Veterinary vaccines are antigen preparations to be used for preventing of diseases in animals caused by virus (Newcastle disease, infectious bronchitis, Marek’s disease, Gumboro disease, …), bacteria (fowl cholera, fowl typhoid, colibacillosis, …) mycoplasma (mycoplasma), fungi and protozoa (coccidiosis) or their toxins. Therefore, there are different types of vaccines based on the antigen used and this in turn determines the processing method.

The essential component of any vaccine is the antigen which consists of a foreign or own-animal substance that can be recognized by its immune system and causes a reaction. In this article, we will focus on the importance of bacterial vaccines.

Bacterial vaccines for veterinary use are obtained from infectious agents against which vaccine, in this case, bacteria. These bacteria are subjected to different modifications to eliminate its capacity to cause disease, while maintaining its immunogenicity. That is, they activate the immune system of the animal, which generates antibodies that specifically react against the antigen.

Types of bacterial vaccines

Bacterial vaccines can be divided into two classes: live-attenuated vaccines and inactivated (killed) vaccines.

Live-attenuated vaccines

Live-attenuated vaccines are composed of one or more alive bacteria whose virulence has been weakened (attenuated) and induce immunity in animals against these bacteria. These vaccines can be produced from:

• Culture media containing toxic against bacteria. E.g. vaccine against Bacillus anthracis (agar culture with 50% serum in CO₂ rich environment).
• Successive passes from the bacteria through a series of animal embryos (e.g. chicken embryos). E.g. avian tuberculosis vaccine produced by Mycobacterium avium

As bacteria are in an hostile environment different to original, the vaccine decreases its pathogenicity.

The advantage of live-attenuated vaccines is that its immune response is faster, because the microorganism is the same as that causes the disease, only that it is weakened. However, as the pathogen is alive, there is a danger that the disease can develop in immunosuppressed animals or reverse its pathogenicity after continued use.

Therefore, live-attenuated vaccines are unsafe in a long-term because there may be a reversion of virulence.

Inactivated vaccines

Inactivated (killed) vaccines are generated from the growth of the infectious agents in a static culture medium (fermenter) or with stirring, generating the normal bacteria more the toxins excreted by the bacteria. Bacteria are inactivated by using physical and/or chemical agents, maintaining its structure.

The most widely chemical methods used for inactivating vaccines are formaldehyde, thimerosal (merthiolate) or chelating agents such as ethylene oxide, propiolactone and ethylenediamine. Temperature is the most commonly physical method used.

Inactivated vaccines are divided into two kinds:

• Inactivated vaccines without toxoids are prepared by separating the bacteria from the culture medium by centrifugation, so that toxins excreted by the bacteria are removed. The animal is protected only against bacteria but not against toxins excreted by bacteria. Eg porcine colibacillosis vaccine generated by coli
• Inactivated vaccines with toxoids are prepared including the whole broth, that is, both bacteria and toxins excreted by the bacteria. These toxins are some toxic compounds that affect the animal’s immune system, that is, the toxins can be immunosuppressive. Therefore, the animal vaccinated with toxoids is protected against bacteria and their toxins, getting an improved immune outcome. E.g. ALQUERVAC NEUMO, pneumonia vaccine generated by Actinobacillus and Bordetella.

The advantage of inactivated vaccines in front of live-attenuated vaccines is that there is no danger that animals suffer disease because the pathogen can not be replicated. Inactivated vaccines may or may not contain adjuvants.

The advantage of inactivated vaccines with toxoids in front of inactivated vaccines without toxoids is that the animal’s immune system remains intact and acts more effectively, as there is a double protection against bacteria and toxins.

Therefore, inactivated vaccines with toxoids are safe in application and highly effective against bacteria and toxins that cause disease.

Adjuvants

The use of adjuvants is recommended to slow down the release of antigen from inactivated vaccines with toxoids. Adjuvants are chemicals, materials of microbiological origin or mixtures that, when administered together with the antigen, contribute to the production of a longer immune response.

The most widely used adjuvants are:

• Aluminum salts and calcium: delay release of the antigen, obtaining a prolonged immune response. AlPO₄, Al(OH)₃, Ca₃(PO₄)₂
• Immunostimulants: increase or restore the immune system. They may be materials of biological origin (Corynebacterium, Bacillus Calmette, Bordetella), vegetable (ellagic acid from Emblica officinalis, tinosporine from Tinospora cordifolia) or chemical (fatty acid salts, isoprinosine).
• Emulsifiers: “Freund’s complete adjuvant” is an emulsion of water in mineral oil with killed mycobacteria
• Liposomes

Conclusions

From the first vaccine developed by Louis Pasteur to the current vaccines, there has been a great evolution in the development of vaccines against different diseases of bacterial origin, using different methods to attenuate the virulence of infectious agents (live-attenuated vaccines) as inactivate them (inactivated vaccines). The choice of one vaccine or another will provide a greater immune response with more effective and safe results.

It is recommended to develop immunization plans according to the epidemiology of the geographic area where the animals are located.

Picture: REUTERS/Crack