Cattle vaccines are generally available in one of two different forms, either modified-live or attenuated products, or killed or inactivated products. Each type of vaccine has its unique properties, advantages and disadvantages, and its place in a herd health program. Many commonly used viral cattle vaccines, such as infectious bovine rhinotracheitis (IBR), bovine viral diarrhea (BVD), bovine respiratory syncytial virus (BRSV), rotavirus and coronavirus, are available as either modified-live or killed products. However, many bacterial pathogens including E. coli, H. somni, leptospira, moraxella, etc., are only available as killed products.
Modified-live vaccines
Modified-live vaccines contain an altered version of the actual virus or bacteria that can replicate in the body but won’t cause disease in healthy cattle. Historically, it is believed that the best immunity is achieved by an individual getting a disease and surviving that disease. The next best method of achieving long-lasting immunity is for an animal to receive a modified-live vaccine. Modified-live organisms more closely resemble the naturally occurring organisms and mimic a natural infection more closely than do killed pathogen vaccines. Modified-live vaccines trigger both a strong antibody and cell‑mediated immune response, typically providing long-lasting, durable protection after just one or two doses.
Essentially all modified-live vaccines come in the form of a freeze-dried powder containing the organisms and are reconstituted with an included liquid diluent. The liquid diluent may be sterile water, saline or a solution of killed organisms. For example, the killed leptospira portion of a 10-way cattle reproductive vaccine is contained in the liquid diluent. It is always very important to use the diluent included with the vaccine and not substitute it with a different diluent.
Modified-live vaccines should be kept cool and in the dark after mixing. Manufacturers' recommendations vary as to how soon a vaccine should be used after mixing, but generally, modified-live vaccines should be used within two to four hours after mixing. Modified-live vaccines should never be exposed to sunlight, nor should they be kept overnight and used the following day; this is a disadvantage of modified-live vaccines.
Adverse reactions to vaccines have been covered in a previous article. Although adverse reactions to modified-live vaccines are fairly uncommon, one of the more frequent adverse reactions is when modified-live infectious bovine rhinotracheitis (IBR) vaccines are given to naïve, unvaccinated heifers. Several studies have shown that vaccination of naïve animals within 30 days of breeding can cause diminished pregnancy rates. Modified-live IBR vaccines can negatively impact the corpus luteum of the ovary of these vaccinated heifers. Affected heifers will have lower first-cycle conception rates, but the subsequent heat cycles and fertility are normal. Another adverse reaction seen with modified-live IBR vaccines is when the vaccine is administered to unvaccinated pregnant heifers, abortion can be seen three to six weeks later. It is common for 10% to 30% of the heifers to abort beginning about three to six weeks post-vaccination; there is no effective therapy to stop the abortions. It is always important to follow the manufacturer's label directions regarding the administration of any vaccine.
A potential concern for some people is that modified-live vaccine organisms can revert to virulence and cause disease; this is an undocumented event. Some producers also believe vaccination is responsible for causing illness in their cattle a few days after vaccination. Post-vaccination sickness is usually due to the stress of processing or weaning, not due to the modified-live vaccine directly causing infections or sickness. Be aware that modified-live vaccines may be stressful or ineffective in highly stressed or nutritionally compromised animals.
An advantage of modified-live vaccines is cost. Modified-live vaccines per dose tend to be more economical than killed vaccines, for a few reasons. Modified-live vaccines tend not to be as highly adjuvanted as killed vaccines, and they have fewer organisms per dose since they rely on the natural replication of the organisms in the animal after administration.
Killed vaccines
Killed vaccines consist of pathogens or parts of pathogens, like proteins or toxins, that have been completely inactivated by way of heat, chemicals or radiation. Killed vaccines usually contain more organisms per dose than modified-live vaccines since the organisms in a killed vaccine cannot replicate. Additionally, killed vaccines tend to be highly adjuvanted, which can contribute to an increase in injection-site lesions and adverse reactions. Recall from previous articles that adjuvants are compounds added during the manufacturing of vaccines to increase the duration and exposure of the vaccine to the immune system. Adjuvants are required for killed vaccines to produce an acceptable immune response since the organisms do not replicate and are not a very “natural” way of exposing the immune system to pathogens.
Killed vaccines primarily induce an antibody response with a more limited cell-mediated response than can be obtained with a modified-live vaccine. The immune system's mediated mechanism is the primary way that the body protects against an infection from intracellular pathogens like viruses. Killed vaccines require multiple doses to build and maintain immunity, and often require periodic boosters due to the limited cell-mediated immunity they invoke.
Killed vaccines usually come in a convenient liquid, ready-to-use form, which gives producers the flexibility to use a single dose taken from a multiple-dose bottle. One exception is some killed Mannheimia haemolytica vaccines, which can come with a freeze-dried powder and a liquid diluent format, just like modified-live vaccines. The reason for this is that these are bacterin-toxoid products, and the toxoid component is more stable if freeze-dried. They are mixed just like a modified-live vaccine but are completely killed and inactivated. Mannheimia vaccines should be treated and handled just like modified-live vaccines, even though they are killed products.
Autogenous or custom vaccines are only available in a killed format, despite the fact that they are produced from live cultures obtained from the producer’s animals. USDA regulations prevent the manufacture of modified-live autogenous vaccines.
How do we choose when to use a modified-live or killed product?
The risk for disease in cattle depends on many factors, some of which are out of our control, such as weather, etc. How a vaccine is selected depends on factors that include what organism we wish to vaccinate against (Table 1). Vaccines for some organisms, such as BVD, IBR and BRSV, are available in both modified-live vaccines and killed products. Other pathogens, such as clostridial, leptospiras and Histophilus somni are only available as killed vaccine products. Another consideration is the anticipated stress level of the cattle to be vaccinated. Some ultra-high-stress cattle may benefit from a killed vaccine due to vaccine safety. Mid- to low-risk cattle may benefit from a modified-live vaccine because of better immunity and disease protection obtained from a modified-live vaccine.
Another factor in vaccine selection involves the cattle marketing opportunities associated with vaccine usage. Many branded preconditioning programs are available through the vaccine manufacturer, trade channel facilities or video auction markets. These branded preconditioning programs add value to cattle over and above the cost of vaccination.
Always remember to consult with your herd veterinarian; there are no cookie-cutter herd health programs. Your local veterinarian knows the differences between products and how to use the vaccines safely for maximum efficacy, and the prevalent diseases that your cattle may be at risk for.
