A Skin-Patch Test for FIV Immune Competence
1. Preconditions
2. Procedure
3. Interpretation
I will forward a copy of the article describing this test to anyone who requests it: Cynthia M. Otto
, Cathy A. Brown , Patricia A. Lindl , Donald L. Dawe . Delayed hypersensitivity testing as a
clinical measure of cell-mediated immunity in the cat. Veterinary Immunology and
Immunopathology, 38 ( 1993 ) 91-102.
1. Preconditions
(1) Type 4 hypersensitivity response necessitates previous exposure to the antigen(s)
expected to provoke the response. The FVRCP vaccine, a combination vaccine, was chosen
because of the great likelihood that any given cat would have previously had one of the
diseases against which it immunizes (feline herpesvirus, feline calicivirus, and feline parvovirus)
or would have been previously vaccinated against one or more of these pathogens. Exposure
results in the creation of a long-lasting pool of memory T cells for quick response should the cat
ever be exposed again to the same antigens. A previous study had already established that feline
herpesvirus provokes a hypersensitivity skin response in cats.
(2) Willingness of the owner to make two veterinary visits. 72 hours are required for an
optimal skin response to the vaccine.
(3) Willingness to sedate the cat if necessary. The test requires the
injection to be intradermal, not subcutaneous. Intradermal injections are more difficult to do, and
it is distinctly possible (though not a certainty) that an unanaesthetized cat would not be still
enough to allow the injection. When asked for an opinion on this point, one veterinarian of note responded, "The test would be easy to do in some cats, hard in some, impossible in others. Using sedation, easy in all cats. It is really just like Tb testing in cattle." One of the cats from the FIV- control group of the vaccine study
was disqualified because an attempted intradermal injection was instead subcutaneous. A routine
dental procedure requiring anaesthesia might be a good time to run the test.
(4) The cat should not be suffering at the time of testing from a transient debilitating disease besides
FIV or conditions which might temporarily affect immune competence. Otherwise the test will be one of
immune competence at that time and under those circumstances, rather than of FIV-related immune competence. In this regard,
preconditions are analogous to those holding for the T cell blood testing by flow cytometry
described on the main page (Determining Immune Status.)
2. Procedure
(1) Shave the dorsal pinna of one ear using an electric clipper with a No. 40 blade.
(2) With a tuberculin syringe and a 25 gauge needle, inject 0.1 ml of FVRCP vaccine
intradermally (not subcutaneously) into the dorsal pinna. (The product used in the study was
Felocell CVR, Norden, Lincoln, NE.) One source at a veterinary school notes that a 30 gauge
needle is more commonly used by veterinary dermatologists at the school for intradermal
injections.
(3) Prior to injection, measure a double skin thickness at the projected injection site using a
Vernier constant tension caliper. Mark the injection site post-injection with a waterproof
marking pen.
(4) 72 hours post injection, re-measure the injection site and calculate the percentage increase
in thickness at the injection site.
For clarity’s sake, here is a verbatim description of the procedure from the published report of
the vaccine study:
"Cats were anesthetized with an intravenous (IV) injection of 15 mg ketamine (Ketoset, Bristol
Laboratories, Syracuse NY, 100 mg ml- ~ ) and 0.15 mg acepromazine (ProAce, Fort Dodge
Laboratories, Fort Dodge, IA, 10 mg ml- i ). Blood was drawn from the jugular vein for CBC and
LBT in Groups 1 and 2, and FeLV, and FIV testing in all cats. The dorsal pinna of one ear of
each cat was atraumatically shaved using an electric clipper with a No. 40 blade in Groups 1 and
2. The pinnae of both ears were shaved in Group 3. Two intradermal injections of 0.1 ml each
were made using a tuberculin syringe and 25 gauge needle. The injections consisted of FVRCP
vaccine (undiluted) and vaccine diluent (negative control). In Group 3, one injection was made in
each ear to allow for biopsy. Injection sites were marked with a waterproof marking pen. In
Groups 1 and 2, using a Vernier constant tension caliper (Fisher Scientific, Pittsburg, PA),
double skin thickness measurements were taken prior to injection and at 72 h by the same person
(CMO). One cat was eliminated from the study because of subcutaneous injection. The percent
change in skin thickness (%D) was calculated by subtracting the original thickness from the
thickness at 72 h and dividing by the original thickness and multiplying by 100."
3. Interpretation of Results
The difference between what often passes for a trial in feline medicine vs human medicine can be
demoralizing. 18 uninfected cats as a normal control group in the vaccine study is a decent sized
sampling. 5 infected/immunocompromised cats (2 of them tested twice for 7 total data sets) are
hardly an adequate sampling for the comparison group. Of these, 4 had FeLV, and only one had
FIV. (A guess would be that since the FIV+ – 2yrs old & probably not infected very long – was
diagnosed at the time of testing, the original intention was to use only 4 FeLV-infected cats, with
the FIV+ intended for the uninfected control group). The study does claim that a statistical test
(called a t-test) of the result tends to validate the findings. Still . . .
If you accept the size of the infected group for the sake of argument, the results do suggest the
FVRCP vax test differentiates between immune competence and lack thereof. No uninfected cat
showed less than a 70% increase in injection-site thickness; only 4 of the 17 showed less than
86% increase. Of the infected cats, only 1 had an increase in thickness higher than 70%. The
mean (average) thickness in the infected cats was 55.8% +/- 19.7%. The uninfected cats
averaged a 109% (nearly double) increase in thickness +/- 35%. The +/- figures are standard
deviations, or the tendency of the individual values to vary from the mean (average). In this case
adding 19.7 to 55.8 (=75.5) and subtracting 35 from 109 (=64.0) does produce an overlap of 11
or so, but still the doubling of the infected mean in uninfected cats is generally persuasive. In
other words, it is much more likely than not that the result of this test would accurately predict
immune competence or the lack thereof.
Worst case would be a result that fell within the 11+ point percentage overlap (64-75.5) or
somewhere just above it. The only FIV+ in the study, unfortunately, scored within this range (at
70%). The fact of the cat’s youth and of the high probablility that it had not been infected for the
2+ year period established by other studies as the low end of the emergence point of a distinctly
abnormal lymphoproliferative response is worth keeping in mind, though. This is not a perfect
test of FIV-related immune competence, but like other immune testing, adds useful information
to a determination of immune status.
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