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The Optional Vaccines Your Cat May Really Need

Designing a vaccination program that best suits your cat's needs is a two-step process.

The first step is easy: it consists of including in the program the core vaccines, the vaccines that all cats should receive whatever their exposure to pathogens or their lifestyle.

These vaccines are the core vaccines. They are covered in the Chapter 5 of this guide. They are namely, vaccines against panleukopenia, calicivirus, herpesvirus, and rabies.

The second step is a bit trickier. You’ll have to choose from the optional, non-core vaccines list those your cat really needs.

For this purpose, you’ll need to find relevant information (with the help of your vet) about the risk your cat runs to answer the following questions:

  • What are the most dangerous viruses and bacteria?
  • How likely is it that my cat gets infected?
  • Could my cat be especially susceptible to one of these viruses or bacteria?
  • In case it is a bacterium, is the infection easily curable?

This chapter is meant to help you.

Your cats' age, lifestyle and environment are the key parameters that help answering these questions.

You also need to know that the distinction between core and non-core vaccines comes as a result of the thorough work of the Vaccination Guidelines Group (VGG), an international panel of veterinary experts specialized in infectious diseases.

The group was put together by the World Small Animal Veterinary Association (WSAVA). Its mission was to design guidelines on vaccination in cats and dogs.

Non-core vaccines as defined by the VGG protect against:

  • Feline leukemia virus (FeLV)
  • Feline immunodeficiency virus (FIV)
  • Chlamydia felis bacterium
  • Bordetella bronchiseptica bacterium

Feline Leukemia Virus (FeLV)

In the 50s-70s, the feline leukemia virus was the cause of 70% of lymphoma, the main form of cancerous tumor in cats.

Nowadays, thanks to vaccination and simple hygiene measures, the number of cats infected by the FeLV virus has dramatically decreased.

However, the virus may still cause very serious disorders in cats and especially in kittens. Some recent studies have shown that the virus causes a marked decrease in life expectancy.

FeLV - Feline Leukemia Virus Identity Card


The feline leukemia virus IS NOT very contagious because it can't survive for a long time outside cats' organism. It has to find a new host within 24 hours after it has been shed by an infected cat through its nasal or oral discharge.

Moreover, the quantity of FeLV viruses necessary to infect a cat, the infective dose, is quite high.

This is why contamination most often occurs by direct contact from one cat to another. For instance, in case of mutual grooming or by a bite wound. Cats may also be infected when they share the same feeding dish or use the same litter box.

Thus, the risk for cats of being infected by the FeLV virus closely depends on their lifestyle. The risk is higher for those cats that live in communities of cats, or are authorized to wander around.

Queens may also transmit the virus to their kittens before or after birth.

FeLV virus progression in the body

The behavior of the feline leukemia Virus in the body is highly variable. It may even have opposite effects.

In some cases, the feline leukemia virus induces lymphocytes proliferation and initiates cancerous tumors formation in many organs.

On the opposite, the virus can reduce lymphocytes production and depress the immune system.

The virus may also have no effect at all, if the infected cat's immune system takes over.

The high variability of the virus' genome and of the cat's immune system reaction to the infection explain the wide spectrum of the diseases potentially caused by the infection

Genetic variability of the Feline Leukemia Virus

The Feline Leukemia virus is a retrovirus. It is a single strand of RNA that is reverse transcribed into DNA inside the infected host cell. When it is in its DNA form, a retrovirus is called provirus.

Only proviruses can replicate. A provirus incorporates itself in host cells' DNA and forces the cell to produce new viruses, as shown in this video (source: Khan Academy):

Errors may sometimes occur during the transcription from RNA to DNA. They accumulate and lead to the high variability in retroviruses genome.

4 subtypes of the Feline Leukemia Virus have been identified: FeLV-A, FeLV-B, FeLV-C and FeLV-T:

  • FeLV-A subtype does not cause any symptom at the time of infection, but may induce, on the long run, a lymphoma of the thymus gland. The thymus is a very important lymphoid gland located in the chest.
  • The role of the FeLV-B subtype is not fully elucidated, although it is close to FeVL-A. It is also asymptomatic during the initial phase of infection. It can be found in great number in lymphoma tissues.
  • FeLV-C subtype causes anemia
  • FeLV-T subtype attacks lymphocytes T and weakens its host's immune system

Cat-virus interaction

Very early in the course of infection, the virus replicates in the lymphatic tissues of the oral and nasal cavities. This phase lasts about 3 weeks.

Then the virus may reach other lymphatic tissues in any part of the body. Bone marrow, the thymus and lymph nodes are prime targets.

At that point, and depending on the immune response, the infection may develop in 4 different ways.

Abortive infection

In this unfortunately rare situation, the immune system is fully effective. The virus is eliminated. No virus' antigen or DNA can be found in the blood flow or in the body cells.

The cat is cured for good. No relapse is to be feared.

Regressive infection

The immune system holds back the virus infection. The virus is still present in its provirus form, but in small number in cells of the lymphatic system, especially in the bone marrow. It does not replicate.

The infected cat does not contaminate its environment and does not develop symptoms. Thus there is no clinical difference with an abortive infection.

Nevertheless, the virus can still reactivate later, at any time. In this case, the infection would resume to the next phase, the progressive infection.

The progressive infection

It is the clinical form, when the cat displays symptoms and contaminates the environment.

After the initial period of replication in lymphatic tissues of the oral and nasal cavities, the virus reaches the bone marrow, and then the tissues of some organs: the lymphatic nodes, the spleen, the bladder, the intestinal tract, and the salivary glands.

The progressive infection is the most frequent form of the infection.

Focal infection

The infection focuses on the organs where the virus preferably replicates: the eyes, the mammary glands, or the bladder.

Viruses' production is permanent, but varies in intensity. FeLV virus may not be detected if a blood or a serum test is performed at a time when the virus replication rate is very low.


During the first weeks that follows contamination, there are no symptoms or they are very mild.

If the infection is progressive, symptoms occur after a few months. They are unclear and not specific: loss of appetite, weight loss, lethargy, vomiting...

Overtime, they become more precise and indicative of the organs that are affected by the infection.

Leukemia is not the only possible consequence of the infection, even though the virus is named after it. The FeLV virus may cause many other diseases:


The FeLV virus reduces the number and the activity of immune cells: macrophages, dendritic cells, leukocytes, lymphocytes.

Cats become very susceptible to infections by any type of other microbes: bacteria, viruses, fungi or parasites.

In this case, the FeLV virus acts in a very similar way as do FIV virus in cats (see in the next part) and AIDS in humans.


FeLV virus may also attack red blood cells, the cells that bring oxygen to body organs, thus depriving the cat of energy.

It causes pallor of mucous membranes, extreme and prolonged fatigue, increase in heart and respiratory rates.


It is the most frequent cancerous tumor in cats. Decades ago, the FeLV virus was the most frequent cause for this kind of tumor.

But, as much less cats are now infected by the FeLV virus, it is no more the main cause for lymphomas. However, a cat infected with FeLV virus has 62 times more chances of getting a lymphoma than a non-infected cat.

Lymphomas may affect numerous organs:

  • Organs of the digestive tract
  • The thymus
  • Lymph nodes
  • The kidneys
  • Multicentric, including liver, spleen and bone marrow

Autoimmune diseases

Autoimmune diseases result from the impairment of the immune system that becomes over-active and considers some of the body cells as foreign invaders. Thus, the immune system attacks these cells and induces disorders in the organs they belong to.

The FeLV may induce autoimmune reactions in the kidneys or in the joints. The result is a glomerulonephritis leading to a chronic kidney disease, or a polyarthritis.

Reproductive disorders

FeLV may cause fetal resorption, abortion or stillbirth.

Neurological disorders

They are caused by the growth of a tumor compressing the brain or the spinal cord, or also by a direct action on the peripheral nerves.

The symptoms are: impairment of the eyes pupils or eyelids, blindness, urinary incontinence, abnormal vocalizations, increased sensitivity of all the senses, paresis or paralysis.


If your cat experiences some of the FeLV symptoms or if its lifestyle is considered at risk, your vet will advise you to perform some tests.

At risk cats are those that live permanently or occasionally in cats collectivities or that are allowed to wander outside the house.

FeLV detection relies on 3 types of tests.

ELISA: enzyme linked immunosorbent assay

This is the first test your vet will propose. It detects in the blood a specific antigen, the protein p27.

This test is usually performed at the vet clinic.

It can be misleading though, because of the numerous false negatives. As the replication of viruses may vary a lot over time, you may come across a period when the rate of replication is very low and when there are very few viruses in the blood.

This is the reason why your vet will probably ask you to repeat the test 2 weeks later, and/or to perform another type of examination.

IFA: immunofluorescence assay

Your vet will take your cat's blood or serum and send it to a laboratory. This test is used as a confirmation of the previous test.

It detects the same p27 protein antigen, but, this time, in blood white cells. It tells if the infection is already in a late stage and affects the bone marrow.

This test may have false positives. It can diagnose regressive or abortive infections.

If these 2 tests give discordant results, the cat is presumed infected. The tests must be repeated until your vet reaches an acceptable level of certainty.

PCR: DNA or RNA amplification test

This test makes it possible to detect the virus' DNA and RNA in the infected cells. It can be used as a confirmation of the other 2 tests and tells if the bone marrow cells are affected.

FeLV infection management

Until today, there is no antiviral treatment that can clear the FeLV virus from a cat, even though some preliminary experimentation with molecules from the human medical research are promising.

Infected cats should be kept away from healthy cats to avoid contamination.

If an infected cat develops symptoms, it should be treated for the disease that the FeLV virus has caused, whether it be a lymphoma, an anemia or a superinfection.


It is neither necessary nor recommended to vaccinate infected cats.

This is the reason why a test should be performed before any vaccination.

FeLV vaccines are inactivated or recombinant vaccines. Live vaccines are not used because of the risk that an attenuated virus might regain its virulence.

Primary vaccination consists of 2 injections, 3 to 4 weeks apart.

In kittens, the first injection must be made at 8 weeks of age.

A booster is injected one year later and then, every 2 to 3 years.

FeLV vaccines are not 100% effective. This is why it is recommended, in addition, to comply with hygiene measures that prevent contamination:

  • Keep infected cats away from other cats
  • Try to prevent infected cats wander in the neighborhood with the risk of infecting other cats
  • Test any cat new member of the family
  • Don't let cats share the same feeding bowl or litter box

Main references


Feline Immunodeficiency Virus (FIV)

The feline immunodeficiency virus (FIV) shares a lot of characteristics with the FeLV. They are both retroviruses, they are not resistant in the outside environment and they can induce immunodeficiency in cats.

However, FIV can only cause immunodeficiency. This is where it takes its name from. This is also the reason for its alternative name: the cat AIDS.

The FIV virus is very similar to the HIV virus that causes AIDS in humans. They have got the same structure, the same lifecycle and provoke the same disorders in their respective hosts.

Fortunately, the FIV virus is not transmissible to humans!

The FIV virus is a simple encapsulated RNA strand accompanied with an enzyme, a reverse transcriptase. In an infected cell, the RNA strand is reverse transcribed to DNA, with the help of the enzyme, and incorporated into the cell's genome.

The DNA form of the virus is called provirus. It can be found only inside a host's cell. It is the form of the virus that the most advanced detection techniques are searching for.

There are 5 variants of the FIV virus: A, B, C, D, and E. This distinction has no consequence on the management of the disease. But it is important for the design of vaccines or diagnostic tests.

FIV Feline Immunodeficiency Virus Identity Card


The FIV virus can only survive for a couple of minutes in the outside environment. It is present in large quantities in the infected cats' saliva and is transmitted mainly by the bite wounds of cats. Thus, it affects primarily aggressive male cats.

The virus is so slightly contagious that cats infected with FIV may live for years with healthy cats without transmitting the virus.

Likewise, pregnant queens may transmit the virus to their kittens. But in the same litter, only some of the kittens will be infected.

FIV progression in the body

The FIV virus attacks primarily the adaptive immune system cells (see Chapter 1: The Immune System), that is to say lymphocytes and antigen presenting cells (APC): macrophages and dendritic cells.

After FIV viruses enter the body, many of them are quickly hunted and absorbed by macrophages and dendritic cells that bring them to the main organs of the lymphatic system: the thymus, the lymph nodes and the spleen.

APCs, as it is their role, present the virus to T-lymphocytes which become infected this way. This is the time for the acute stage of the infection. Viruses replicate actively and infect, in great numbers, the blood.

If the cat's immune system is strong enough, it will produce enough antibodies within 2 to 4 weeks to clear the blood from the viruses.

Then, surviving viruses enter in latency. They keep on replicating in lymphocytes and lymphatic tissues.

The latent stage may last for years and even for the rest of the cat's life.

Then, in some infected cats, when the immune system declines (because of stress, age or a disease), it opens the door for a new phase of virus development that provokes immunodeficiency, and death.


The infection includes 3 phases:

  • The acute phase
  • The latent phase
  • The terminal illness

The acute phase

It corresponds to the rapid replication of the virus in lymphocytes and in lymphatic tissues cells.

It takes place just after the cat has been contaminated. It lasts from a few days to 3-4 weeks.

The symptoms include fever and lymph nodes enlargement. They are mild or even non-existent.

The latent phase

This is the asymptomatic stage. There is no disorder, no clinical manifestation.

The virus is present but in a latent stage. It is waiting for the weakening of the immune system.

Generally, this phase lasts several years. The infection may well stop at this point and never induce any disease or symptom.

The terminal illness

This is the feline acquired immunodeficiency syndrome, the AIDS of cats.

It affects cats older than 4 years of age. Because of their weakened immune system, aging cats are more often affected.

The cat becomes very sensitive to any kind of infection whatever its origin: viral, bacterial, fungal, parasitic or protozoal.

The veterinarian will be alerted by the frequency and the severity of these infections. She will suspect an immunodeficiency and test the cat for FIV or FeLV.

She will also suspect the FIV or the FeLV if she notices some symptoms that occur more often in infected cats: inflammation of the gum and of the oral mucosa (gingivostomatitis) or of the kidneys, rhinitis, inflammation and enlargement of lymph nodes or weight loss.

Gingivostomatitis is the most frequent cause of consultation of cats infected with FIV. It causes intense pain, bleeding and prevents the cat from eating normally. In this case, the veterinarian will necessarily suspect a FIV infection.

The cat often passes away on the medium term, after the terminal illness has started. However, and unlike human AIDS, temporary or definitive remission is still possible.


There are no symptoms that clearly signal a FIV infection. However, some symptoms, such as gingivostomatitis, may raise suspicion about the presence of the virus.

The veterinarian will propose you a blood/serologic test that can be performed at the clinic.

ELISA or IFA tests detect FIV antibodies.

These tests are effective. But they are not perfect. They should be interpreted with some caution, for two reasons:

  • It takes up to 2 months for the immune system to produce FIV specific antibodies. If the tests is performed just after contamination, the result will be wrongly negative
  • Tests are effective: their precision (sensitivity) is about 99%. It means that there is a false positive in only 1% of the cases. Nevertheless, as the actual prevalence of the virus is also about 1%, a positive result out of 2 will be a false positive.

Both considerations should lead your vet to repeat the test about 2 months after the first one.

You should also be cautious when interpreting the tests performed on kittens of less than 6 months of age. A positive test may be due to the maternal antibodies from the mother queen if she was infected. This is why the test should also be repeated.

Of course, there is no point in testing a cat that has been vaccinated against FIV. Available tests can't make the difference between the antibodies induced by the vaccine and those induced by the wild virus.

FIV infection management

There is no registered drug against FIV virus.

AZT and AMD 3100 are molecules used in human medicine to treat AIDS. They have shown positive effects in experimental treatments of FIV.

These treatments are not generally used in veterinary medicine. They are maybe useless since many cats infected by FIV may live a long and normal life.

If your cat is infected with FIV, preventive measures should be considered.

Infected cats should not leave the household: on one hand in order to avoid the transmission of the virus to other cats and on the other hand to reduce the risk of being infected by a pathogenic microbe.

An infected cat, in the terminal phase, is very susceptible to any kind of infection. A superinfection, with another pathogen may accelerate the development of the FIV infection.

A FIV infected cat requires close monitoring. A follow up visit at the vet clinic and a blood test every 6 months are recommended.

Vaccination against FIV virus

Because FIV is an animal model for the HIV virus, responsible for AIDS, many experiments have been conducted to design an effective vaccine.

One of the issues is the extreme variability of the FIV's genome exemplified by the five different variants of the virus (A, B, C, D, and E).

There is only one vaccine. It contains the entire virus, inactivated. It includes an adjuvant. It is available in the U.S., in New Zealand and in Australia only. It only protects against the A and D variants of FIV virus. It seems that it has also got a cross-effect on variant B.

A yearly booster is required after the initial primary vaccination including 3 injections.

FIV vaccine is not a core vaccine. Many experts recommend not using it because they think it is not effective against some of the variants, and because the vaccination requires too many injections in an animal, the cat, prone to developing tumors at the injection site.

FIV vaccine is an option for cats living in collectivities of cats or cats allowed to wander outside, in areas where they can meet other cats.

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Chlamydia felis

Chlamydia felis is an intracellular bacterium. It needs to penetrate into one of its host’s cells to be able to replicate.

Chlamydia felis colonizes epithelial cells. It can colonize any kind of epithelial cells, in the intestines, in the lungs. But it is more often found in the epithelial cells of the eye conjunctiva.

Chlamydia felis is the pathogen the most frequently associated with conjunctivitis in cats.

More rarely, it can cause digestive, respiratory or reproductive disorders.

Chlamydia Felis in Cats Identity Card

Chlamydia felis transmission

Chlamydia felis can't survive for a long time in the outside environment. In most situations, contamination is direct from a cat to another cat. Mutual grooming is a typical example.

Chlamydia felis is shed in the oral, nasal and ocular discharge of an infected cat. It seems that transmission through ocular secretions is the most frequent method of infection.

Occasionally, a cat may be infected in highly contaminated area, a litter box containing feces from other cats, for instance.

Chlamydia felis infection progression

Chlamydia felis has to enter its host's epithelial cells to find the substances it needs to replicate.

The bacterium comes in 2 different forms:

  • The elementary body is 0.2 to 0.6 microns long. It is the infectious form, the one that can be transmitted from a cat to another and freely floats in the blood stream and in lymphatic systems.
  • The reticulate body is 0.5 to 1.5 microns long. The bacterium turns from the elementary body into the reticulate body once it has entered a host's cell. It then uses the resources of the cell (lipids and proteins) to replicate actively. At the end of the replication phase, the reticulate bodies change back to infecting elementary bodies which leave the host's cell by destroying parts of its membrane.

Initially, Chlamydia felis replicates mainly in the epithelial cells but may also infect white cells and smooth muscles cells.

After a 2 to 5-day incubation period, Chlamydia felis spreads through the body and reaches the spleen, the lungs, the digestive tract, the liver, the kidneys


Chlamydia felis acts mainly upon the conjunctive tissues of the eyes.

Conjunctivitis develops within 2-5 days following the beginning of the infection. It affects first one of the eyes and then both eyes after just one to two days. This is referred to as bilateral conjunctivitis.

It may be serious and cause an excess of blood flow in the nictitating membrane (a third eyelid, transparent that protects the eyes of some animals including cats). It gives a red color to the eyes. It is quite impressive!

An additional eye edema is suggestive of an infection with Chlamydia felis. There may also be blepharospasms, muscular spasms of the eyelids that cause a rapid, involuntary and repeated spasm of the eye.

Far more rarely, coughing and nasal discharge occur. These disorders are temporary.

However, affection of the eyes may last several months as a moderate conjunctivitis, in case the infected cat is not well treated.


Suspicion of infection with Chlamydia felis is raised by suggestive symptoms: conjunctivitis, eye edema and/or vascular congestion of the nictitating membrane.

It then needs to be confirmed by a laboratory exam on ocular secretions to differentiate this infection from feline calicivirus or herpesvirus.

The biologic test of choice is the PCR (Polymerase Chain Reaction) which detects Chlamydia's ADN fragments.

Ocular secretions are collected by rubbing a swab on the eyes of the infected cat. It may be anesthetized to make sampling easier.


Chlamydia felis is a bacterium. It is sensitive to antibiotics.

Tetracycline and doxycycline are the antibiotics of choice against Chlamydia felis.

Vaccination against Chlamydia felis

As it always possible to treat Chlamydia infections with antibiotics, vaccination against Chlamydia felis is less useful than against viral infections.

As often, vaccination is recommended for cats living in collectivities. An additional risk factor is the inclusion in a group of a new cat without immunization records.

Vaccination is also recommended for kittens whose mother is already infected.

Vaccination against Chlamydia felis does not prevent the infection. It reduces the bacteria replication rate and avoids the disease.

The vaccination schedule includes two doses to be given 4 weeks apart, from 9 weeks of age on.

An annual booster is necessary.

Main reference


Bordetella bronchiseptica

Bordetella bronchiseptica is a pathogenic bacterium that affects the respiratory system.

Unlike many other opportunistic respiratory bacteria that take advantage of a primary viral infection, Bordetella bronchiseptica is able to cause alone all the symptoms of an infectious respiratory disease.

Bordetella bronchiseptica in Cats Identity Card


Infected cats shed Bordetella bronchiseptica with their oral and nasal secretions. However, the bacteria can hardly resist to outside conditions.

Thus, cats are usually infected by direct contact with another cat, or a dog!

A cat has more chances to get infected in collectivities of cats such as rescue shelters or breeding catteries. Occasionally, cats can be infected in highly contaminated area such as litter boxes.

Bordetella bronchiseptica may develop and survive in pets without causing symptoms. But these healthy carriers keep on shedding bacteria and potentially contaminate other animals.

Bordetella becomes virulent and starts causing symptoms when body condition deteriorates (by a viral infection, for instance), or when an infected cat is stressed out (overpopulation of cats, presence of dogs, travels, parturition...).

Bordetella bronchiseptica infection progression

Bordetella bronchiseptica is able to move in liquids thanks to its flagella.

Bordetella bronchiseptica 02

Bordetella bronchiseptica secretes a certain type of proteins, the adhesins that facilitates its attachment on ciliated epithelial cells of the respiratory tract, despite the presence of mucus on the surface.

Once it adheres to the epithelium, the bacterium releases numerous toxins that cause the destruction of ciliated cells.


They cover the entire spectrum of respiratory signs. From the milder ones that disappear within a few weeks (fever, coughing, sneezing, nasal and oral discharge, swollen lymph nodes) to the most severe symptoms such as pneumonia, especially in younger kittens, and death.

Although it is a very virulent bacterium, most of the time Bordetella bronchiseptica doesn't act alone. It is often associated with other pathogens, especially viral, such as the calicivirus or the herpes virus (see Chapter 5: Core vaccines for cats).


You vet will try to collect samples containing the bacterium, using one of these two techniques:

  • Broncho-alveolar lavage explores the lower airways: bronchioles and alveoli.
    The cat is anesthetized. Your vet may get the help of an endoscope.
    A tube and the endoscope probe are introduced through the cat's mouth to the lungs. A solution of sodium chloride is instilled in the lungs via the tube and then aspired back with a syringe.
    Finally, the collected liquid is examined in a veterinary laboratory.
  • Samples from the pharynx and/or form the nasal cavity give information on the infection of the upper airways. A swab is passed on target zones and then stored in a test tube for later analysis.

Collected samples by either method are analyzed either by DNA amplification technique (PCR) or by bacterial culture.

Serologic tests, which detect Bordetella's specific antibodies, are rarely used. They indicate if the cat got infected at some point in its life. They do not give a clear indication of the reasons for the symptoms.


Bordetella bronchiseptica is a bacterium. It is susceptible to doxycycline, enrofloxacine or the combination amoxicillin-clavulanic acid.


Vaccinating your cat against Bordetella bronchiseptica is only an option. It is recommended for cats highly exposed to infectious respiratory diseases. They are cats that live in large communities of cats or kittens whose mother is infected.

The available vaccine is a live attenuated vaccine. It is administered nasally.

Primary vaccination consists of a single administration given at 4 weeks of age, the soonest.

A yearly booster is necessary.

Main references