Applied Bovine Immunology (Vaccinology)

Timothy J. Goldsmith DVM, MPH, DACVPM

College of Veterinary Medicine, University of Minnesota

Summary

1. Basic Bovine Immunology Review
2. Vaccine Types
3. Vaccine Efficacy
4. Vaccine Safety
5. Timing of Vaccine Administration
6. Vaccine Handling / Administration
7. Developing Vaccine Programs

Introduction

The understanding and application of what is known on the bovine immune system and how we manage immunity in cattle is continuing to evolve.  This review will attempt to cover some of the basics and concepts that would apply as a practitioner considers the application of immune concepts in the development of vaccine approaches.

Content

Bovine Immunology Review

The purpose of this chapter is not to get to deep into immunologic details, but to focus on the application of principles as it relates to vaccination and the management of the immune system of the bovine.  It is helpful to keep the basics in mind, the following diagram is an attempt to summarize the key aspects of the immune system, adding in where we introduce vaccines.

Graphic Representation of Immune Basics Related to Vaccinology – T. Goldsmith

Vaccine Types

The different types of vaccines and methods to manage or stimulate acquired immunity is important to understand as a basic step in optimally managing the immunity of animal populations. For this conversation, we will review Passive Immunity and Active or Acquired Immunity as the major buckets.

Passive Immunity

The bovine is born with limited or no acquired immunity, which makes passive immunity very important.  While is not a vaccine, methods to provide passive immunity are important factors in managing the immunity of the bovine for early life survival but also for overall immune system development and management.  The primary methods to provide passive immunity are through colostrum or blood/plasma transfusion, as well as some antibody and antitoxin products that are available in the market.

Colostrum management practices are key for management of passive immunity in the young bovine.  While we will not focus on those practices here, a practitioner must be aware that colostrum management is the first immune management step in the bovine.  More details on passive immunity considerations related to vaccine administration will be covered in the Timing of Vaccination section.

Bovine Neonatal Immunology - Review

• The normal neonate is agammaglobinemic when born
• Immunocompetent at birth.

• Fetal Calf: Respond to viruses at 90-120 days. >180 days responds to bacteria.
• Usually consider 150 days of gestation breakpoint for immunocompetence

• Colostrum supplies immediate specific and non-specific immunity.

• Antibodies – Primarily IgG
• Anti-inflammatory cytokines- TGF-beta, IL-4, IL-10

Components of Immunity in the Calf

Acquired Immunity

When we think of vaccinating animals we are primarily discussing the stimulation and management of acquired immunity.  Vaccination is essentially the act of introducting a pathogen in a way that will stimulate an immune response with limited to no active disease.  The goal of inducing immunity occurs when vaccination is done properly and an individual develops an immune response that is protective against disease as well as immune memory for future exposure.  We should also recognize that individuals do see common pathogens in their environment and these host pathogen interactions also result in acquired immune stimulation which are important.  In the bovine we traditionally will think of vaccines in two categories; Killed/Inactivated or Live/Attenuated.

A great resource for a listing of the available products and pathogens contained in them is available through the Compendium of Veterinary Products – US edition.  Go to Charts, choose biological Charts and choose species of interest.  https://mwi.cvpservice.com/

Killed / Inactivated Vaccines (KV)

KV are available for both viral and bacterial pathogens and primarily target the Humoral Immune system to stimulate antibodies and memory B cells targeted at the pathogens of interest (bacterins) as well as in some cased the toxins (toxoids) produced by pathogens that cause disease symptoms. Types of vaccines included in this category include; Bacterins, Killed Viruses, Subunits (Toxoids), Recombinant, Conjugate and Polysaccharide products.  Autogenous vaccines are also usually a Killed or Inactivated Vaccine.  

As a general rule of thumb KV will require adjuvants for an effective immune response as well as an initial booster series.  However , there are some KV that use specific antigens to achieve effective immune responses with one dose.  In addition there is starting to be some technologies that allow for a delayed release of a second antigen exposure, effectively getting a two dose series with one administration.  

Live / Attenuated Vaccines

Live vaccines are available for both viral and bacterial pathogens and primarily target the Cell Mediated Immune system.  Types of vaccines most common in this category are Modified Live Virus (MLV) or Attenuated Live Bacteria.  As a general rule of thumb live vaccines do not require “boosters” to achieve acceptable levels of vaccine, but often apply the “revac” concept to achieve a higher % of responders in the population for herd immunity.  Rather than relying on and adjuvant live vaccines rely on replication for stimulation and antigenic load, but we are seeing more MLV including an adjuvant to enhance specific response.

Adjuvants

Adjuvants are components added to vaccines to “nonspecifically” increase vaccine effectiveness, enhancing the final immune response to a given vaccine. In general, this enhancement can occur due to a shortening in the time of onset, an increase in total response (antibody or T cells), or influence the quality of the response in terms of antibody isotype, cellular response, or a combination of both. 

Vet Clin Food Anim 35 (2019) 391–403 https://doi.org/10.1016/j.cvfa.2019.08.005

The primary mechanism that adjuvants use to enhance response are;

1. Physical regulation of antigen release

1. increase, prolong, and enhance exposure of the immune system to antigen
2. “antigen-depot” effect

2. Enhanced Response to Antigen Exposure

1. Receptor targeting to effectively boost the production of specific antibody and T cells.

3. Regulation of the Quality of the Immune Response

1. Regulation of signaling to affect response

Immunomodulators

Future Section

Routes of Administration

Commercial vaccines available in the US for cattle today primarily come in two routes; Intra Nasal (IN) / Mucosal Vaccination available as Intra Nasal (IN) delivery products or Parenteral Vaccination available in Subcutaneous (SQ) or Intramuscular (IM) delivery products.  In addition, there is one MLV oral vaccine for neonatal scours control; this would be acting primarily as a mucosal vaccination.  

Practitioners should recognize that the difference in these delivery methods does matter.  IN products are primarily stimulating a mucosal immune response (IgA), while IM and SQ products are by passing the mucosal immune system barriers and stimulate the systemic immune system both Humoral and Cell Mediated responses.  While there is some independence of these two portions (mucosal and systemic) of the immune system, there is also some relationships related to memory and targeting of immune functions.  The level of knowledge related to the relationships between delivery methods and how to manipulate those for optimal immune management is evolving and approaches will likely be different in the future.

An example of using different delivery methods is the “prime-boost” approach.  Conceptually this involves exposing the immune system to different forms of antigen by different routes to achieve a superior immunologically balanced and durable response.  In practice currently, this involves the use of early mucosal priming (IN) dose followed by a boosting and immune memory maintenance dose of a parenteral vaccine (most commonly a MLV).

Vaccine Efficacy

Vaccine efficacy is a measure of how a vaccine works in a population related to the vaccines ability to alter disease status in vaccinated individuals when exposed to an infectious agent.  

Label Claims

Since 2002 licensed biological products in the US were given one of four, efficacy claims related to efficacy level.  Starting in 2015, products have been moving to a single uniform label format. Rather than grant an efficacy level to the label claim, all biological products approved in the US are required to have a standardized summary, of the efficacy and safety data in support of a product license or conditional license given by USDA.  

History of the Single Tier Label Claim https://www.aphis.usda.gov/animal_health/vet_biologics/publications/STLC_History.pdf

All label claims for veterinary biologicals are based on statistical significance and clinical relevance of efficacy data provided by manufacturers. Historically, veterinary biologicals were assigned one of 4 tiers of effectiveness, depending on the magnitude of the observed protective effects:

• for the prevention of infection,
• for the prevention of disease,
• as an aid in the prevention of disease, or
• as an aid in the control/reduction of disease.

Distinctions between these levels were not well understood by many users of biological products, resulting in potential misinterpretation of label information.

In response to this concern, the USDA is now using a single statement (e.g., “This product has been shown to be effective for the vaccination of healthy animals X weeks of age or older against * * *”) to replace the previous four tiers of effectiveness. The rigor with which the USDA evaluates efficacy data for statistical and clinical meaningfulness has not changed.

In addition to a standardized claim statement, a standardized Product Compilation Summary of the efficacy and safety studies used to support licensure is available for review on this website.

https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/veterinary-biologics/ct_vb_licensed_products

Assessing Effectiveness of Vaccination

For the purpose of product licensing the vaccine efficacy measured is most commonly the direct effect, or the impact directly on the vaccinated population as compared to the non-vaccinated when challenged in challenge study model.  The mathematical formula for direct vaccine efficacy would be:

Vaccine Efficacy = Incidence of disease in unvaccinated – Incidence of disease in vaccinated

                                        Incidence of disease in unvaccinated

In a production system setting, we seldom live in controlled challenge study environment, and the impact of a vaccine is the combination of direct and indirect effects.  To expand this we need to remember that a vaccine rarely has the ability to prevent infection, but rather reduces/limits the susceptibility to clinical disease caused by infection as well as the virulence of the disease in the vaccinated individual, what would be considered direct effects.  The indirect effects a related to the decrease in duration and shedding of pathogens into the environment that decrease the exposure likelihood or level for those other individuals that may have no or limited immunity.  This combination of effects is commonly referred to as “Herd Immunity”.  

Vaccine Value / Return on Investment

Future section

Vaccine Safety

Similar to efficacy, licensed vaccine products in the US must have safety studies completed which demonstrate the product is safe.  The summary of these studies can be accessed from the same website as the efficacy summaries. The definition used by USDA for safety approvals is what they refer to as an adverse event:

Definition - An adverse event (AE) is any observation in an animal that is unfavorable and unintended and occurs after the use of a veterinary product or investigational veterinary product, whether or not test operators consider it product related.

For the purpose of this review, we will consider safety not only to include the animal but also those administering the product as well as any food safety / injection site aspects.

Adverse Events

Adverse events experienced following vaccination in cattle most commonly include the following:

• Immune system / inflammatory process related

• Soreness/swelling/abscesses/knots at injection site
• Fever
• Anaphylaxis (allergic reactions) including death

• Production related

• Off –feed,
• Drop in milk production

• Reversion to virulence for MLV

• Abortion

Immunologist generally agree that some level of inflammatory response impact is necessary if a vaccine is stimulating and immune response, and should be expected.  Anaphylaxis is always a risk and can happen, however this should be a rare event.  It is recommended that epinephrine and supportive care be available when administering vaccines and for a time following the administration.  

The production related impacts are generally related to the activated immune systems large glucose requirement.  This can result in a state of hyperglycemia followed by hypoglycemia, followed by anorexia, etc.  This is most acute in gram – LPS vaccines, but is seen at some level with any vaccine that stimulates an active immune response, as well as acute disease that stimulates and immune response.

Reversion to virulence is a potential risk with any MLV but in the bovine is most commonly dealt with in MLV containing Bovine Herpes Virus – 1 (BHV-1) also referred to as IBR.  We will discuss that more below.

Endotoxin Stacking

Endotoxin stacking is the use of multiple vaccines that contain gram – cell wall and/or LPS fractions to stimulate immunity at the same time.  The administration of multiple vaccines in this category can result in severe adverse events and production impacts including anaphylaxis and death.  These severe adverse events are reported to occur more commonly in the Holstein breed of cattle as well as pure or straight bred cattle.  

The following is a list of gram – or gram – acting vaccines that are reactive in this way from most reactive to least reactive, provided by Dr. Chris Chase:

• E.coli Mastitis vaccines
• Pinkeye (Moraxella bovis)- Whole cell LOS very reactive
• H. somni  Whole cell LOS very reactive
• Salmonella-Whole cell LPS
• E.coli Scour vaccines -Whole cell LPS
• M. hemolytica- Whole cell LPS
• P. multocida

The general recommendation in the bovine industry is to never give more than 2 gram – vaccines at one time and if need to give more than 1, administer each vaccine in opposite sides of the neck (different lymph node chains) to reduce risk.  Subunit vaccines such as leukotoxoids, etc. seem to have no issues.

MLV BHV-1 (IBR) Considerations

The administration of MLV IBR vaccines to naïve animals have been associated with the following adverse events.  An individual should be considered naïve to IBR if it has not received a two dose series of MLV IBR previously for reproductive animals, and no history of any IBR vaccine in non-reproductive animals.  

• Abortion in pregnant animals

• Due to IBR infection of the fetus

• Negative effects on reproduction when administered within 30 days of breeding.

• Due to luteal necrosis from local IBR infection

• Decreased first service conception rates
• Irregular return to estrus

• Immunodominance / Interference Issues        

• Decreased immune response to antigens given concurrently in naïve animals

General recommendations in the bovine industry related to MLV IBR are;

1. Administration of MLV IBR in pregnant animals, even when following label recommendations, has a level of risk,
2. Do not administer a MLV IBR vaccine within 30 days prior to first breeding (some have gone to a min of 45 or 60 days prior), and
3. If administering MLV IBR vaccine in naïve calves you should administer concurrent vaccines (IBK, Bangs, M. haem, etc.) in the opposite side of neck.  

Safety of the Administrator

In addition to general safety related to use of needles and injection as well as adequate restraint of cattle when administer products, there is a safety issue for administrators, mostly related to attenuated live bacterial vaccines.  The most common is:

BRUCELLA ABORTUS VACCINE (STRAIN RB-51)

Professional Biological

BRUCELLA ABORTUS VACCINE STRAIN RB-51, LIVE CULTURE

https://mwi.cvpservice.com/product/view/1425000

Food Safety / Injection Site Concerns

All licensed cattle vaccines in the US carry a meat withdrawal on the label.  While this may not be primarily due to violative residues in the tissues, it is likely due to a local reaction / injection site that would require trimming and product loss.  Practitioners should be aware of this and account for it when developing protocols.  

Vaccine Labels and Safety Warnings

Vaccine labels contain a wealth of information related to safety, practitioners should be aware of the label on products they use.  Below is an example, with all the different statements related to safety highlighted in yellow.

https://mwi.cvpservice.com/product/view/3690207 

Timing of Vaccination Administration

The timing of administration of vaccinations depends on a variety of factors and is an important consideration in the development of vaccination protocols.  This review will cover what the author considers the major considerations related to vaccination timings.  

Passive Immunity / Maternal Interference

As discussed in the Vaccine Types section above, good passive immunity is vital to the survivability of the young bovine as well as maximizing lifetime production.  When considering timing of vaccine delivery in the young bovine passive immunity is often brought up as a limitation or challenge to successful vaccination in young bovines.  This challenge is mostly summarized in the common acceptance that the presence of maternal antibodies prevents seroconversion from parenteral vaccination.  This is challenged by the following findings in literature:

• Calves with maternal AB do not seroconvert but were primed for a secondary response after subsequent dose of vaccine; Brar JS, et al. AJVR 1978;1098-101
• Calves vaccinated with MLV develop specific memory T-cell response (Cell Mediated) in the presence of high Maternal AB; Endsley JJ, et al. Biologicals 2003 Jun;31(2):123-5
• Presence of T-cell and B-cells resulting from parenteral vaccinations have been demonstrated to be protective/disease sparing; Endsley JJ, et al. Biologicals 2003 Jun;31(2):123-5
• Protection should be demonstrated through challenge studies; R.W. Fulton, et al./Vaccine 22 (2004) 643-649

The above summary statements highlight that the historic acceptance (based on lack of seroconversion) that you cannot successfully vaccinate In the Face of Maternal Antibody (IFOMA) was lacking the consideration of the Cell Mediated arm of the immune system.  

In addition the type of vaccine and vaccine delivery needs to be considered, KV are less likely to stimulate a Cell Medicated response and are less likely to be successful, while MLV are demonstrated to establish memory.  IN vaccination route is also a key consideration IFOMA as this route of delivery targets the mucosal immune system, which is somewhat separate from the systemic immune system and thus less affected by the circulating IgG acquired via passive immunity.

Duration of Passive Immunity

The level and duration of passive immunity is directly related to the amount/level consumed and thus the length of time a calf is protected is highly variable among individuals.  The charts below demonstrate the AB titer decay over time (AB decay by half-life’s) and the distribution of AB titers in a group of calves with a history of good colostrum management.

Understanding when passive immunity is gone is an important indicator for when vaccination may be needed in a group of calves.  The chart below is the distribution of titers above decayed by ½ life to no AB level, suggesting no protection (level of protection needed would depend on challenge).  This shows us that calves with a good colostrum management history, start to reach levels of zero AB for the measured viruses at around 60 days of age.

Immune System Maturity

While a bovine is immune competent before birth the immune system “matures” over time, with full maturity thought to occur around the time of puberty to one year of age.  This is demonstrated in the graphic below.  A key time is represented by the “window of susceptibility”, where the calf’s passive immunity is waning and active/acquired immunity is increasing, leaving an valley of lower overall immunity that depending on when that occurs can be a very “high risk” time, such as weaning.

In addition, there is believed to be a level of immune dysfunction in the neonate bovine, which occurs in the first week of life.  This is show by diminished blastogenic responses and lower lymphocyte development.  The resulting application of this is the general recommendations that vaccination should be avoided between day 3 and 7 of age in general, and parenteral bacterins should be avoided in the first 3 weeks of life.

Timing of Booster Vaccinations

Information around the timing of booster vaccination doses has increased in recent years.  For optimal response, adequate time is needed for the initial immune response and subsequent apoptosis of lymphocytes to occur.  This time is dependent on the type of vaccine and adjuvant used we are learning.  The graph below represents what the minimum times should be.  It appears that in field application, that greatly extending the time may have limited effects, but has not been demonstrated.  We do know however, that going too short can render the booster dose ineffective, resulting in a non-protected individual.  Vaccine labels do have the recommended time for the booster dose on them; labels should be reviewed and followed.

Stress

Stress in the context of immunity can be considered anything that reduces the immune response capability.  In reality, there are aspects of intensive production systems and life stages of animals that can be considered stressful.  These can include but are not limited to:

Anything that improves adaption to “stressful events’ will reduce costs and improve production and health, specifically with regards to the immune system vaccination prior to or avoiding stressful events is thought to positively affect immune response.

Vaccine Handling / Administration

There are many effective vaccines available to use to manage immunity in cattle today.  In order to maximize the value of these products they have to be handled and administered in the proper manner.

Storage

Vaccines should be stored and maintained at the appropriate temperature range the entire time prior to use.  Common issues related to storage include:

• Lack of refrigeration or use of ice packs and coolers while transport from purchase location to farm location or from storage to use area
• Faulty refrigerators on farm (old, temperature set incorrect, door left open, power failure, etc.)
• Multi-use refrigerator

• example; same refrigerator used for cooling colostrum and vaccine storage will have large temp swings

• Overstocked refrigerator that does not allow air flow for cooling
• Not updating or rotating inventory, discard or return out of date products to manufacturer

Handling

Vaccines should be kept cool and out of the sunlight until used.  In addition vaccines should not be allowed to freeze, (this can be common issue in norther climates).  Development of coolers that accommodate keeping vaccines cool as well as not allowing them to freeze are important vaccine handling tools.

Build you own vaccine cooler: https://ucanr.edu/sites/BeefCattle/files/315807.pdf

When mixing vaccines they should be rolled to mix and not shaken.  Violently shaking vaccines can actually damage the product, create foam, etc.  In extreme cases, a violently shaken bacterin can have a much higher endotoxin load due to the damage of the shaking.

MLV vaccines should not be mixed until they are needed.  Once mixed they should be used up as soon as possible, within four hrs. max.  If beyond four hrs. should be discarded.  When mixing MLVs clean transfer needles should be used.

Administration

Vaccines should always be administered according to label and not mixed with other vaccines unless indicated on the label.  For BQA purposes vaccines should always be given SQ if have a choice between SQ and IM.  IN vaccines should use a clean cannula.  Needles should be replaced often, and anytime they become damaged or dirty.  Always use a clean needle when drawing off from the bottle.

Cleaning / maintaining equipment

Multiuse vaccine guns/syringes and transfer needles should be cleaned with boiling water and never with soap or disinfectants on the internal parts.  Disinfectants inactivate MLV’s.  Maintaining multiuse syringes so they are giving accurate doses is also important.

Developing Vaccine Programs

The goal of any vaccination program is to strategically manage an individual or group of individuals’ immunity level to prevent or minimize the impact of clinical disease.  The production system, class of animals, and management details are important components for consideration and thus most vaccination programs should be unique to the system being considered, although many will have some common components.  The level of immunity (protective threshold) needed is dependent on the disease exposure level.  Any level of protection can be overwhelmed by exposure, thus it is key to consider and manage exposure as part of any vaccination program.

Designing Vaccine Strategies

There are three basic principles that become part of most vaccination strategies.

1. Need: Is risk of exposure to a given disease high enough, or the level of stress great enough to become a health issue?
2. Efficacy: Is the vaccine effective and is enough evidence available to support claims?

3. Safety: Is the vaccine safe and timing of administration appropriate?

To put this in context the author approaches the designing of vaccine strategies by the Who, Why, What, When and How approach.

Who? – This is the population description or animal class you are targeting with the vaccination program.  Consideration include age, production status, etc. as well as the relation / interaction to the overall production system being considered.  For example, you cannot develop a wet calf vaccination program without considering the colostrum management components of the dam and the source farm.

Why? – A good first step in any program development is to ask: why are we doing this, what are the goals?  Common considerations related to this would be, History of a known issues/challenge, Risk Factors related to management constraints, animal flow issues, seasonal challenges, etc. as well as risk management goals from a risk tolerance perspective and other producer driven goals.  

What? – What goes closely with Why, with the detailed focus on two components.  

1. What pathogen or resultant disease process are you targeting?

• Vaccines target pathogens or products of pathogens such as toxins, the more specific you can describe your goals the better you can refine.

2. What vaccine or product will you choose?

• Is there a product available?

1. Is product safe and efficacious
2. Does data exist? (Approval data vs. Field use data)
3. Cost effective?

When? – When will vaccine be administered?  Considerations of age, animal class and production status, management and production systems, potential stressors, logistics, season and weather factors will help establish when vaccines can be administered to make a doable/feasible program.  Practitioners should recognize that “ideal” is not often achievable and designing programs that take into account the production systems and take advantage of leveraging other management tasks or handling events when possible and logical, help to achieve good compliance.  At the same when necessary additional handling events can be necessary to achieve immunity goals; practitioners can and should help prioritize areas where this is valuable and necessary.

How? – This is a vaccine protocol.  This should be written in a way that is clear on the Who, What (product and target pathogens), and adding in the details on the How (dose, route, withdrawal, etc. as necessary).  Developing a clear and concise protocol can help assure compliance and consistent application, which in turn can allow evaluation and changes based on outcomes as relevant.

Additional Resources

Ruminant Immunology

Veterinary Clinics: Food Animal Practice, Volume 35, Issue 3

https://www.vetfood.theclinics.com/issue/S0749-0720(19)X0003-1

Acknowledgments

All attempts to include references as applicable in the above text were made by the author.

Dr. Goldsmith would like to thank and acknowledge the following for their guidance and input both in personal communications and materials.

Dr. Chris Chase; South Dakota State University

Dr. Gerald Stokka; North Dakota State University

Dr. Vic Cortese; Zoetis Animal Health