How the Immune System Works
When a foreign agent gains access to the body of the dog, the intrusion is detected through the body's security network called the lymphoid system. Lymph nodes are strategically located to guard portal entries into the body. An enemy invader will eventually reach the circulation and be filtered out through the lymph nodes or the spleen. In the lymph nodes, white blood cells called macrophages, surround and degrade the foreign agent and eventually expose antigens. The immune system then responds to the antigens in two ways. B lymphocytes, cells originating in the bone marrow, have proteins on their surface which will bind to the antigens. Binding, in turn, activates the B lymphocyte to mature into a plasma cell that multiplies and then is released into the blood circulation. Once circulating through the body, the plasma cells synthesize and secrete specific antibodies that target and destroy all invaders displaying that particular antigen. Once the infectious material is destroyed, the mature B lymphocyte, or plasma cell, remains in circulation as a "memory cell". If the body becomes invaded again by the same foreign agent, the memory cell produces antibodies to the antigen so rapidly that the infectious agent does not have an opportunity to multiply and produce symptoms of infection in the dog. In addition to the B lymphocyte, the immune system is composed of another cell type which can recognize and bind to antigens. However, these cells do not secrete antibodies. Instead, T lymphocytes that mature through the thymus gland, have proteins on their surfaces called T cell receptors which may bind to the antigen. Additionally, these immune cells release certain biological factors that attract macrophages to the area of infection.
There are three types of T cells involved in immunity: the cytotoxic or killer T cells bind to and destroy other cells which display antigens on their surface; the helper T cells which assist B cells to stimulate the growth and secretion of antibodies; and suppressor T cells which reduce B cell activity and thereby play a role in reducing the possibility of an autoimmune response.
Vaccines: Preparing the Immune System In Advance For Invasion
Canine infectious diseases are caused by organisms which gain access to the body, multiply, and in the process of their life cycles cause severe and in some cases irreparable damage to the cells which make up organs and tissues of the body. Even in animals with normal immune function, invasion and damage can proceed at a rate faster than the immune system's ability to destroy the invader. In cases where organ function is severely compromised, the dog may succumb to the disease before the immune system can eradicate the infection, or in cases where the infection is eliminated, death or debilitation may still occur as a result of irreparable cellular damage. Based on the knowledge that the immune system responds much more rapidly if it encounters an invading organism that it has already battled and defeated, the theory that introducing just enough antigen into the body to illicit an immune response without causing disease would protect the body from contracting the disease at a later time gave rise to the procedure of vaccinating. Therefore, "vaccination," also known as "active immunization" refers to the procedure whereby administration of an antigen results in protective immunity to the disease associated with that antigen. Types of Vaccines Killed vaccines. Killed vaccines are composed of "inactivated" microorganisms which cause a particular infectious disease. Because these microorganisms are dead they are unable to replicate once introduced into the dog's body and therefore are incapable of producing disease. However, their presence will induce an immune response. Therefore, in terms of some adverse reactions, killed vaccines are considered to pose fewer risks. In terms of protective immunity, however, killed vaccines produce weak immune responses and provide a shorter duration of protective immunity. In many cases, killed vaccines must be administered in large or frequent antigenic doses to induce a sufficient immune response to yield protection in the event of disease exposure. Modified live vaccines. Modified live vaccines are composed of "attenuated" microorganisms; that is, these microorganisms associated with a particular disease are altered so that they do not cause infection in most dogs, but they are still capable of replicating and inducing a protective immune response. Because these microorganisms are still capable of replicating and spreading throughout the body like an infectious agent, they elicit a stronger protective immunity of longer duration. As such, however, a higher frequency of adverse reactions is associated with use of modified live vaccines (discussed below) and therefore, not all dogs are good candidates for immunization with modified live vaccines. Subunit vaccines. Subunit vaccines are composed not of the whole microorganism but only a component of the microorganism which will produce an immune response. Therefore, subunit vaccines are similar to killed vaccines in that they are not infectious and therefore, also present a low risk for adverse reactions. However, as with killed vaccines, subunit vaccines cannot replicate and, therefore, do not provide strong protective immunity for long periods of time. Because of these factors as well as the higher cost for production, subunit vaccines are used less frequently than modified live and killed vaccines.
The Vaccine Controversy: To Vaccinate or Not to Vaccinate
Even vaccines that have been proven to be safe and effective in a majority of individuals may cause adverse reactions when administered to certain individuals. In most instances and from a historical perspective, however, when one examines the correlation between introduction of a particular vaccine and decline in incidence of the respective disease subsequent to initiation of inoculation, the benefits of immunization for preservation and protection of health are clearly evident. For example, in 1990 the number of laboratory confirmed cases of dogs infected with rabies were approximately 150 for the year. Prior to local government enforced vaccination of dogs against rabies, the reported confirmed cases of this disease were approximately 7000 cases/year. Since, on the average, modern-day canine vaccines pose only a 1:60,000 risk that an individual dog will develop an adverse reaction; the benefits of administering the rabies vaccine for protection against disease far outweigh the risks of occurrence of adverse reactions. However, some early modified live rabies vaccines posed a high risk for bringing about active disease in both the immunized host and other non-immune individuals exposed to virus shed by the vaccinated animal because the injected microorganisms reverted back to an infectious state. Under these circumstances, the risks associated with vaccinating for rabies were unacceptable. Therefore, whenever deciding whether or not to immunize it is important to take all of the following factors into consideration: the risk of infection, the consequences of the disease, the availability of a safe and effective vaccine, and the duration for which the vaccine will provide protective immunity.
Risks and Consequences of Canine Infectious Diseases
(Reprinted from R. Foster and M.R. Smith, The Doctors Foster & Smith Catalog)
Canine distemper is a widespread, often fatal viral disease in which the early symptoms are similar to those of an upper respiratory infection in man. Fever, cough and nasal discharge occur regularly. If left untreated, signs of neural involvement may appear, including localized muscle twitching (chorea) and convulsions. Distemper is often resistant to treatment, but can be prevented through vaccination.
Canine adenovirus type-1 and type-2 cause infectious hepatitis and respiratory infection, respectively. Hepatitis caused by adenovirus type-1 may cause severe kidney damage or death. Common signs of this disease include listlessness, fever, loss of appetite, vomiting, excessive thirst, and discharges from the eyes and nose. Adenovirus type-2 is an important factor in kennel cough.
Canine bordetella may contribute to kennel cough. This bacterial infection can occur alone or in combination with distemper, adenovirus type-2 infection, parainfluenza, and other respiratory changes.
Canine leptospirosis is a bacterial infection which may lead to permanent kidney damage. The disease is easily spread to other pets and to humans. Depression, fever, and loss of appetite appear suddenly, and jaundice, vomiting, dehydration, excessive thirst, and excessive urination may indicate liver and kidney damage.
Canine parainfluenza is another cause of kennel cough. Although parainfluenza is often a mild respiratory infection in otherwise healthy dogs, it can be severe in puppies or debilitated dogs.
Canine parvovirus is a disease of widespread distribution which may cause severe dehydrating diarrhea in dogs of varying ages. Parvovirus infection is especially dangerous for puppies and very old dogs. In some instances, this disease leads to secondary heart disorders. Canine coronavirus infection is highly contagious intestinal disease causing vomiting and diarrhea in dogs of all ages. Especially in young puppies, dehydration from coronavirus infection can be life-threatening.
Lyme disease, a bacterial disease caused by Borrelia Burgdorferi, may be spread by insects such as flies, fleas and ticks. Arthritic-like symptoms may occur.
Rabies, a disease which has reached epidemic proportions throughout the United States, is almost always fatal. Rabies virus attacks the brain and central nervous system, and is transmitted to humans chiefly through the bite of an infected animal.
Kennel Cough - There is no vaccine for complete protection against infectious canine cough. Thirteen different viruses and bacteria are implicated as its cause. Currently vaccines are available for 3 of the 13 known components of the disease complex. These three include Parainfluenza, Adenovirus Type 2, and Bordetella. By vaccinating for these 3 diseases, 90% of the cases of kennel cough can be eliminated. Canine cough is usually a mild, self-limiting disease, but it can develop into a severe bronchopneumonia, especially in younger dogs. The most common sign of this disease is a harsh unproductive cough that leads to gagging or even vomiting.
(This information was taken from the Internet.)
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