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Troubleshooting Persistent Effusions

Occasionally while monitoring treatment, you may have a cat with thoracic or abdominal effusion that continues to persist.  This page describes the possible causes, ways to differentiate, and courses of action.


Persistent Effusions -- when to become concerned

Abdominal effusions usually begin to resolve between 7-14 days of treatment are no longer apparent after 21-28 days. Small or scant amounts of persistent fluid may still be detectable by ultrasound.  If there has been no reduction of abdominal fluid after 14 days, or a large amount of fluid persists after 4 weeks of treatment, it is time to begin investigating the cause.


Thoracic effusions clear more rapidly, with improvement in breathing within 24-72 hours and disappearance usually within less than 7 days.  If there has been no improvement after a week of treatment, it is cause for concern.

Causes of Persistence

There are six primary causes of effusions which do not resolve as expected during treatment for FIP:

  • Dosing is inadequate

  • There is an issue with the specific formulation of the antiviral 

  • Failure to properly administer the antiviral dose 

  • Damage to venules due to fibrosis

  • Viral resistance to the antiviral drug

  • The effusion is caused by a disease process other than FIP

Determining the cause and solution

In order to determine the cause and find a solution to resolve the effusion, it is necessary to work through the possible differentials.   

Inadequate Dosing

One of the first things to verify is that the correct dose is being administered.  Check that the dose has been calculated correctly using a current (and correct) weight, and that the person administering the dose (usually the owner) is measuring it properly.  For example, cases have been seen where an owner confused measurements on a syringe, giving 0.3 cc instead of 3.0 cc.

Antiviral formulation

Some of the FIP antivirals are available from multiple suppliers, often obtained on the "grey/black market" by owners.  If not obtained from a reputable supplier, it is possible that the quality of the drug is poor -- for example, low concentration or purity.  In cases where this is a concern, switching to a different formulation is advised.

Administration Failure

Failure to reliably administer full doses is a common cause of treatment failure.  Often owners are not experienced at administering injections or pills, and some cats can make the process particularly challenging.  Additionally, some owners may be unaware of how to compensate for incomplete injections, and may not know to look for post-injection leakage, greasy spots at injection sites, or pills which may have been evaded or regurgitated by the cat.  Verify with the owner their confidence level that doses were fully administered, and if there is low confidence, advise them on proper administration technique.


In some cases the effusions may persist despite the antiviral treatment successfully inhibiting replication of the FIP virus due to residual fibrosis causing chronic damage to venules and increased capillary pressures.  If this is the case, the effusion will often ultimately resolve, but at a slower rate than usual -- sometimes requiring months.  No specific treatment or intervention is required, just additional time.


Cats with persistent effusions typically show improvement in other aspects, for example improved activity levels, appetite, and bloodwork values.  Analysis of the effusion fluid can also aid in identifying fibrosis as the cause, especially if there is an initial cytology report or fluid analysis to compare to.  The following changes in the fluid relative to fluid sampled prior to treatment would be indicative of fibrosis:

  • Color: lighter in color

  • Protein: Lower in protein (around 2-3 g/dl)

  • Cell Count: Lower cell count (cell counts <200) with fewer white blood cells

Viral Resistance

Some cats may be infected with a viral strain which naturally has mutations that confer partial or complete resistance to a particular FIP antiviral.  In such cases, it may be possible to overcome the resistance by aggressively raising the dose and continuing to treat with the same antiviral.  In other cases, the level of viral resistance to the antiviral may be too high for this to be successful and may necessitate adding or switching treatment to a different antiviral to which (hopefully) the virus is not resistant.

Other Disease Processes

If other explanations for the persistence of effusion fluid do not seem appropriate, if the initial diagnosis was in doubt, or if the cat received minimal diagnostic workup prior to treatment it is important to consider that there may be a disease process other that FIP at work -- either instead of, or in addition to FIP.  In this case, diagnostic imaging and analysis of the fluid (including FIP PCR testing) may be indicated.  (Keeping in mind that up to 30% of FIP PCR negative results are false negatives, and that if the cause is fibrosis the level of FIP virus may now be below detection.)

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