune globulin (tetanus antitoxin) is given to animals and humans to confer immediate protection against tetanus.