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Frequently Asked Questions Related to Vaccination of Cats
Veterinary & Aquatic Services Department, Drs. Foster & Smith
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Frequently Asked Questions Related to Vaccination of Cats Frequently Asked Questions Related to Vaccination

by Holly R. Frisby, DVM
Drs. Foster & Smith
Veterinary Services Department

Should a cat who is ill or recovering from an illness be vaccinated?

Multiple vials of vaccineThe decision as to whether a cat who is showing signs of disease should be vaccinated or not needs to be made on an individual basis. One must consider whether the benefits outweigh the risks. Results of a physical examination and possible laboratory tests will aid in the decision. Signs of illness in a young kitten or puppy may need to be interpreted differently than similar signs in an adult animal. For example, in an older catl with a skin disease unassociated with an immune problem, vaccination is probably OK.

Will vaccination of an animal being treated with glucocorticoids be effective?

Short-term and low doses of glucocorticoids such as prednisone most likely would NOT interfere with the effectiveness of a vaccination. Animals on high doses of glucocorticoids and other immunosuppressants, however, may not respond adequately to a vaccination and may have an increased risk of adverse effects from the vaccine, especially if it is a modified live vaccine.

Should cats infected with FeLV or FIV be vaccinated differently?

Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) can suppress the immune system in some cats. It is generally preferable to use a killed vaccine instead of a modified live vaccine in animals with immunosuppression. If the FeLV or FIV-infected cat appears healthy, the cat should be vaccinated according to the vaccination schedule for non-infected cats.

Why do young animals need a series of vaccinations?

Kittens receive antibodies from their mother through the placenta and after they are born, through the colostrum (the first milk). The age at which kittens can effectively be immunized is proportional to the amount of antibody protection the young animals received from their mother. 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.

High levels of maternal antibodies present in a kitten's bloodstream will block the effectiveness of a vaccine. When the maternal antibodies drop to a low enough level in the young animal, immunization by a commercial vaccine will work.

The antibodies from the mother generally circulate in the newborn's blood for a number of weeks. The complicating factor is that there is a period of time from several days to a couple of weeks in which the maternal antibodies are too low to provide protection against the disease, but too high to allow the vaccine to work and produce immunity. This period is called the 'window of susceptibility'. This is the time when despite being vaccinated, a 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. Since the length and timing of the window of susceptibility varies so widely, it is impossible for us to determine when is the best time to vaccinate each individual kitten. There are just too many variables. For this reason, young animals are given a series of vaccinations in hope that we can vaccinate the animal as soon as it leaves the 'window of susceptibility.'

What is the earliest age at which an animal can be safely vaccinated?

In general, six to eight weeks is the earliest age at which a routine vaccination program should be started. Exceptions occur for animals who are in environments which would likely expose them to disease, e.g., animal shelters, breeding facilities, etc. These animals may benefit from earlier vaccinations. Vaccinating kittens less than 4-5 weeks of age, especially with modified live vaccines, can cause severe problems.

What should be done if a young animal misses a dose of vaccine?

The age at which the kitten received the last vaccination is the most important factor in determining boosters. To provide the best response, a kitten needs multiple vaccinations for feline distemper, calicivirus and rhinotracheitis until he reaches at least 16 weeks of age. He should be given one booster again when he reaches one year of age. If his last vaccination as a kitten was before he reached 12 weeks of age, it is generally recommended that he be given a series of two boosters. The reasoning behind this: we do not know in an individual kitten how long the maternal antibody levels persist and may interfere with the kitten's response to the vaccine. In many kittens the maternal antibody is still present at 12 weeks of age so there may be inadequate response to the vaccine at that age.

Should an animal who had a severe vaccine reaction be vaccinated in the future?

The decision to vaccinate an animal who has had a prior vaccine reaction must be made on an individual basis. The age and health status of the animal, severity of reaction, and risk of exposure to disease all need to be taken into account.

If there is minimal risk that an animal would be exposed to the disease, vaccinations for that disease are discouraged in an animal who has experienced a prior severe vaccination reaction. For instance, an adult, totally indoor cat may not need to be vaccinated against panleukopenia. Likewise, an adult dog with a history of a severe vaccine reaction, who has minimal exposure to other dogs, would probably not need to be revaccinated with canine distemper.

In situations in which there is a good likelihood that the animal with a history of a vaccine reaction will be exposed to the disease, vaccinations should probably be given, but as infrequently as possible. In addition, separate monovalent vaccines may decrease the possibility of an adverse reaction and should be given at least 3 weeks apart. Modified live vaccines may be less likely to cause a reaction. If an intranasal vaccine is available, that may be preferable.

An animal with a history of a vaccine reaction should always be vaccinated in a veterinarian's office. Certain vaccines may be excluded from your pet's vaccination schedule or a different type of vaccine may be used. Administration of antihistamines and short-acting corticosteroids prior to vaccination may decrease the chance of a reaction. The veterinarian may place a catheter in the pet's vein so if a reaction does occur, medications and fluids can be given immediately. The animal should be closely monitored for a period of time (1/2 to several hours) at the veterinarian's office and then also at home. Even with these precautions, life-threatening reactions could still occur.

Should younger and smaller animals receive less than the full dose of vaccine?

The entire dose of the vaccine should be given at one time. Vaccines are not developed to give different doses to different size or age of animal. All cats regardless of age, body weight, breed, and gender are given the same vaccine dose.

Why do some vaccinated animals still get the disease?

There are three main reasons why a vaccinated cat would still get the disease: First, the animal's immune system may not have been functioning adequately at the time of vaccination, so a proper response was not achieved. Secondly, and much less likely, there may have been a characteristic of the vaccine that produced a suboptimal response. For instance, the strain of virus in the vaccine may have been different than the strain that caused disease in the animal. Finally, there is always the possibility of human error such as improper storage or mixing of the vaccine. When a vaccinated animal still gets the disease, some term this 'vaccine failure,' although it is more likely a failure of the immune system to respond than a problem with the vaccine itself.

How long does it take for a vaccine to work?

Vaccines do not stimulate immunity immediately after they are administered. Once a vaccine is administered, the antigens must be recognized, responded to, and remembered by the immune system. In most animals, disease protection does not begin until five days after vaccination. Full protection from a vaccine usually takes up to fourteen days. In some instances, two or more vaccinations several weeks apart must be given to achieve protection. In general, modified live vaccines and those vaccines administered intranasally provide the fastest protection.

Is one method of administering a vaccine better than another?

As with other factors relating to vaccinations, the best route of administration of a vaccine depends on the situation. Vaccines must always be administered as stated by the manufacturer. Sites at which the vaccine can be administered will appear on the vaccine label. Intranasal vaccines should NEVER be injected, and vaccines designed for injection should never accidentally get into the eyes, nose, or mouth.

In general, if an intranasal product is available, it will provide faster protection than an injectable vaccine. Intranasal vaccines are less likely to cause allergic reactions, and are more likely to provide protection if maternal antibodies are still present.

If more than one vaccine is being given, the vaccines should be given at separate sites, and never be mixed together.

Veterinarian vaccinating against rabies in the right rear leg In cats, it is customary to give the FeLV vaccine far down on the left rear leg, rabies vaccine subcutaneously far down on the right hind leg, and other vaccines are given over a shoulder. Veterinarian vaccinating against FeLV in the left rear leg

Is one type of vaccine better than another?

Again, the answer to this question depends on the situation. In general, immunity from a modified live vaccine will develop faster and last longer than that produced by a killed vaccine, so modified live vaccines are recommended over killed vaccines in most circumstances. The newer recombinant vaccines are less likely to cause allergic reactions.

Should pregnant or nursing mother cats be vaccinated?

In general, avoid using any medications, drugs, or vaccines in pregnant or lactating (producing milk) animals. Any vaccine component that is modified live could conceivably cross the placenta and cause defects or deaths of the fetuses. This is known to be true in non-immune cats vaccinated with a modified live panleukopenia during pregnancy. Some vaccines, however, have been tested in pregnant or lactating animals and can be used in circumstances in which the animal does not have immunity to a specific disease and needs to be immunized. If vaccination must occur, use a killed vaccine. Your veterinarian will be able to help you determine which vaccines can or should be given to your pregnant animal.

Vaccinating a lactating animal will not provide protection to her nursing offspring. The mother's immune system will take at least a week to react to the vaccine and produce higher levels of antibody. And remember, the newborns can only receive antibodies from their mother through the colostrum in the first 36 hours of their lives. By the time the mother's antibody level has increased, the offspring will no longer be able to absorb them. The vaccination, then, may help protect the mother, but it will not provide antibody protection to her nursing offspring.

What are vaccine-associated fibrosarcomas?

A fibrosarcoma is a tumor of the connective tissue. These tumors tend to invade deeply into the underlying tissues. The frequency of these tumors is increasing in cats and has led researchers to believe some of the tumors may be caused by a local reaction to a vaccine. Although these tumors are seen more frequently, they are still rare. Current estimates on the incidence of these tumors are about one tumor per 5,000 to 10,000 cats vaccinated. These tumors are most commonly associated with the FeLV vaccine.

Research is ongoing to determine the exact cause of these tumors and how they can be prevented and treated. Possible explanations for the tumors include abnormal reactions to the adjuvant in vaccines, genetic predisposition, and infections with another virus.

The possible risk of vaccine-associated fibrosarcomas has led to a reevaluation of vaccination protocols. This is why it is recommended that adult cats without potential exposure to FeLV may not need vaccination against that disease. In cats with potential exposure, the risk of FeLV infection is greater than the risk of developing a sarcoma so vaccination is advised.

A small, painless swelling sometimes develops at the site of a recent vaccination. This should disappear in several weeks. If it persists, however, it could mean it is developing into a sarcoma and should be checked by your veterinarian.

The warning signs for a vaccine-related fibrosarcoma are:

  • A lump persists for more than three months after vaccination.
  • It is larger than two centimeters in diameter (2.5 centimeters = 1 inch).
  • One month after vaccination it is still increasing in size.

If you have any concern regarding a lump at a vaccination site, do not hesitate to contact your veterinarian.

If a vaccine-associated fibrosarcoma develops, surgical removal is attempted, but generally, this tumor is so invasive it is difficult to remove it all. Radiation or chemotherapy are often recommended in combination with surgery.

Generally, the FeLV vaccine is now given in the left rear leg, in an area distinct from where other vaccinations are given. If a tumor would develop, knowing which vaccine was given where will help us determine with which vaccine the tumor is associated. Also, if a tumor would develop, amputation of the leg would, in many cases, be curative. Cats do incredibly well on three legs, and many owners prefer this to having their cat succumb to a tumor.

If the vaccination history of a cat is unknown, how many vaccinations should the cat receive?

There are many variables that could influence the answer to this question, including the type of vaccine being used, the cat's risk of being exposed to disease, and the age of the cat.

For cats older than 4 months: For a cat who has never been vaccinated or whose vaccination history is unknown, it is generally recommended that the cat receive two distemper (panleukopenia) combination vaccinations 3-4 weeks apart. Revaccination should occur one year after completion of the series and then as recommended by the local veterinarian. Cats should be tested for feline leukemia and have a negative test result prior to vaccinating against that disease. In addition, vaccination should be based on the cat's risk of being exposed to the disease. If FeLV vaccination is recommended, the cat should receive two vaccinations against FeLV at a 3-4 week interval. Annual boosters for feline leukemia should be continued for those cats at risk of exposure. Rabies vaccinations should be given by the veterinarian according to local law.

For kittens less than 4 months of age: Kittens less than 16 weeks of age should receive distemper (panleukopenia) combination vaccinations consistent with the kitten vaccination schedule. A minimum of two vaccinations should be given at a 3-4 week interval, with the last vaccine in the series being given at age 16 weeks of age or later. Revaccination should occur one year after the last booster and then as recommended by the local veterinarian. Kittens should be tested for feline leukemia and have a negative test result prior to vaccinating against that disease. Because of their high susceptibility, FeLV vaccination is highly recommended for all kittens. The kitten should receive two vaccinations against FeLV at a 3-4 week interval starting at 8 weeks of age. Annual boosters for feline leukemia should be continued for those cats at risk of exposure. Rabies vaccinations should be given by the veterinarian according to local law.

Cat Vaccinations and 'Kitten Shots': Cat Vaccine Schedule for Routine Vaccinations
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