When is a ‘Fever’ a ‘Fever of Unknown Origin’? 

Presenter: Professor Ian Ramsey, University of Glasgow, Small Animal Medicine (Small Animal Clinical Sciences)

A 16 week old male Irish Setter called ‘Brogan’ presents with lethargy and a temperature of 40.5c. After performing a full clinical examination, there was no obvious underlying cause for Brogan’s fever – what would you do now? This was the question asked by Professor Ian Ramsey in last week’s webinar covering ‘Fever of Unknown Origin (FUO)’ with 46% of the participating audience opting to give treatment with a follow up re-examination, 38% opting to perform tests and give treatment and 17% opting to perform tests only.

Nobody (including myself) opted to only monitor Brogan for 4-5 days without reaching for treatment. Controversially perhaps, this is exactly the path Professor Ramsey advocated, explaining that Brogan’s own body had developed a fever through a series of chemical reactions as a means of self protection. Increasing body temperature for a protective response has developed as an evolutionary process across the animal kingdom. For example bees will deliberately increase the temperature of their hive by increasing activity levels if invaded by a life threatening fungus. Fever within humans and our veterinary patients fights infection by enhancing WBC activity, decreasing viral excretion and inducing the production of acute phase proteins. This mechanism is so effective that in humans suffering sepsis that it has been shown that the survival rate is greater for those allowed to remain pyrexic compared to those who had their pyrexia treated.

However there is a price to pay for developing a fever which includes an increase in energy consumption and a shift in haemoglobin – oxygen dissociation. Temperatures over 41.1C can also lead to cell death. For this reason Professor Ramsey advises monitoring patients without treatment only if they are suffering from a mild pyrexia (less than 40.5C) lasting no more that 4-5 days. Beyond this point treatment and tests should be initiated which is exactly what happened when Brogan returned 5 days later having shown no signs of improvement. 

At this stage Professor Ramsey asked another question, ‘at what point do we classify Brogan’s condition as ‘fever of unknown origin’? Certainly not now, according to Professor Ramsey, who defines FUO as ‘pyrexia for which no cause can be established after two weeks of examinations’. Brogan had yet to go through any form of diagnostics other than a full clinical examination and his condition can only be defined as  FUO once ‘2 weeks’ worth of tests have been performed. These tests should not just involve bloods, urinalysis and X-rays but should include repeats of tests previously performed and if still nothing is found further diagnostics should include abdominal and thoracic ultrasound, joint and CSF taps as well as CT and MRI scans. If all these tests have been performed and still nothing is found, a diagnosis of FUO can finally be made. 

Fortunately for Brogan, this extensive list of tests was not necessary as a simple set of X-rays revealed he was suffering from metaphyseal osteopathy and after 8 weeks of treatment Brogan went on to make a full recovery. Of course there will be a proportion of clients who will not be able to afford this extensive list of tests. If financial restraints mean only a minimal amount of tests can be performed and still nothing is found, it is not unreasonable to consider treatment with antibiotics and steroids. This is based on the assumption that most diseases causing a fever are likely to be neoplastic, immune mediated or infectious/inflammatory. How long to treat is also an unknown and Professor Ramsey advises measuring the acute phase proteins CRP and haptaglobin which tend to return to normal levels when a patient has made a fully recovery, indicating that treatment can be stopped. 

This was a thought provoking veterinary webinar which made me stop and consider whether reaching for a bottle of NSAIDs is always the right approach for my pyrexic patients. Whether I will have the courage to do ‘nothing’ other than monitor these patients is another matter but with good client communication and a belief this is the best approach, it should certainly be possible. 

The Stethoscope (MRCVS)