The challenge to produce effective and safe
vaccines for the prevalent infectious diseases of humans and animals has
become increasingly difficult. In veterinary medicine, evidence implicating
vaccines in triggering immune-mediated and other chronic disorders (vaccinosis)
is compelling. While some of these problems have been traced to contaminated
or poorly attenuated batches of vaccine that revert to virulence, others
apparently reflect the host’s genetic predisposition to react adversely upon
receiving the single (monovalent) or multiple antigen “combo” (polyvalent)
products given routinely to animals. Animals of certain susceptible breeds
or families appear to be at increased risk for severe and lingering adverse
reactions to vaccines.
The onset of adverse reactions to conventional vaccinations (or other
inciting drugs, chemicals, or infectious agents) can be an immediate
hypersensitivity or anaphylactic reaction, or can occur acutely (24-48 hours
afterwards), or later on (10-45 days) in a delayed type immune response
often caused by immune-complex formation. Typical signs of adverse immune
reactions include fever, stiffness, sore joints and abdominal tenderness,
susceptibility to infections, central and peripheral nervous system
disorders or inflammation, collapse with autoagglutinated red blood cells
and jaundice, or generalized pinpoint hemorrhages or bruises. Liver enzymes
may be markedly elevated, and liver or kidney failure may accompany bone
marrow suppression. Furthermore, recent vaccination of genetically
susceptible breeds has been associated with transient seizures in puppies
and adult dogs, as well as a variety of autoimmune diseases including those
affecting the blood, endocrine organs, joints, skin and mucosa, central
nervous system, eyes, muscles, liver, kidneys, and bowel. It is postulated
that an underlying genetic predisposition to these conditions places other
littermates and close relatives at increased risk. Vaccination of pet and
research dogs with polyvalent vaccines containing rabies virus or rabies
vaccine alone was recently shown to induce production of antithyroglobulin
autoantibodies, a provocative and important finding with implications for
the subsequent development of hypothyroidism (Scott-Moncrieff et al, 2002).
Vaccination also can overwhelm the immunocompromised or even healthy host
that is repeatedly challenged with other environmental stimuli and is
genetically predisposed to react adversely upon viral exposure. The recently
weaned young puppy or kitten entering a new environment is at greater risk
here, as its relatively immature immune system can be temporarily or more
permanently harmed. Consequences in later life may be the increased
susceptibility to chronic debilitating diseases.
As combination vaccines contain antigens other than those of the clinically
important infectious disease agents, some may be unnecessary; and their use
may increase the risk of adverse reactions. With the exception of a recently
introduced mutivalent Leptospira spp. vaccine, the other leptospirosis
vaccines afford little protection against the clinically important fields
strains of leptospirosis, and the antibodies they elicit typically last only
a few months. Other vaccines, such as for Lyme disease, may not be needed,
because the disease is limited to certain geographical areas. Annual
revaccination for rabies is required by some states even though there are
USDA licensed rabies vaccine with a 3-year duration. Thus, the overall
risk-benefit ratio of using certain vaccines or multiple antigen vaccines
given simultaneously and repeatedly should be reexamined. It must be
recognized, however, that we have the luxury of asking such questions today
only because the risk of disease has been effectively reduced by the
widespread use of vaccination programs.
Given this troublesome situation, what are the experts saying about these
issues? In 1995, a landmark review commentary focused the attention of the
veterinary profession on the advisability of current vaccine practices. Are
we overvaccinating companion animals, and if so, what is the appropriate
periodicity of booster vaccines ? Discussion of this provocative topic has
generally lead to other questions about the duration of immunity conferred
by the currently licensed vaccine components.
Veterinary vaccinologists have recommended new protocols for dogs and cats. These
include: 1) giving the puppy or kitten vaccine series followed by a booster
at one year of age; 2) administering further boosters in a combination
vaccine every three years or as split components alternating every other
year until; 3) the pet reaches geriatric age, at which time booster
vaccination is likely to be unnecessary and may be unadvisable for those
with aging or immunologic disorders. In the intervening years between
booster vaccinations, and in the case of geriatric pets, circulating humoral
immunity can be evaluated by measuring serum vaccine antibody titers as an
indication of the presence of immune memory. Titers do not distinguish
between immunity generated by vaccination and/or exposure to the disease,
although the magnitude of immunity produced just by vaccination is usually
lower (see Tables).
Except where vaccination is required by law, all animals, but especially
those dogs or close relatives that previously experienced an adverse
reaction to vaccination can have serum antibody titers measured annually
instead of revaccination. If adequate titers are found, the animal should
not need revaccination until some future date. Rechecking antibody titers
can be performed annually, thereafter, or can be offered as an alternative
to pet owners who prefer not to follow the conventional practice of annual
boosters. Reliable serologic vaccine titering is available from several
university and commercial laboratories and the cost is reasonable (Twark and
Dodds, 2000; Lappin et al, 2002; Paul et al, 2003; Moore and Glickman,
2004).
Relatively little has been published about the duration of immunity
following vaccination, although new data are beginning to appear for both
dogs and cats.
Our recent study (Twark and Dodds, 2000), evaluated 1441 dogs for CPV
antibody titer and 1379 dogs for CDV antibody titer. Of these, 95.1 % were
judged to have adequate CPV titers, and nearly all (97.6 %) had adequate CDV
titers. Vaccine histories were available for 444 dogs (CPV) and 433 dogs (CDV).
Only 43 dogs had been vaccinated within the previous year, with the majority
of dogs (268 or 60%) having received a booster vaccination 1-2 years
beforehand. On the basis of our data, we concluded that annual revaccination
is unnecessary. Similar findings and conclusions have been published
recently for dogs in New Zealand (Kyle et al, 2002), and cats (Scott and
Geissinger, 1999; Lappin et al, 2002). Comprehensive studies of the duration
of serologic response to five viral vaccine antigens in dogs and three viral
vaccine antigens in cats were recently published by researchers at Pfizer
Animal Health ( Mouzin et al, 2004).
When an adequate immune memory has already been established, there is little
reason to introduce unnecessary antigen, adjuvant, and preservatives by
administering booster vaccines. By titering annually, one can assess whether
a given animal’s humoral immune response has fallen below levels of adequate
immune memory. In that event, an appropriate vaccine booster can be
administered. |
|
| W. Jean Dodds, DVM
| | |
