Some notes about over-vaccination from a immunologists David W. Horohov, Ph.D. This issue of whether we are vaccinating horses (or other species) too frequently leading to unintended consequences is often raised. Below are my thoughts, from an immunologist perspective. Please feel free to share this with your colleagues. If anyone has additional questions, they can contact me directly using the information below. Can vaccines overstimulate the immune system? As noted below by Dr. McClintock’s initial query, the immune system daily encounters many thousands of antigens, some novel and many seen previously. Those that are identified as foreign could, under the right set of circumstances, lead to the induction of an immune response. The immune system is capable of recognizing something like 100 billion antigens, so its recognition capacity is quite large. The daily exposure to antigens in the gut or airways is also large, whereas the antigen content of vaccines is, by contrast, quite small. Thus it is quite unlikely that any vaccine is capable of overloading the immune system, per se. Can adjuvants overstimulate the immune system. particularly when multiple vaccines containing different adjuvants are administered? All killed (inactivated) vaccines contain adjuvants that enhance the immunogenicity of these vaccines. The purpose of any vaccine is to stimulate both cellular and antibody responses and inclusion of an adjuvant enhances this function primarily by prolonging the immune systems exposure to the antigen. This allows for the use of less antigen in a given vaccine. Adjuvants, particularly modern adjuvants, also stimulate macrophages and other cells of the immune system to produce those accessory signals that are required for the induction of an immune response. They often do this by mimicking the molecular patterns of bacteria and other microbes that the immune system recognizes. This recognition leads to the increased expression of surface molecules and secreted proteins that enhance the induction of an immune response. One consequence of this activity is that there is often an immune infiltration into the injection site. In some individuals, this can be quite pronounced leading to local swelling with pain and systemic signs, notably fever. The fever and other systemic signs are the result of the production of pro-inflammatory cytokines induced by the vaccine. It is not known why some individuals appear to be to particularly sensitive to this response, though genetic background and vaccination history may play a role. Minimizing adjuvant exposure could lessen this response, so spreading out vaccine administration over time or using a combination vaccine is one way to limit this exposure. Can vaccination lead to allergies or other immune pathologies? It is important to remember that allergies and other immune-mediated pathologies are directed against specific antigens. While allergic responses to vaccine components can occur, these are very rare events and likely the result of a genetic susceptibility. I am not aware of any evidence that indicates a vaccine adjuvant could promote sensitization to an antigen not included in the vaccine. Indeed, in order for an adjuvant to work it has to be mixed directly with the vaccine antigen. This is in contrast to other immune modulators which are thought to modify immune responses beyond the injection site. As such, immune modulators are formulated to be dispersed throughout the body post-injection, unlike adjuvants which are formulated to remain at the injection site. Recent work has shown that many cases of immunopathology (allergies, chronic inflammatory conditions, autoimmunity, etc) result from a failure of T regulatory cells. These cells play a role in of maintaining homeostasis and serve to limit the magnitude and duration of most immune response while maintaining a population of memory cells at the ready for the next exposure. Like the immune response they are regulating, they are antigen-specific. The underlying mechanism responsible for the failure of these cells in allergies and other conditions remains unknown, but there is no evidence to suggest that vaccination precipitates this failure. This is this not to say that vaccines carry no risk and should be used freely. Vaccination is a medical treatment used to prevent a specific condition. The decision to vaccinate a horse should be made only after considering the risk and benefits of that procedure.
Posted on: Wed, 06 Nov 2013 12:19:14 +0000
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