08/08/2026
Have you noticed these two FeLV (feline leukemia virus) cases?
In the past, I often wondered why some FeLV cases responded very well to treatment and continued to improve, while other FeLV cases showed little or no response at all.
Over time, I began to realize that there seem to be FeLV cases that respond well to treatment and others that are much more difficult to treat successfully. One way we may be able to recognize this difference is by looking at parameters such as IPF, MPV, and HFLC.
And this is exactly why I want younger veterinarians to study the newer hematological parameters available in modern CBC analyzers.
You may find that these new parameters can help us differentiate prognosis, treatment response, and even the risk of mortality in cats with FeLV and FIV.
Why can two cats have almost identical Ct values, with both having very low Ct values indicating a high viral load, yet show completely different clinical outcomes?
In one case, the red blood cell parameters remain excellent and the cat appears clinically healthy.
And I should mention that in the cat named Jew, I have followed the Ct value for several years, and it has remained consistently similar throughout that period.
Sometimes we need to understand the limitations of PCR and Ct values.
They can tell us about the amount of viral material or viral activity detected in the blood, but they cannot directly tell us whether the bone marrow stem/progenitor cells have already been damaged, or how severely the immune system is beginning to lose the battle.
What I would like to propose is a simple way of looking at disease severity, treatment response, and possible mortality risk in cats with FeLV and FIV:
And these parameters may even help us make a rough estimation of the possible FeLV subtype, because:
- IPF (Immature Platelet Fraction) may provide information about bone marrow suppression or platelet regenerative activity, which may make us suspect a pattern compatible with FeLV-C.
Important: IPF should never be interpreted from %IPF alone. We need to understand and evaluate both absolute IPF ( ) and %IPF together.
- MPV (Mean Platelet Volume) is actually an old CBC parameter that many of us tend to overlook.
I have collected data from more than 30 cases, and I have noticed that cats with poor treatment outcomes or higher mortality frequently have MPV >15.
But when we look deeper, MPV appears to be closely related to the IPF pattern as well.
- HFLC (High Fluorescence Lymphocyte Cells) is another very interesting parameter.
Under normal circumstances, HFLC should be essentially absent or extremely low in cats.
However, when the immune system is strongly stimulated by antigens—for example from viral infections, cancer, hemoparasites, or autoimmune disease—HFLC may begin to appear.
The problem is that many conventional CBC analyzers cannot clearly distinguish these highly fluorescent/reactive lymphoid cells from large mononuclear cells such as monocytes.
And there is another important concept to understand:
When the body develops severe inflammation, white blood cell populations tend to shift toward greater fluorescence or altered cellular characteristics.
This means that conventional CBC analyzers may begin to mis-gate or misclassify cells.
So, in a very simplified clinical approach, these hematological parameters may help us estimate:
survival probability, mortality risk, and likelihood of successful treatment.
Even if we do not perform expensive and time-consuming testing to definitively identify the exact FeLV subtype, we may still be able to make a reasonable clinical prediction from the CBC pattern.
For example:
FeLV-B
Palpate the lymph nodes.
Look at HFLC.
Look carefully at the scattergram, especially for cell populations showing increased fluorescence.
FeLV-C
Look closely at IPF and MPV, together with evidence of bone marrow dysfunction.
If these patterns are not present, then the case may potentially behave more like FeLV-T, because this subtype primarily affects T cells.
If the cat does not develop severe secondary infection, the overall lymphocyte population is not markedly depleted, and infections are treated aggressively and appropriately, then FeLV-T is the subtype that I personally suspect may have the best chance of long-term survival.
...
After 5–7 years, when antibody-mediated protection and immune control begin to decline...
that may be the point when the disease returns.
For now, I would like to end this discussion here.
This is a clinical concept based on my observations and experience, and I hope it encourages veterinarians to look beyond traditional CBC values and pay more attention to the new parameters and scattergram patterns that modern hematology analyzers can provide.
