Over
the years, we have found that there are many, many,
misconceptions about the immunity that puppies and kittens
derive from their mothers. When a puppy or kitten is born,
its immune system is not fully
developed. Therefore, on its own, this animal would be
completely susceptible to almost any infectious condition.
If a serious disease were encountered, the animal would
probably die. Fortunately, this is not the case, as Mother
Nature has devised a method to provide newborn animals
with protection.
Active
Immunity
When people or animals are
exposed to a disease-causing organism by natural means or
vaccination, the organism or a part of it interacts with
the cells of the animal's immune system. These cells then
make antibodies (large protein molecules) that reside in
the animal's body and will recognize foreign organisms and
destroy them. And the body activates cells which can kill
the disease-causing organism more directly. When an
individual has an immune system that will effectively
protect it against a disease-producing organism, it is
said to have 'immunity' or be 'immune' to that organism.
When an animal’s own immune system provides that
protection, it is referred to as 'active immunity.'
Passive
immunity
When an animal receives
another animal's defense mechanisms (antibody
and/or lymphocytes), rather than
developing its own defense system, we refer to it as
'passive immunity.' Examples of passive immunity include
the antibodies received by a fetus through the placenta,
antibodies the newborn receives from its mother through
colostrum, antivenins to treat snakebite, and bone marrow
transplants which help replace the lymphocytes. A
disadvantage of passive immunity is that the animal's body
does not have the ability to replenish it (except in the
case of a bone marrow transplant). As the antibodies the
animal received break down through natural aging, or are
used up destroying disease-causing organisms, the animal's
body cannot replace them. However, in the case of
active immunity, more antibodies
are produced whenever the immune system comes in contact
with the same organism again. Active immunity is
self-perpetuating. Passive immunity is not.
Two types of passive
immunity protect young puppies and kittens. All antibodies
derived from the mother, either via her blood or colostrum
(first milk) are called maternal antibodies. It must be
noted that the puppy or kitten will only receive
antibodies against diseases for which the mother had been
recently vaccinated against or exposed to. As an example,
a mother that had NOT been
vaccinated against or exposed to parvovirus, would not
have any antibodies against parvovirus to pass along to
her puppies. The puppies then would be susceptible to
developing a parvovirus infection.
Passive Immunity In Utero (Through the Placenta):
In some species, protective antibodies pass through the
placenta (the organ by which a fetus is connected to its
mother) from the mother’s blood system into the fetus
while the unborn animal is still in the uterus. A young
animal, therefore, carries this protection with it when it
is born. It has immunity to protect it against disease
conditions that it may encounter before its own system is
operating. Humans get most of the immunity they receive
from their mothers in this same manner.
Passive Immunity through Colostrum:
Dogs and cats, like many other mammals, pass the majority
of the mother's antibodies to the newborn via
colostrum.
Defined as the first 36 to 48 hours of milk flow following
birth, colostrum is a highly concentrated mixture of large
protein antibody molecules, vitamins,
electrolytes, and nutrients.
The puppy or kitten absorbs
the colostral antibodies into its blood system through the
intestine. The ability to absorb such large protein
molecules unchanged across the intestinal wall is one of
the peculiarities of newborns. As animals mature, they
cannot absorb these large molecules and their digestive
systems break down these large protein molecules into
smaller pieces. Newborn puppies and kittens, through some
process that we do not quite understand, are able to
absorb the large antibodies unchanged. This is important
because if the antibodies are broken down into small
pieces, they lose their ability to destroy bacteria or
viruses. Usually, before the
puppy or kitten is one week of age, it loses this ability
of absorption, and all large proteins are broken down.
Therefore, even if the mother continued to produce
colostral antibodies, they would be destroyed and not
provide any protection to the newborn.
It is important that we now
clarify one often-misunderstood point. As stated,
colostrum with its antibody protection is only present in
the first 36-48 hours of milk flow. Puppies and kittens
can only gain immunity from colostrum if they nurse during
that time frame, and they are less than two days old.
After that, it makes no difference how much or how little
they nurse, they will not receive any more antibodies.
Many breeders and pet owners
believe that as long as the puppy or kitten is nursing, it
is gaining more protection. Wrong! Others feel that by
allowing older animals to nurse on a new mother
immediately after she gives birth, she will give these
older animals another dose of antibodies. Wrong again!
Remember, the puppies and kittens cannot absorb antibodies
after their digestive tracts lose the ability to absorb
large unaltered protein molecules. All the colostral
protection the puppy or kitten has is what it received in
that first day or two of life. Later on, we can only
augment this by vaccination.
Level of
protection from colostrum
The
amounts of immunoglobulins (antibodies) that are present
within the milk are directly proportional to the levels of
antibodies present in the mother. We often speak of 'titers'
as a way to qualify the levels present in an animal. Tests
are run on blood from the animal in question, and in
simple terms, the higher the titers, the more antibodies
are present. Mothers with high titers pass higher
concentrations of antibodies across the placenta and
through their milk. Puppies and kittens that have taken
milk with larger quantities of antibodies are able to
absorb more antibodies, and therefore, have higher
concentrations in their blood. Newborns, that start out
with higher levels of these colostral molecules carry this
protection for longer periods of time. This explains why
we want to be sure the mother has a high antibody titer
before breeding since she will be able to pass more
protection to her offspring. Her offspring will then
possess higher levels of protection for longer periods of
time against the diseases that we commonly vaccinate for
such as distemper,
parvo,
and
coronavirus
for dogs, and panleukopenia (feline distemper) and
calicivirus for cats.
Window
of susceptibility
The age at which puppies and
kittens can effectively be immunized is proportional to
the amount of antibody protection the young animals
received from their mother. High levels of maternal
antibodies present in a puppy's or kitten's bloodstream
will block the effectiveness of a vaccine. When the
maternal antibodies drop to a low enough level in the
puppy or kitten, immunity (protection from disease) can be
produced through vaccination.
The antibodies from the
mother generally circulate in the newborn's blood for a
number of weeks. There is a period of time from several
days to several weeks in which the maternal antibodies are
too low to provide protection against the disease, but too
high to allow a vaccine to work. This period is called the
window of susceptibility. This is the time when despite
being vaccinated, a puppy or kitten can still contract the
disease.
The length and timing of the
window of susceptibility is different in every litter, and
even between individuals in a litter. A study of a cross
section of different puppies showed that the age at which
they were able to respond to a vaccine and develop
protection (become immunized) covered a wide period of
time. At six weeks of age, 25% of the puppies could be
immunized. At 9 weeks, 40% of the puppies were able to
respond to the vaccine and were protected. The number
increased to 60% by 16 weeks, and by 18 weeks, 95% of the
puppies could be immunized.
As you can see, it is really
impossible for us to determine, when in the presence of
passive immunity, an individual puppy or kitten should be
vaccinated. There are just too many variables. Even if we
did blood tests on them, each animal in the litter would
probably have a different titer. Some will have absorbed
more antibodies, the antibodies may have broken down more
quickly in others, or some may have used a portion of
their antibodies if they encountered harmful bacteria or
viruses. Additionally, a young animal may have a
protective titer (level of antibodies) for one disease,
but not enough for another.
Progress is being made. Some
of the newer vaccines can stimulate active immunity in the
young animal even when maternal antibodies are present.
These are called 'high titer, low passage vaccines.' These
modified live vaccines contain a
higher number of virus particles (high titer) which are
less attenuated (low passage) than the 'average' vaccine.
High titer, low passage vaccines can generally elicit an
immune system response in young animals who have a
maternal antibody level that
would prevent them from responding to an 'average'
vaccine. A common way to describe this is "the vaccine
'breaks through' the maternal antibody." This vaccine
technology is used most often with parvovirus. As vaccines
improve, we will hopefully be better able to protect
puppies and kittens throughout their early life.
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