Veterinary Advice: Vaccination Failure - When Vaccines Don't Seem to Work.

Vaccines are products containing minimally harmful viral or bacterial antigens(surface proteins and sugars) similar to those contained on the surfaces of wild-typeinfectious-disease-causing organisms which, when administered to an animal, are designed to induce a protectiveimmune system response against disease-causing organisms that might infect that animal.

Sometimes, however, despite our best efforts, a vaccinated animal will developsigns of the disease that we are trying to protect it from. This is termed vaccination failureor failure of immunization.

Generally, it occurs because of one of two reasons:

1) The clinical disease is being caused by the vaccine itself (live vaccines only) OR
2) The clinical disease is being caused by a wild-type, infectious disease organism that has infected the animal from its local environment.

Clinical disease caused by the vaccine:
Live vaccines are functional, replicating strains of the exact same viruses and bacteria that we are trying to protect our pets from. They only differ from the wild-type disease causing agents in that they been specially 'bred' and selected for their avirulent properties (e.g. they might not replicate as quickly as the wild viruses or they might not access the host's cells as well). Consequently, when live vaccines are manufactured and used correctly, it is very unlikely that they should ever cause disease in a normal animal with a normal immune system (even a naive immune system which has notyet been exposed to the vaccine viruses, like that found in our puppies at first vaccination) ball gown wedding dresses with train

In some cases, however, live avirulent vaccines can cause the diseases they are trying to prevent. This happens for the following reasons:

1) There has been a failure of quality control at the manufacturer and a virulent strain of virus has made it into the vaccine.
The effect of injecting a virulent strain of virus (e.g. parvo, distemper) is the same as if you had just taken a small puppy and let it lick up the poo from a dog with parvo! The animal will develop disease because its immune system is naive and not able to cope with the virulence of the virus strain.

2) The vaccine was given by the wrong route.
The most notable occasion when this happens is with the injectable cat-flu vaccines. Live, injectable cat-flu vaccines are often moderately virulent strains of virus. The main reason they do not normally induce disease is that they are inoculated at a site that they do not normally grow well in(under the skin). If, however, they are accidentally inhaled (this can occur when a vaccine is drawn up and aerosolized in the cat's presence), they will get access to the cells of the respiratory tract (which is their preferred site for replicating and causing disease). Cat-flu will result with all of the sneezing, fever, eye discharge and nasal discharge associated with the condition.

3) Mild clinical disease is a known side effect of the vaccine route.
Sometimes, mild signs of the disease are common and accepted side effects of the vaccine. This is particularly true of the intranasal vaccines (kennel cough and cat flu intranasal spray vaccines): animals will often get mild signs of coughing, sneezing and watery eye and nose discharges after these vaccines. It is also a known side effect of the feline calicivirus injectable (sometimes kittens will show lameness and fever after vaccination, a condition sometimes seen in the calicivirus disease).Generally, these mild side effects are self-limiting and do not need medication.

4) Vaccination of the pregnant animal with a live virus vaccine.
Foetuses are unprotected against live vaccine viruses (their immune systems are very immature) and many of the organisms will preferentially replicate in the rapidly-dividing cells of the foetus. Damage tofetal cells can result in stillbirths, abortion, fetal abnormalities or puppies and kittens born with clinical disease. Well known examples of this: fetal kittens whose mother received a live panleukopenia vaccine can be born showing signs of anunderdeveloped cerebellum (they will tremor whenever they move).

5) Vaccination of neonatal (newly born) puppies and kittens that did not get their colostrum with a live vaccine.
Although live vaccines are fine for use in animals over 4 weeks old (which have a functional, though naive immune system) and newborn animals protected by their mother's colostrum, newborn animals (termed neonates) unprotected by maternal antibodies have a greatly reduced immune response (not much better than the late-stage fetus) and live vaccines may produce disease in them.

6) Vaccination of animals with a poor immune system.
Although live vaccines are not as virulent as wild-type vaccines, they do replicate in cells and damage them(it just takes them longer). By the time they grow numerous enough to do any real harm,the immune system has generally come along to kill them off, thereby preventing vaccine-associated disease and acquiring the needed immunity. If, however, the immune system response fails to 'show up', the live vaccine viruses will continue to replicate as the wild-types do and they will eventually produce symptoms of disease.

Situations where animals might have a poor immune system:

- Animals born with a bad immune system.
Some purebred dogs and cats (and other species) have a genetic predisposition to immunodeficiency (they lack parts of their immune system).
An example of this is CID (Combined Immuno Deficiency) in Arabian horses. These foals are born with non-functional T cells and, as a result,not only do their T cells not work, but their B cells also can not be activated to make antibodies (immature B cells initially need T cells to activate them). They suffer from a lack of both humoral and cell-mediated immunity (see How Vaccines Work page for details on what these terms mean). Live vaccines would induce disease in these animal sand they often die young from overwhelming infection - they essentially have no immune system.

- Animals that acquired a bad immune system.
There are many diseases and drugs which damage (permanently or transiently) the immune system, making it inadequate to fight off wild-type diseases and live vaccine organisms. These include:
A) Radiation therapy - can destroy the bone marrow which makes most of the white cells.
B) Bone-marrow-invasive disease - diseases such as leukemia (bone marrow cancer can invade and destroy the bone marrow so that white blood cells can't be made.
C) Bone-marrow-suppressive drugs - some medications stop the bone marrow making white cells. These include most chemotherapy drugs (e.g. vincristine, azathioprine, cyclophosphamide),corticosteroids (e.g. prednisolone) and long-term estrogen products in dogs (normally used to treat incontinence).It is a reason we don't vaccinate dogs on immune suppressive drugs such as prednisolone.
D) Auto-immune diseases that kill the white blood cells in the blood or the cells in the bone-marrow that make them.
E) Viral diseases - FIV in cats (feline AIDS) kills off many of the T cells. Feline leukemia virus can result in auto-immune diseases (point D) and leukemia (point B).Canine and feline parvoviruses also damage the bone marrow during clinical illness (note that a veterinarian would never vaccinate a dog or cat this unwell).
F) Bone marrow suppression can occur in undesexed female ferrets (they get estrogen poisoning).This is why vets recommend desexing female ferrets.
- There are also some situations where the condition of the animal may induce hormonaland chemical changes in its body that might result in a poor immune system response anda potential for vaccine-induced disease signs. These include:

A) Excessive stress (including pregnancy) - natural cortisols produced during times of excessive, prolonged stress (we are not talking about the otherwise well, shivering chihuahua sitting on the consult table) will suppress the immune response to vaccination.
B) Very old animals - over time and age, the immune system of most animals starts todeteriorate. This can result in a subnormal immune response.
C) Fever - certain elements of the immune system do not work as effectively during times of high body temperature.
D) Subnormal temperatures - as with fever, some immune cells do not work as well when body temperatures are very low. It would be rare for vets to vaccinate a cold animal,however, one situation in which this could occur is when vaccines are given to animals that are under anaesthesia.
E) Severely malnourished animals - animals need nutrition for immune cells to replicate and for antibodies to be made.