Looking Beyond the Antigen in Livestock Vaccines to Adjuvants

Livestock

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Many manufacturers focus on the antigen component of their vaccines, but adjuvants also play an important role in creating immunity. 

Most livestock vaccines on the market today focus on the antigens contained inside the vial. One product may help protect against the latest strain of influenza while another may tout its protection against the most virulent strain in circulation. Looking beyond the discussion about antigens, there’s another component just as critical for killed vaccines: the adjuvant.

“Most killed, or inactivated, antigens don’t stimulate a strong immune response by themselves,” said Randy Shirbroun, DVM, director of the ruminant business unit at Newport Laboratories, Inc. “The purpose of the adjuvant is to help stimulate a stronger immune response to that antigen and sustain a longer immune response to that antigen. Basically, it’s providing another stimulation to the immune system.”


How adjuvants help

In killed vaccines, adjuvants can provide artificial signals to the immune system to initiate the immune response. This helps to minimize the number of immunizations needed for a good immune response. In addition, it helps decrease the amount of antigen needed – making the vaccine less expensive.1

The proprietary nature of adjuvants can make them difficult to differentiate, Shirbroun said.

“Companies will develop their own adjuvants. That’s part of the secret sauce, and those are closely guarded formulations,” he explained. “Often, you will see a proprietary adjuvant marketed under a brand name so they can talk about it and not divulge what it contains.”

Evolving formulations

While adjuvants can help, they can also cause adverse reactions like inflammation and abscesses at the injection site. In livestock, these reactions can result in carcass trim losses. Improvements in both vaccine administration practices and adjuvant technology have helped minimize these losses, Shirbroun noted.

“Adjuvants have evolved over the decades to simulate immune responses without stimulating reactions,” he said. “Some adjuvants work better with different antigens, too. For example, we can choose from different adjuvants based on the antigen, or combination of antigens, in a specific custom, or autogenous, product.”

Ease of use is also an important factor for livestock producers, Shirbroun said. For example, it’s typically the adjuvant portion of the vaccine that can make the product easy – or difficult – to administer chuteside.

“Different adjuvant formulations have their merits and, in some cases, maybe some disadvantages,” he said. “A manufacturer can evaluate these adjuvants, how well they perform in terms of an immune response and even in the syringeability of the product. Does drawing out the vaccine feel like molasses in January? Or, is it nice and fluid?”

Adding up the information

For commercial vaccines, there is always publicly available data on vaccine safety, efficacy and adverse events. This information can help veterinarians – and distributor sales representatives – make a conclusion among vaccine choices. Yet, comparing products can quickly become complicated when considering the proprietary nature of adjuvants. In addition, the convenience to the producer should be considered, Shirbroun added.

“Typically, most inactivated products – bacterins, for example – require two doses to get a good level of immunity. There’s the sensitizing dose to ‘tickle’ the immune system, and then you come back two to three weeks later with a booster to really kick that immune response up,” he said. “In the vaccine world, there are some adjuvants that are somewhat unique and can produce a good immune response with a single dose.”

A single-dose vaccine can provide benefits to livestock producers seeking to reduce processing and handling stress. On the other hand, the duration of immunity might be shorter compared to products that require a booster dose. However, this may not be a drawback to products that should be re-administered prior to seasonal challenges, Shirbroun noted.

“Sometimes, we – collectively as the cattle industry – make overly optimistic assumptions for the duration of immunity for the things we use. For example, a pinkeye bacterin has a duration of immunity that is typically not very long. For pinkeye vaccines, I recommend giving a booster at weaning because you can’t depend on the shot they got at branding.”

1 Spickler AR, Roth JA. Adjuvants in veterinary vaccines: modes of action and adverse effects. J Vet Intern Med. 2003 May-Jun;17(3):273-81.

What’s in a vaccine?

There are two basic ingredients in killed vaccines: antigens and adjuvants. Antigens are viruses or bacteria that have been inactivated so they don’t cause disease. But even in an inactivated state, antigens set off an alarm within the immune system, signaling that a foreign invader is present. It’s the job of the adjuvant to amplify that alarm, which allows the immune system to begin building up its defenses against the invader, making it better prepared to fend off the real thing.

Closeup photo of vet reading shot for milk cattle representative of adjuvants in livestock vaccines

Key points:

  • Vaccine adjuvants are chemicals, microbial components or proteins that enhance the immune response to vaccine antigens.1
  • Adjuvants are commonly used in killed vaccines. Modified-live virus vaccines usually do not require the inclusion of an adjuvant.
  • Despite years of research, adjuvants’ mechanism of action remains somewhat speculative.1
  • Most adjuvants appear to enhance antigen presentation, improve antigen stability or act as immunomodulators.1

Photo credits: istockphoto.com/mustafagull, istockphoto.com/Jevtic