Persistant cat flu
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Persistant cat flu

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Lol that's okay, it does actually highlight the key points quite well.

Okay, I don't know how much you know about cat 'flu and cat noses, so I apologise if this comes across as patronising in any way. This is a good opportunity to review the various causes of snotty cats. I'll just go back to basics for the benefit of any other people reading this thread in the future. This includes how I approach cats with chronic snotty noses - but note that this is just my approach, and that others may approach them differently in an equally good or better way.

You might want to put the kettle on because this is long.

There is also a big photo of a skull at the bottom.

Cat 'flu 101 - the infectious causes of snotty cats

Cat 'flu is an all-encompassing term that describes the typical snotty cats we see on such a frequent basis - runny nose and eyes, sneezing, sore throat, sometimes a fever and inappetance if signs are severe. Some of the bugs that cause it can also cause conjunctivitis and eye ulceration, mouth ulcers and lameness - either in conjunction with the snotty signs or on their own.

There is no one infectious agent that causes 'cat 'flu'. Several different viruses and bacteria may be involved, either on their own or in combination. As a summary, these are:

Feline herpesvirus (FHV1) - almost certainly involved in every case
Feline calicivirus (FCV)

Mycoplasma - the involvement of this bug is controversial
Chlamydophila felis (commonly known as 'chlamydia')
Bordetella bronchiseptica
Overgrowth of nasal commensals* such as Pasteurella, Klebsiella, Streptococcus etc

A quick note on fungi - these are a very uncommon cause of snotty cats in the UK

*commensals are normal bacteria that are usually found in the nose (or bowel, mouth, skin - anywhere really) of healthy cats. They usually don't cause any problems unless something else upsets the balance and lets them grow out of control. In the case of our snotty cats, this is usually a viral infection that damages the nose lining and lets the commensals burrow in and go nuts, causing a secondary bacterial infection.

Note that secondary bacterial infections with nasal commensals are different to infection with Mycoplasma and Chlamydia, which are believed to be able to infect the airways all on their own without any viral involvement. Bordetella usually follows a viral infection, especially FCV.

Confused? Just think 'cat 'flu = potentially a big smelting pot of various bugs' and you won't go far wrong.

Treatment of cat 'flu varies according to what bugs are involved

Just like in people, respiratory viral infections get better on their own. Usually no treatment is required beyond TLC and general nursing care, though some cats require hospitalisation for fluids and the like if they're really unwell with it and stop eating or drinking. Cats with conjunctivitis or eye ulcers need specific treatment for that, and many cats benefit from anti-inflammatories to help a sore throat and bring their temperature down, but just as many recover uneventfully on their own.

Once bacteria get involved, on the other hand, obviously antibiotics are needed. It's not always straightforward deciding whether or not a cat has a bacterial infection. Tests are not foolproof (as I'll explain in a minute), and going by symptoms alone (such as 'bacterial infections cause thicker nasal discharge') is justified and sensible but not an exact science. Antibiotics often end up being given on a case-by-case basis, and in some of those cases they're probably not necessary.

Antibiotics are also sensible in severe viral infections even if there is no evidence of bacterial infection, because the ulceration inside the nose puts the cat at risk of bacterial infection.

The choice of antibiotic is important!
1. Some antibiotics work better against certain bacteria than others. This is particularly important in the case of Mycoplasma and Chlamydophila, which are quite difficult to kill.
2. Some antibiotics 'penetrate' the airways and their secretions better than others.
3. When dealing with kittens in particular, not all antibiotics are licensed or safe.

A commonly prescribed antibiotic for snotty cats is potentiated amoxicillin (various trade names such as Synulox, Clavaseptin etc). For cats for whom oral medication is not considered possible, the fortnightly injection Convenia (a cephalosporin) is often given. In my hands, none of these work particularly well. They can clear some secondary infection by commensals and do have some power against Chlamydophila, but do absolutely naff all about Mycoplasmas and most Bordetella strains tend to be resistant to them anyway. They also don't get into respiratory secretions very well, so it's questionable how well they actually reach the site of infection.

My favourite antibiotic for these cats is doxycycline (trade name Ronaxan), which gets into the respiratory tract really well and is effective against all the bacteria on our list. Other options are the fluoroquinolones (various trade names including Marbocyl, Veraflox, Baytril - I avoid Baytril personally) and azithromycin (Zithromax, a human antibiotic). In my experience, doxycycline does the job most of the time.

Common situations when antibiotics might not work to clear cat 'flu:
- the symptoms are not caused by bacteria - for example viral damage to the nose, a fungal nasal infection, or something else going on entirely such as a nasal tumour.
- the wrong choice of antibiotic has been used - as already mentioned, Synulox isn't going to work as well as Ronaxan or Marbocyl, for example.
- the course wasn't long enough... see below.

How long should antibiotics be given for?
This is important. A five or seven day course of antibiotics will probably shift commensals, but Mycoplasma needs at least seven weeks and Chlamydophila needs 6-8 weeks - they're persistent little toerags. Obviously, such long courses of antibiotics can be daunting for both cat and owner and the cost can add up, and side effects can be a concern - so this is a situation where it's usually preferable to get a firm diagnosis of these infections before embarking on two months of pills! This leads us onto the next hurdle when handling our snotty cats - testing...

Testing for infectious causes of cat 'flu

Testing for respiratory viruses and bacteria can be achieved from a variety of samples. The most commonly performed sampling methods are swabs, either of the nose, conjunctiva or throat. Tests can also be performed on crud flushed out of the nose under anaesthesia, or chunks of tissue sampled via a biopsy.

Note that not all sampling methods are equal in terms of their reliability. Personally, I don't swab noses. There are so many commensal bugs up the average cat's nose that you're virtually guaranteed to get things like Pasteurella and Streptococcus (which, as we know, can be normal) and then you'll be staring at a piece of paper wondering whether the result actually means anything. Throat swabs can pick up FCV quite well and conjunctival swabs are essential for Chlamydophila. Nasal biopsies are the best way of sampling because they come deep from the nose, but they are invasive to get.

As for the viruses, they're not simple either. Personally I don't test for FHV1. Ever. In my opinion it's a waste of time and money and here's why:

FHV1 tests

FHV1, like the herpesvirus that causes cold sores in humans, is generally a lifelong thing. Once you get it, it stays in your body forever. In cats, it sits in the trigeminal ganglion, a big nerve in the head, and becomes active during times of stress when the immune system isn't on the ball. Flare-ups of FHV1 can occur during cattery visits, building work, other illnesses... and giving birth - this is why it's common for kittens to catch FHV1 from their mum.

The exact prevalence of FHV1 in cats is unknown but figures up to 100% have been reported in some studies and it's generally accepted that the majority of cats carry it. Sure, you can test a snotty cat for FHV1 and it might be positive, but you could also test twenty normal cats and get a positive result. I suppose it's nice to get something concrete down on paper, but it rarely changes anything you're going to do to help the cat.

As for negative results, I don't find those particularly helpful either. We know that most cats carry FHV1 and that the virus can spend significant amounts of time hiding in the trigeminal ganglion, so I don't usually believe a negative result when I get one. Personally, I prefer to assume that all snotty cats I see have FHV1 - and if they've got corneal ulceration with their snots then it's pretty much a given anyway as FHV1 is the only major infectious cause of corneal ulcers in cats. Obviously all of this is just my opinion, and others may and probably do disagree with me.

FCV tests
Unlike FHV1, cats can clear FCV from their bodies after they recover. This usually happens within about a month - however, a proportion of cats carry the virus in the back of their throat and continue to shed it for long periods (sometimes years!). I keep reading different things about what percentage of cats carry FCV - I've seen all sorts of numbers quoted from 30% to 80%. However, based on current knowledge these 'chronic shedders' aren't actually ill any more, just infectious. These cats will come back positive for FCV even though they're actually fine - another potentially confusing result!

Is a positive Mycoplasma test significant?
Maybe, maybe not. Sorry to be vague, but the role of mycoplasmas in UPPER airway disease in cats (snotty noses and sore throats) is currently controversial. At the moment it's believed that they are actually a normal commensal of the feline nose, so while testing and treating for them isn't wrong, it might not be necessary. This is contrast to the current state of play with LOWER airway disease (pneumonia and bronchitis), where Mycoplasma is very much treated as an abnormal result and always considered significant if found.

Now to the crux of the matter - why cats can suffer with 'permanent' cat 'flu that never goes away...

This is actually not uncommon. These cats are called 'chronic snufflers'. They have snotty noses and noisy breathing, possibly with other signs such as conjunctivitis, that never goes away. It might improve to a certain point, but it never resolves. The majority of these cats are suffering from 'chronic post-viral rhinosinusitis', a horrible tongue-twister of a name that nonetheless explains quite well what it is.

Chronic post-viral rhinosinusitis
(if you look this up, you'll probably also see it called 'lymphocytic-plasmocytic rhinosinusitis or chronic rhinitis')

If I may, I will just take a moment to explain what is inside a cat's nasal cavity. It's not a big empty chamber - it's in fact packed full of tiny, delicate little scrolls of bone. These are called 'turbinate bones' or 'turbinates' and are covered in a fine, velvety moist lining which has several functions including:
- warming and humidifying the air as it is inhaled,
- smelling!
- trapping bacteria and irritants (such as dust or pollen) as they pass into the nose.

Here is a photo I took of a cat skull - you can see the fragile little turbinates filling up the nasal cavity like the folds in a Cadbury's Twirl or Flake.


These turbinates are important when it comes to understanding chronic post-viral rhinosinusitis (you know what? I'm going to refer to this as 'chronic rhinitis' from now on, forgive me).

FHV1, as a virus, loves body linings. The delicate lining of the nasal cavity is one of its favourite places and during an acute FHV1 flare-up, the lining becomes swollen, congested and ulcerated due to viral damage (this is what causes you to have a blocked nose when you have a cold). If bacteria get in on the act, they make everything worse. Once the immune system gets its act together, kills off all the bacteria and clears away all the virus-infected cells, everything settles down and goes back to normal. The lining heals. Everything is hunky dory - just like if you or I get a cold and recover.

There are three common scenarios that take a garden-variety FHV1 infection and turn it into horrible, 'permanent' chronic rhinitis:

1. A really, really severe FHV1 infection, usually in kittenhood...


2. Repeated heavy flare-ups of FHV1 during stress...


3. A compromised immune system, for example in young kittens or cats with FIV (feline immunodeficiency virus) or FeLV (feline leukaemia virus)...

In these cases, the damage caused by FHV1 is more severe than in a 'normal' case. Whether it's one severe infection in a kitten with a delicate young nose, or repeated flare-ups in an adult cat who is prone to getting symptoms of cat 'flu, the virus goes beyond damaging the nasal lining and actually destroys the turbinate bones. Bacteria can lodge deep in the bones and help erode them away. With time, the specialised lining responsible for smelling and humidifying air is complete destroyed and the body replaces it with generic tissue that patches up the damage but isn't really designed to line a nose.

Here's a picture borrowed from the internet that demonstrates the damage to the delicate turbinate bones such as that caused by FHV1.

This is a CT scan of a dog's skull (sorry, couldn't fine a decent cat one). On the left is the nasal cavity of a normal dog, with the scrolls of bone clearly visible. On the right is the nose of a dog who has had their turbinates destroyed by a fungal infection. In cats with chronic rhinitis, this is the kind of damage we see - usually not as severe, but it should help you get an idea of what's happening in the nose.


What are the consequences of the nasal damage for the cat?

Think about when you or I have a cold. Our nose is stuffy from the inflammation. It's full of snot, produced by the body as a defence mechanism to trap infectious organisms and dead cells. We can't smell or taste. Occasionally, we can get a nosebleed or sinusitis.

All of these things are exactly the same for cats with chronic rhinitis, except they're like that all the time.

1. These guys have lost the normal moist squashy lining of their nose, so they can't humidify the air they breathe. They end up breathing dry air, which dries out the nose even more and makes their snot more sticky and difficult to shift. It can become really congealed in there, until they can't even sneeze it out.

3. To make things worse, these cats produce more mucus than a normal cat because the mucus glands become more active in response to the inflammation.

3. They no longer have a good sense of smell. This is important if they stop eating for any reason, because the sense of smell is the biggest drive of appetite in cats. If they can't smell, it's hard to get them to eat if they're poorly.

4. The duct that drains tears from the eyes out of the nose is also affected in these cats. It can be squashed by all the swelling, or it may even get all scarred over. This makes these cats more prone to runny eyes, even if they haven't actually got conjunctivitis (although many do have conjunctivitis as well).

5. Because they've lost the lining that traps bugs and supports the immune cells in the nose, these cats are at increased risk of bacterial infection. They can get secondary infections by commensals more often and more severely than normal cats, and this just feeds into the damage. It's a vicious circle.

NOTE: there are other things that can cause damage to the turbinates. FHV1 is not the only culprit!

Chronic damage caused by severe or repeated FHV1 infection is one of the most common cause of turbinate damage in cats. However, there are other causes including:
- nasal tumours
- fungal infections
- foreign bodies that have been stuck up there for a while (grass blades!)
- nasopharyngeal stenosis (a narrowing of the canal between the nose and throat, usually due to scarring from long-term inflammation)
- polyps
- other weird and wonderful, but uncommon, bugs

Okay, I'm going to stop this post and continue in another as this is getting too long. Next one will be more to the point, I promise - I just wanted you to understand what could be going on in the nose.

Joined: 2025 years ago
Posts: 68

great post