Ask a Vet: Why Is My Cat Sick With a Fever?


A reader named Yvonne recently contacted me through my website (another way to get in touch is through my Facebook page) about a problem suffered by one of her cats.

The cat, who is kept strictly indoors, was suffering lethargy and fever. The veterinarian administered subcutaneous fluids, a long-lasting injectable antibiotic called Convenia, and a non-steroidal anti-inflammatory drug called meloxicam. Yvonne expressed disappointment; she had hoped that he would use a different antibiotic. The next day the cat seemed worse. His temperature was 105 degrees Fahrenheit, and he was vomiting. He was extremely lethargic and was eating and drinking nearly nothing. Yvonne expressed concern that Convenia may have caused the vomiting.

The problems started about a week after the cat was scratched near his eye by a new kitten, who had been in the house for eight weeks. There had been no contact between the cat and the kitten for the first month, and since that time she only allowed supervised play. Yvonne was very worried and wondered whether I had any thoughts on the matter, and especially on the use of Convenia.

Indeed I do. There is a name for the condition suffered by Yvonne’s cat: fever of unknown origin.

This name should not be confused with a diagnosis. A diagnosis is a definitive determination of a disease process. Fever of unknown origin is merely a description of a problem: The cat is sick and has a fever, and the origin of it is unknown.

There are many possible causes of fever of unknown origin. Infection with bacteria or viruses can cause it. Infestation with certain parasites such as Toxoplasma gondii can trigger the condition. Inflammatory processes in the body can trigger fever and lethargy. Organic issues such as gastrointestinal obstruction may cause the condition. Often, the condition is linked to weird single-celled organisms such as T. gondii, Bartonella henselae, or Mycoplasma.

Fever of unknown origin is markedly more common in outdoor cats. This is because they are much more likely to get into fights (developing abscesses are the leading cause of fever of unknown origin). They also are much more likely to be exposed to bacteria, viruses, and parasitic organisms that can cause the syndrome.

However, the condition can occur in indoor cats as well. Inflammatory processes, intestinal obstruction, and many other causes of the condition do not require exposure to pathogens.

Of course, the cat in question did have a possible source of infection: the new kitten. And, although it is not absolutely certain, there is a high chance that the new kitten was the source of a pathogen that caused the older cat’s symptoms. In this case I am highly suspicious of one of those weird single-celled organisms — kittens carry those things in droves.

Now, what about the treatment of the cat in question? I would probably have treated it differently than described, but I’m also in no place to judge the treatments that were implemented.

When I see cats in this type of situation I generally recommend inpatient care (hospitalization) for intravenous fluids, antibiotics, and careful observation. I recommend a battery of blood and urine tests as well as radiographs (X-rays) to try to determine the cause of the fever.

The general-purpose antibiotic Convenia has been available for several years. It is given once as an injection and lasts for about a week. It represents a great step forward for cats who refuse to take oral medications. However, I don’t generally recommend it as a first-choice antibiotic for cats who accept oral medications, since once Convenia has been administered it’s there for a week and it can’t be taken back. Convenia has a very good side effect profile (it rarely — which is not the same as never — causes vomiting), but very good is not the same as perfect. Also, when I suspect a weird single-celled organism my first choice antibiotic is one called doxycyline.

I also don’t generally reach for meloxicam right away in cases of fever of unknown origin. Meloxicam, like all NSAIDs, must be used cautiously in cats. It helps to bring down fevers in some instances, but the kidneys can be stressed by the drug, especially in dehydrated individuals who aren’t consuming adequate water.

Now, with all of that said, I have to be emphatic that I’m not criticizing the care provided by Yvonne’s vet. It’s very tempting to backseat drive in these situations, but it’s wrong to backseat drive in these situations.

First, it’s easy for me to say that something different should have been done after the initial treatment didn’t work. I have the benefit of hindsight. I know that the treatment didn’t work, which makes it easy for me to poke holes in it. If the treatment had worked, I never would have heard from Yvonne — for all I know the other vet has treated 10,000 cats in this fashion and this is his or her first treatment failure.

Also, for all I know the treatment option that was elected in the end was not the other vet’s first choice. Hospitalization and diagnostic testing can get expensive fast. Many people are very reluctant to keep their cats in the hospital. Clients often decline my recommendations for the treatment of fever of unknown origin. In these cases, I generally administer subcutaneous fluids and antibiotics — and if the owners state that they can’t give oral antibiotics, I usually administer Convenia because it’s far better than nothing. However, I advise these clients that if the treatment doesn’t work, then more aggressive care is strongly indicated.

And that is the case for Yvonne’s cat. Conservative care has failed, and the cat is still showing significant symptoms. He needs further care and treatment.

Got a question for Dr. Barchas? Ask our vet in the comments below and you might be featured in an upcoming column. (Note that if you have an emergency situation, please see your own vet immediately!)

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