When making the decision to use a modified-live vaccine (MLV) or a killed vaccine specifically for infectious bovine rhinotracheitis (IBR) and bovine viral diarrhea (BVD), it’s important to balance risk with achieving the greatest benefit from a vaccination protocol.
History of MLV safety claim
To fully appreciate this discussion, it is important to understand the history of the safety claim of MLVs, as well as the USDA ’s regulations that governed their approval and use.
Long before the first safety approval of MLVs in pregnant cows, progressive cattlemen recognized their calves could be safely vaccinated while nursing pregnant cows previously vaccinated with a MLV.
They pursued this practice because they discovered that calves vaccinated at branding with a MLV had reduced disease and sickness at weaning when compared to calves vaccinated with killed vaccines or non-vaccinated calves.
The killed vaccine did not provide the same broad-spectrum immune protection against various field strains of IBR and BVD that was seen with MLVs.
This was demonstrated clearly during the 1990s when a severe disease outbreak of a highly virulent strain of BVD type 2 made its way from Canada into the U.S. Calves vaccinated with killed viral vaccines were much more susceptible to infection than those given MLV vaccines.
Killed vaccines also tended to cause greater reactions due to the amount of antigen they carried and their adjuvants.
Given the efficacy advantage of MLVs in suckling calves, manufacturers began to pursue USDA approval of products for use in suckling calves.
To ensure that post-vaccination shedding of IBR or BVD virus from vaccinated calves was not an issue for pregnant cows, USDA required manufacturers to administer MLVs directly to pregnant cows during each trimester of gestation.
It was recognized that safety, under these circumstances, demanded that cows be previously vaccinated to prevent abortion, and that they would have to maintain a sustained immune response through the entire pregnancy.
This required that cows be vaccinated close to breeding to reach a level of immunity capable of preventing viremia and stopping viruses from crossing the placenta barrier and infecting the developing fetus.
The first MLV approval came in 2004 – safe for use in suckling calves and pregnant cows. Since this initial claim was granted, a number of additional vaccines have been granted this safety claim.
Safe use dependent on immune response
While all manufacturers have demonstrated in USDA-approved trials that their MLV vaccines were safe for use in pregnant cows and nursing calves suckling pregnant cows that had been previously vaccinated, the risk of abortion remains.
While most information surrounding the safety of MLVs is anecdotal – such as a rise in diagnostic lab IBR abortion or a herd that has had improved calving rates after stopping the use of MLVs in pregnant animals – there have been reports of abortions even in previously vaccinated cows. This is because the safe use of MLVs depends on a prior and sustained immune response.
Unfortunately, the immune response to a vaccine never is 100 percent, and the ability to sustain an immune response is influenced by many factors, including nutritional deficiencies and genetic variation.
It is important for veterinarians and producers to fully assess this risk and determine when or if the use of a MLV is justified in and around pregnant cows.
The fundamentals of MLV use
Before a decision is made to utilize a MLV, a few fundamentals regarding vaccinology and disease protection in cows and neonates should be considered.
1. The administration of vaccines containing IBR and BVD are the primary concerns when a MLV is considered for use in a pregnant animal or non-pregnant commingled herdmates.
2. When administered, MLVs naturally replicate in the animal until the immune system responds and removes the virus. The level of replication will be reduced greatly if the animal has been vaccinated previously.
The viral replication that is demonstrated in a naïve beef calf at branding will be far greater than that seen in a dairy cow vaccinated postpartum.
3. The level of viral shedding as a result of vaccination will be reduced in the animal that has been vaccinated previously. This is why postpartum, open dairy cows receiving a MLV can be commingled safely with pregnant animals that have been previously vaccinated.
4. The risk of fetal loss in an annually vaccinated cow, commingled with a herdmate or calf that has been vaccinated, is minimal.
5. A pregnancy is most at risk from IBR and BVD infections in the first half of the gestation period. For this reason, the best time to vaccinate a heifer or cow is 14 to 30 days prior to breeding.
6. There is no evidence that boosting the IBR and BVD titers late in pregnancy of a cow vaccinated postpartum has a positive impact on calf health due to colostrum quality.
7. Unless there is a persistently infected calf in a group of calves, there is a very low BVD or IBR disease risk in calves younger than 3 months. These diseases can best be addressed through neonatal vaccination. Many of the current MLVs marketed today provide disease protection through most, if not all, of pregnancy for IBR and BVD.
Vaccinating pregnant cows not recommended
With a continual stream of heifers and cows calving and the daily individual animal contact on dairies, there is no good argument to vaccinate a pregnant dairy cow.
Heifers should receive at least two MLVs prior to breeding with the final vaccine administered 30 days prior to breeding. After heifers and cows freshen, they should be vaccinated between 21 and 35 days postpartum.
With the strong herd immunity created by continual postpartum vaccination and the long duration of immunity demonstrated by MLVs, even animals with delayed breedings are well protected throughout their pregnancies.
Because dairy calves are raised away from their dams, there is no viral shedding concern, and commingled postpartum vaccinates pose little concern to the pregnant cow that has been vaccinated throughout its life.
If other abortion risks, such as leptospirosis, are a concern on a dairy, these issues can be addressed with a monovalent vaccine later in pregnancy rather than addressing these issues with a MLV combination.
One of the questions that often arises surrounding the use of a MLV is: How close to breeding can it be used without affecting conception rate? This concern is based on early vaccine work that demonstrated a reduction in ovarian function when a vaccine was given within one week of breeding.
This work was performed in naïve animals at a dose 10 times the recommended dosage. More recent work has demonstrated that if the animals have received prior vaccinations, there is no negative effect on reproductive performance.
When considering the use of MLVs in pregnant cows and heifers, it is very important to fully be aware of the risks even with approved products. The safety of these products depends on a previous immune response from vaccination that is sustained in that animal throughout subsequent exposure.
Unfortunately, the factors that ensure an adequate and sustained immune response often are out of your control. For this reason, always consult your veterinarian when considering the use of MLVs in pregnant cows and heifers. PD
Nordstrom is a veterinarian and the director of dairy technical services for Merck Animal Health. Click here to contact him via email or call (540) 578-2041.
Scott Nordstrom
Director Dairy Veterinary Technical Services
Merck Animal Health
