One explanation for this delay is the phenomenon called maternal antibody interference, according to Felippe. "The theory is that the circulating antibodies that came from the colostrum interfere with the ability
of the foal to produce new antibodies in response to vaccination. How would that occur? A few mechanisms have been proposed.
They would interfere by perhaps neutralizing the vaccine antigens. So if you're trying to put a vaccine antigen to stimulate
the foal's immune system, and it is immediately neutralized by an antibody that is circulating against it, it doesn't do what
it should. It doesn't stimulate the cells. It doesn't make them develop immunity against it."
Felippe says she questions this theory based on some fundamental principles of "mother nature." "The horse is an animal that
has been selected for millions of years. One of the interesting things about horses is that they don't transfer immunoglobulins
during gestation," she says. "Instead, the foal needs to get its immunoglobulins from the colostrum intake after birth. This
is a major pressure in their selection process. If this foal is not healthy enough to nurse quickly and the mare is not healthy
enough to produce good-quality colostrum, that transfer doesn't happen in a timely manner, and the foal will be very susceptible
"It would be a major selection pressure for colostrum, which essentially gives foals survival during the early weeks of life,
to paradoxically prevent their own ability to produce protection for continued survival. The species would possibly be in
major trouble if that was the case. Therefore, simplistically, I question it," says Felippe.
What is the basis for this hypothesis? Studies done across species have shown some limitations of neonatal response to some
vaccines, whereas other vaccines were good at inducing immunity in the neonate despite circulating maternally derived antibodies.
"The first thing that comes to mind is that one of the limiting factors here is not necessarily what is going on in the background,
but it is the quality of what you are using to elaborate an immune response," says Felippe. "There is good research showing
that depending on what you use to stimulate the immune system of the neonate, you are going to have a good response or not-so-good
Dosage has also been studied. "In fact, smaller doses may do a better job than higher doses for some pathogens," Felippe says.
"We cannot generalize across antigens, but these are some observations that we know may play a role."
As for maternal antibody interference, there have been some conflicting results in the literature, and it depends on the type
of model that has been used. "But some of the literature points to the fact that in the presence of circulating maternal antibodies,
the immune response to some vaccine products is not as good," says Felippe. "If you vaccinate a foal that doesn't have that
much circulating immunoglobulin specific to that antigen, the immune response could be more robust.
"I question some of the conclusions from some of the studies, because if these studies are basing their results on total serum
immunoglobulin levels, I think there may be some limitations in understanding what the true response is," she continues. "Because
when you measure serum immunoglobulin levels against an antigen, you're measuring the maternally derived and the foal's at
the same time. There is no way to discern whose is whose. You're trying to identify an immune response that is going to be
minor because it was from a neonate, a naïve individual, in the mix with a lot of other antibodies that belong to the mare.
So to find that very small amount within that large sample can be difficult technically."
It does take time for an immune system to expand the population that will respond to an antigen, according to Felippe. With
a more trained adult immune system, you can anticipate that fewer doses would be necessary for that population expansion.
But in an untrained immune system, such as the foal, you would probably need more boosters to achieve the same level of population
expansion that you would have with an adult horse. "We go back to principles that are not new, but the use of boosters, and
perhaps more than one or two, are necessary for some antigenic stimulation to accomplish what we consider protective levels,
or standard adult horse levels," Felippe says.