Last month I wrote a blog on the science behind vaccination. Several colleagues contacted me to find out my views on vaccinating pregnant cows. I am aware that many veterinarians still believe that pregnant cows should not be vaccinated for fear of causing abortion. That was what we were taught in veterinary schools. I, therefore, decided to spend some time researching on this topic, both on how and from where this rule of not vaccinating in pregnancy came from and secondly, is there a valid answer to this query based on scientific evidence. This article provides information about a cattle vaccination schedule & benefits.
What is true for human may not be so for cows
It seems that the rule of not vaccinating in advanced pregnancy came from human medicine since causing abortion or deformities to the growing fetus were major concerns. In humans and other monogastric animals, there is a single layer barrier between maternal and fetal circulation hence exchange of molecules, small proteins, and organisms such as viruses have been shown to cross to fetal circulation. It is, therefore, possible that antigenic and virulence factors from vaccine when given in pregnancy can pass through the placenta and cause harm including abortion. In contrast, in cows and other ruminants, the barrier is three-layered hence except for elementary nutrients, other molecules cannot cross the placental barriers. This was proved from the studies demonstrating that bovine viral diarrhea (BVD) which is known to be the smallest pathogenic virus could not infect fetus when pregnant cows were experimentally infected. When the fetus was infected with BVD by administering in amniotic fluid, the mid-gestation fetus (> 150 days) was found to respond and produce BVD antibodies demonstrated in blood after birth. These studies prove the point although > 150 days old cow fetus has developed immune system blood of new-born calf is free from antibodies since antigens can’t pass the placental barrier. This should eliminate the myth that vaccine antigens would cause harm to the growing fetus in cows.
Vaccinate cows to protect calves too
An exactly an opposite viewpoint has emerged that vaccinating heavily pregnant cows might be of benefit to calf health. Let us examine this based on science. In the case of ruminants during the first three days, the cow secretes milk, called colostrum, which is rich in antibodies, growth factors, vital nutrients, and immune cells. Interestingly on feeding colostrum to the new-born calf, all these molecules are absorbed intact. This wonderful phenomenon happens only in the first few days of calf-life and later on the gut is closed to intact absorption. Not much is known on metabolic pathways of synthesis of colostrum, how it is triggered and then a whole lot of genes responsible for colostrum is turned off within 3 days and the udder starts producing normal milk of interest here is to understand the origin of antibodies and other molecules in colostrum.
My studies prove that under trigger from certain calving process factor(s) there is the mobilization of secretory antibodies (IgG1) from the immune system to blood and then to udder. We found that this mobilization of antibodies against all possible infections the cow was exposed during its life starts at around 14-21 days to calving whereas maximum antibodies could be demonstrated in blood during the last 3-5 days to calving. When consumes these antibodies are absorbed intact in the newborn calf and provide passive immunity and priming of the immune system which has been shown to ensure protection against important infections as well as optimum growth. So, when a pregnant cow is vaccinated the memory cells, which on one hand will provide immunity to mother cow and besides large amounts of antibodies will also be secreted in colostrum to provide passive immunity in early calf life. This was proved in a study done by me on a farm that was having a high incidence of E. coli metritis and calf-scour. The farm was losing good cows to repeat breeding and superior genetic calves would succumb. We prepared vaccines from E. coli isolates and vaccinated, actually hyperimmunized, cows in advanced pregnancy. We found colostrum was rich in E. coli antibodies which protected calves reducing morbidity and mortality and also helped cows as the incidence of metritis came down significantly. This also led to further studies on hyperimmune health milk. In this study, pregnant cows during 7 and 8 months of gestation were hyperimmunized with H. pylori (common causal factor for gastric ulcers) antigens. Colostrum from such cows was found to be rich in H. pylori antibodies. When these cows were further vaccinated during lactation, milk was also found to be rich in H. pylori antibodies. These and other published studies prove the point that vaccination in pregnancy would be of benefit to even calves. For example, in areas, where FMD is endemic and there is associated calf mortality, vaccinating pregnant cows with FMD would confer passive immunity in young calves fed on colostrum from such hyperimmunized cows.
Are such vaccines available commercially?
In the USA and Canada, and possibly other developed countries, entro-pathogen vaccines such as E. coli, Salmonella, Rota and Coronavirus vaccines are available. To protect the calf from these entero-pathogens, the cow should be vaccinated in later pregnancy leading to high antibody levels in colostrum and better passive immunization of the calf.
In conclusion, veterinarians should come out of the myth that pregnant cows should not be vaccinated as vaccination-associated abortion is a remote possibility. Abortion, however, could occur due to maternal factors and infections which is largely due to placental pathology. It will be pertinent to mention here that studies show that as far as killed vaccines are concerned, these are safe at any stage of pregnancy whereas, live modified vaccines should be handled carefully in pregnancy. Studies have shown that in the case of live modified vaccines, the first vaccine dose must be given before breeding and in that case, subsequent vaccination in advanced pregnancy would be safe and highly beneficial to a cow as well as the calf.