Newborn puppies and kittens receive disease protection
from their mother through the transfer of antibodies.
These antibodies are transferred from the mother through
the placenta and through
colostrum,
the first milk the newborns receive. Antibodies are small
disease-fighting proteins produced by certain types of
cells called 'B cells.' The proteins are made in response
to 'foreign' particles such as bacteria or
viruses. These antibodies bind
with certain proteins (antigens) on foreign particles like
bacteria, to help inactivate them.
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. This window of
susceptibility can vary widely. The length and timing of
the window of susceptibility is different in every litter
and between animals in the same litter. Let us take canine
parvovirus as an example.
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.
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.
Insufficient time between vaccination
and exposure
A
vaccine does not immediately provide protection. It takes
from several days to a week or more for an animal's body
to respond to the vaccine. For some vaccines, coronavirus,
for example, an adequate level of immunity usually does
not occur until 2-3 weeks after the second vaccination in
the series. A young animal is susceptible to a disease if
it is exposed to the disease before a vaccination has had
time to stimulate the body's immunity. A puppy vaccinated
against parvovirus, and then exposed to the virus several
days later, would probably develop the disease. In the
same way, a vaccine will not provide protection to a young
animal who was already exposed to the disease.
We have
seen that too short an interval between vaccination and
exposure to disease can result in the animal developing
the disease. In some cases, the same is true if the length
of time between vaccination and exposure to disease is too
long. Some vaccinations may protect the animal for life.
Other vaccines produce a protection that lasts only a
short time (short duration of immunity).
These vaccines will need to be boostered. These include
almost all vaccines for dogs and cats, and some human
vaccines such as tetanus. The length of protection from a
vaccine varies by the disease, type of vaccine, age at
vaccination, and the immune system of the individual
animal.
Antibody Titer: We can try
to determine if a person or animal has protection from a
disease by measuring the amount of antibodies in the
blood. The result is often expressed as as a 'titer.' The
test is usually performed by making a number of dilutions
of the blood and then measuring at what dilution there is
sufficient antibody to react in the test. For example, a
titer of 1:8 (one to eight) means the blood can be diluted
to one part blood and seven parts saline and still produce
a positive reaction in the test. The higher the titer
(1:16 is higher than 1:8), the more antibody is present.
It
becomes complicated when we try to interpret these titers.
The protective titer for canine distemper is 1:20,
however, the protective titer for feline panleukopenia is
1:100. You can not compare titers between diseases - a
protective titer for one disease will be different than
that for another disease. Also, the titer is only a
measurement of one part of the immune system - the
antibody level. It does not test the remainder of the
immune system which can play a very large role in
preventing disease.
Some
veterinarians suggest we should measure antibody titers
before revaccinating an animal. If the animal has a
protective titer, a vaccine would not be given. At this
time, the protective titer of many diseases is unknown.
For some diseases, the level of antibody would not
accurately assess the immune status of the animal because
other parts of the immune system are more important for
fighting off that particular disease. Another problem with
titers is that the test will only tell us the animal's
status at that point in time. It can not tell us what the
animal's status will be 6 months from now. So, how often
should we test? Finally, there is always the possibility
of laboratory error. A test result may erroneously suggest
an animal has a protective titer when it really does not.
Different strain of bacteria or virus
Vaccines
only contain specific strains of the virus or bacteria
that causes disease. A vaccine produced from one strain
may not adequately protect against another strain. As an
example, until recently, vaccines against leptospirosis
only protected against two types of the bacteria. It would
not protect an animal against the other types. A newer
vaccine manufactured by Fort Dodge now protects against
four types of leptospirosis.
Damage to vaccine
If
not handled properly, it is possible that a modified live
vaccine could be inactivated. This is a very uncommon
occurrence, but could occur if the vaccine was exposed to
ultraviolet light, if there was a long time period between
when it was reconstituted and when it was used, or if it
was not stored at the proper temperature. Manufacturers
realize some of the vaccine particles could be reduced
through handling and have made allowances for this when
determining how many vaccine particles should be included
in each vaccine.
Improper administration
Vaccines
are developed to be given by a certain route, either
intranasally (into the nostrils), subcutaneously (under
the skin), or intramuscularly (into the muscle). If a
vaccine is administered by a route different from the
route for which it was developed, it may not be effective
and could cause considerable harm.
The
entire dose of the vaccine should be given at one time.
Vaccines are not developed to give different doses to
different size animals, except in some cases, it is
recommended that the dose of intranasal vaccines for
kittens be reduced.
Non adherence to vaccination schedule
Vaccine interference: If
too short of a time elapses between doses of vaccines,
vaccine interference can occur. It is suggested that if
more than one type of vaccine is to be given, they should
be given at the same time, not several days apart.
Prolonged interval between
vaccinations: To provide the best response, the
first time an animal is being vaccinated against a
disease, repeated vaccinations are usually given 2-4 weeks
after the prior vaccination. The first vaccine
more-or-less primes the immune system, and the subsequent
vaccination(s) increase the immune response. If a period
longer than several weeks occurs between this first series
of vaccinations, the immune system is no longer 'primed'
and less of an immune response will result from the
subsequent vaccination. It is therefore recommended, that
if more than 2-3 months has occurred between vaccinations
in a young animal, or the vaccination status of an animal
is unknown, the animal should be given two vaccinations
2-3 weeks apart. (This does not apply to rabies
vaccinations.)
Breed variation
It
appears that some breeds of dogs and cats may be more
susceptible to certain diseases. Dobermans and Rottweilers,
for instance, tend to be more susceptible to canine
parvovirus, and may need a different vaccination schedule
than other dogs, if they are to be protected through
vaccination.
Immunosuppression/immunodeficiency
To
provide protection from a vaccine, an animal's immune
system must be adequately stimulated by the vaccine. If
the animal's immune system is not functioning adequately
or is suppressed, as would be the case in animals with
certain viral infections, and those receiving certain
cancer treatments or very high doses of steroids, the
vaccine would not initiate a proper immune response, and
would not result in protection from the disease.
Concurrent disease process
Fever has
been shown to inhibit the response of the immune system to
canine distemper vaccination in puppies. Certain viral
infections may also decrease the ability of the immune
system to adequately respond to a vaccination.
Nutritional deficiencies
Animals
who are malnourished, like those who are ill, may not
respond adequately to a vaccination. Poor nutrition, such
as Vitamin A, Vitamin E, and selenium deficiencies, and
restricted protein or calories
can result in suppression of the immune system.
Summary
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When
a vaccinated animal develops the disease against which it
was vaccinated, it is often referred to as vaccine
failure. In the vast majority of cases, however, it is not
the vaccine that has failed, but an inadequate immune
response to the vaccine has occurred. Listed below are
some of the main reasons disease may develop in vaccinated
animals.
