Vaccinating foals is important, but when should you do it and what should you give?
Foals, just as infants, are vulnerable
to disease and infection because their young bodies are
naive to the world of germs and bacteria which can cause
those problems. So the inclination, particularly for
horse owners who choose to vaccinate their own horses, is
to administer vaccines as early as possible or as
recommended by over-the-counter products. New research,
however, seems to indicate that doing so may be a moot
point in the very young.
Currently, the majority of vaccines
offer only limited protection for the highly susceptible
equine neonate, and therefore, attention should be given
to decreasing exposure to pathogens, or those organisms
which can cause disease, through isolation and
sanitation, as well as by eliminating stressors that
reduce the foal's resistance.
Factors that effect a foal immunization
program include the management situation, geographic
location and risk versus cost benefits for the owner.
Although it is impossible to make universal
recommendations for the vaccination of foals, a rational
plan can be developed if the key decision points in
passive or active immunization regimes are understood.
Nature's vaccination
Passive immunity is best understood as
the immunity transferred to the foal by its mother
through the colostrum the foal ingests as it first
suckles. It is critical the foal receive this colostrum,
which is loaded with maternal antibodies, within the
first few hours of life.
The value of passive immunity can be
considerably influenced by vaccination of the mare in
order to maximize the concentration of the antibodies
present in the colostrum. All mares should receive
booster vaccinations four to six weeks before giving
birth using only killed (inactivated) agents. Typically,
this includes vaccinations for tetanus, encephalomyelitis
viruses, influenza and rhinopneumonitis, with additional
vaccines for Streptococcus equi, Potomac Horse Fever, and
in some circumstances, botulism. In previously
unvaccinated mares, an initial vaccination course should
be administered with the last booster given four to six
weeks before foaling.
It is also important to remember that
the value of colostral transfer of passive immunity can
also be considerably increased if the mare is housed on
the farm where she is going to foal for six to eight
weeks before foaling. This allows adequate time for the
generation of immune responses to pathogens present on
the farm and subsequent transfer of these antibodies into
the colostrum. One general rule is that modified live
vaccines (MLVs) are not given during pregnancy. Remember
that an MLV will induce some type of infection itself in
order to achieve immunization.
Passive immunization is also achieved
by the oral administration of immunoglobin-containing
products to foals in the first hours of life or by
injection at any time. It is relatively common practice
to administer tetanus anti-toxin to neonatal foals.
However, this provides relatively short-lived protection
and carries the risk of inducing serum sickness in the
foal. This policy is unnecessary if the mare is
appropriately vaccinated during pregnancy.
For those foals who do not receive
immunity through passive transfer, the administration of
plasma transfusions is a common procedure and offers an
opportunity to influence resistance to specific pathogens
through the choice of product. Commercial equine plasma
products should be chosen that have been prepared from
donors extensively vaccinated against common equine
pathogens.
Timing
A contentious issue in foal vaccination
is the timing of the initial series of vaccinations. The
problem largely results from the effects of maternal
antibodies received through the colostrum and their
variable half life. It is difficult to time these initial
vaccinations in the foal so that it is effective and
administered early enough so as not to leave the young
animal unprotected after the waning of material
antibodies. An additional consideration is the increasing
perception that foals may be relatively immunologically
unresponsive to many of the currently available vaccines.
The first step in determining when to
start a foal vaccination regime is knowing the duration
of maternally derived antibodies. The rate of decline of
maternal antibodies varies for both individuals and
different infectious agents. For many important
pathogens, the concentration of maternal antibodies in
foals falls to nonprotective levels by two to three
months of age. However, the remaining antibody, which one
could view as residual, can still render the foal
unresponsive to vaccination for weeks or even months to
come.
Because, in the case of equine
influenza, maternal antibodies can persist until six
months of age and prevent immune responses in foals
younger than six months, AAEP recommends beginning foal
vaccinations at three to four months of age, followed by
boosters at four week intervals. This is adequate for
many foals, but a significant number are in a high-risk
situation and may remain vulnerable to infection. A more
intensive vaccination schedule would include an initial
vaccination at two months of age and monthly boosters
until six months of age, with further boosters at nine
and 12 months. In the case of tetanus and rabies, an
initial vaccination at three to four months and a booster
for weeks later should be adequate.
Even when intense vaccination regimes
are used in young foals, poor responses can still be
observed. This may be a result of a relative lack of
immune responsiveness in young foals to currently
available vaccines, but an alternative proposal is that
the frequent use of vaccines in the face of persistent
maternal immunity may actually induce a state of
tolerance, which can prevent a satisfactory response to
vaccines past one year of age.
What to use
Currently available vaccines include
many highly effective products that are safe, provide
long-term immunity and are practical to use. In making
choices between products, one must consider both the
antigen contained in the vaccine and the means of
delivery. For example, in the case of equine influenza
vaccines, it is important to look for an equine influenza
type 2 strain with a date from the late 1980s or ideally
the 1990s. This will increase the likelihood that the
vaccine will protect against currently circulating
strains of influenza virus. Similarly in the case of
Equine Herpes Virus vaccines, it is important to include
both EHV-1 and EHV-4 antigens for protection against both
abortion and respiratory disease.
Most importantly, however, horse owners
need to be aware that immunities develop based on
exposure and protection. With the help of your
veterinarian, you should be able to determine the
appropriate vaccination schedule best for your horses,
both young and old.
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