Hi Jacquie,

If Simba had an active FeLV infection, virus or viral antigen would have
been detected when he was tested- the FeLV ELISA  is a very sensitive test.
You might want to have him tested for FIV and retested with both FeLV tests:
the ELISA and the IFA on the possiblity that first test wasn't run properly
and produced a false negative result.  The IFA will determine if he has an
active bone marrow-origin infection.

You might also want to have some x-rays or ultrasounds done- might help
locate some occult sites of infection that weren't detectable during the
general exam.

If all these tests are negative, a bone marrow aspirate and core biopsy
would be necessary to make sure his bone marrow is producing WBCs and to
rule out bone marrow disorders and immune-mediated neutropenia
(antineutrophil antibodies).

He is tested negative but I'm worried

The safest way to increase WBCs while searching for a cause is a whole blood
transfusion.  There is also a drug called Filgrastim a/k/a Neupogen that
stimulates bone marrow to increase neutrophil proliferation.  However,
Neupogen is human DNA-origin product which means cats can mount an immune
response and develop antibodies that can not only neutralize the Neupogen
but also the cat's native neutrophils.  Because of this, the drug can only
be used in short pulses of 3-5 days. I would probably go with a whole blood
transfusion because he may need more than one treatment while you're
searching for a cause.  Also, he'll be very susceptible to infections during
this time because a low WBC count increases a cat's susceptibility to
secondary infections. A whole blood transfusion will strengthen his immune
system and help him fight off infections until his own WBCs can take over.

Since you didn't mention fever, speak to your vet about broad-spectrum
antibiotic (trimethoprimsulfadiazine) prophylactically.

or something to find out foresure

A hidden (latent) infection wouldn't cause leukopenia/neutropenia because
the virus doesn't replicate or release P-27 antigen during latency.  The
only tests I know of that can detect latent FeLV infections are PCR
(polymerase chain reaction) and the Bone Marrow Reactivation Test. The PCR
is only available in diagnostic labs and the Bone Marrow Reactivation test
takes 7-30 days because bone marrow cells must be cultured.  I don't think
either of these tests are necessary right now because a latent infection is
actually dormant and presents no immediate threat.

A very small percentage of cats housed with FeLV-viremic cats develop
sequestered FeLV infections in some nonmyeloid part of the body, but this
type of infection usually releases viral antigens into the blood which are
detectable by ELISA but not IFA.  This type of infection often results in
discordances- i.e., ELISA+/IFA-.  So, I think a sequestered infection is
also unlikely.

I would tell your vet to run an FIV ELISA and another FeLV ELISA and send a
sample to a lab run the FeLV IFA.  I would also order a blood culture to
determine if bacteria or fungus are present in the blood

But first I'd order a whole blood transfusion because a low WBC count leaves
him very susceptible to infections.

I'm so scared for

I known how you feel.  Keep the faith!

Best of luck,

Phil