Understanding and Applying FIV Testing
1. Introduction
2. The Terminology of Testing Accuracy
Sensitivity and False Negative
Specificity and False Positive
3. What is Being Tested
Testing for Antibodies
Testing for Antigen
Antibody Tests Plus Antigen Tests
4. Problem Situations
Kittens With Mothers Known Or Not Known To Be FIV+
Discordant And/Or Equivocal Results
The Possibly FIV-Vaccinated Cat
The Lost Cat Vaccinated For FIV
A Positive Result on an Indoor-Only Cat
A Possibly Newly Infected Cat
Click here to open a Glossary of Terms in a separate window.
1. Introduction
FIV testing is not a simple, straight-forward matter, and some explanation is necessary in order to
understand why. There is no such thing as a perfect FIV test. To date, all forms of testing are
liable to some level of inaccuracy. Errors may be attributed to inherent imperfections in the test,
failure to react to a particular antibody or antigen, reactivity to nonspecific elements in serum, or
incorrect performance or misinterpretation of the test. To maximize the likelihood of getting a
correct result in a given situation, it is important to know how these tests work, how they differ
from one another, what their limitations are, and how they are best used, individually and in
concert with one another, in various situations.
Understanding FIV testing involves several categories of discrimination. One is what is being tested. Another is the terminology of assessing accuracy. Both involve correctly applying this understanding to a given situation.
2. The Terminology of Testing Accuracy
Sensitivity and False Negative
Sensitivity means, How well does a test detect the feline immunodeficiency virus when it is
actually present? A test’s identification of a cat who is truly infected as uninfected is called a
False Negative. The undesirable consequences of this failure in sensitivity can be the unwitting
spread of infection in multi-cat households or to outdoor cats; or the failure to render appropriate
treatment to sick cats whose immunological deficits go unappreciated. Suppose the sensitivity of
a test is being gauged in a study involving 20 cats, 10 of whom have been previously confirmed
as FIV+, and 10 of whom have been confirmed as FIV-. (The gold standard for these
confirmations would perhaps be virus isolation, a laboratory procedure not available for clinical
use in which a sample is cultured for presence of the virus.) A test that correctly identified 9 of
the 10 infected cats would be considered 90% sensitive for FIV, not perfect but pretty good. The
one cat identified as negative, but which is in fact positive, would have been the victim of a false negative result. An
ideal test would have 100% sensitivity
Specificity and False Positive
Specificity means, How well does a test accurately identify an uninfected cat? A test’s
identification of a cat that is not infected with FIV as being infected is called a False Positive.
If, however, the same test previously hypothesized identified 5 of the 10 uninfected cats as
positive, it would be said to have a specificity of 50% because it returned a false positive on half of
the uninfected cats, which does not sound very good at all. Half of the cats would be the victims
of a false positive result. The undesirable consequences of a failure in specificity are owner
anxiety over a cat falsely believed to have a chronic illness; greater difficulty for rescue agencies
and shelters in adopting out cats mistakenly believed to have a chronic disease; and very often
the euthanizing of such cats by shelters. An ideal test would have 100% specificity.
3. What is being tested
Testing for Antibodies
The commonly used clinical FIV tests are for antibodies to the virus, not for viral antigen
(“antigen” being some component of the virus capable of initiating an antibody response).
Testing for antibody is necessary because the FIV-virus itself is too genetically variable to be
detectable by one test kit and because in some asymptomatic cats the level of virus in blood is
too low for reliable antigen detection.
ELISA
The standard screening test for FIV is called an ELISA (Enzyme-Linked Immunosorbent Assay).
All ELISAs use variations of a labeled antigen format in which the test serum is reacted with FIV
antigens. FIV-specific antibodies bind to the antigens to form an antigen-antibody-antigen
sandwich. Addition of an appropriate solution produces a color change in the sample. (Outside
the U.S., several tests are available that us a synthetic peptide from the FIV transmembrane as
antigen, and one study did find better test validity in a head-to-head comparison with a standard ELISA). Interpretation of a positive or negative test is based on a comparison of the optical
color density of the test sample to that of a positive control sample.
The ELISA can be done “in house,” meaning at the vet’s office (Idexx SNAP Combo), or at a laboratory from a blood sample sent to it by a vet (Idexx PetChek). Idexx has held North American rights to the kits used in both the lab and the private veterinary practice, although its exclusive license expired in 2009, presumably allowing for competitors to enter the field. Comparison of the accuracy of various test
kits (other laboratory and clinical kits available outside North America include Virachek [Synbiotics], Witness [Synbiotics], Fastest [MegaCor], and Duo Speed [Bio Veto]) show only minor variations in validity among most. (Click here to view the Idexx SNAP FIV/FeLV test kit used in many veterinary practices. A “weak positive” will fail to achieve the
optical density of the control. A negative result will produce no reaction at all. Recently, a triple option with an additional heartworm component has been made available.)
One might suppose that laboratory results would always be superior in accuracy to the in-house testing, but (surprisingly) at least one study found the reverse to be true, with an overall 22.5%
discrepancy in results on the same samples and the in-house kit proving more accurate. The
reason for the discrepancy was uncertain, although operator error and misinterpretation of results
were speculated. A later study found a slightly higher level of accuracy in the laboratory microtiter ELISA.
Validation studies have found that samples with an optical density clearly
corresponding to the positive and negative control samples have a sensitivity and
specificity in the mid to upper 90 percentile range. Optical Density Ratios just above and
below 0.5 (in the 0.3 to 0.8 range), the standard for a positive result, are considered
problematical. Sensitivity and specificity for samples within this range (perhaps 75 - 80%) are
significantly reduced, and the result is said to be “equivocal.” Equivocal results may occur for a
variety of reasons. (1) Recently infected cats only beginning to seroconvert may test equivocal
for a time because of lack of antibodies. (2) Samples from cats who have been recently
vaccinated or who have been co-infected with other diseases are particularly prone to fall within
this equivocal range. (3) Vets using in-house screening tests for FIV may use whole blood,
serum, or plasma. Weak-positive reactions results are more likely to occur when whole blood is
used. (4) Some samples, despite high FIV antibody concentrations, may test equivocal because
of what is called a "prozone-like effect." That is, at lower dilutions where concentration of
antibody is high, there is no agglutination; lack of agglutination in the prozone (the zone of high
antibody concentration) is due to formation of small complexes that do not clump to form a
visible result that meets the control standard.
A negative result on an ELISA is generally regarded as reliable, and further testing usually is not called for unless some special circumstance (such as a very recent infection) presents itself. However, false negatives do occur and cannot be entirely ruled out. Although a recent European validation study comparing a new product to the Idexx SNAP again reported high rates of specificity for the latter, the ELISA is widely recognized in the field as having a proneness to false positive results, even with some strong positive results. Many rescuers who test strays frequently claim lower overall specificity (and higher false positive) rates than the studies cite. Some independent studies have provided backup for this claim. One study found as many as 20% false positive FIV ELISA test results. Data recently published by National Veterinary Laboratory, an Idexx competitor, found that 32% of FIV SNAP ELISA positive tests were negative by their Western Blot test. The cause of false positive tests, they speculate, may be hospital personnel technical inexperience, use of improperly stored test kits, or innate variation in the tests kits.
To help insure that a positive ELISA result is accurate, other tests must be used. The Western Blot and IFA are antibody tests like the ELISA. Their advantage is that they are less likely to
mistakenly identify an uninfected cat as infected; in other words, the false-positive rate is lower.
According to the AAFP, all positive results on the FIV ELISA should be confirmed by Western Blot or IFA. The European Advisory Board on Cat Diseases, a panel of 17 veterinary virus researchers, recommends that all ELISA FIV positive tests should be confirmed exclusively by Western Blot.
IFA
The IFA (Immunofluorescence Assay) uses infected cells adhered to a microscope slide as the source of FIV antigen. The test sample is incubated with the cells, allowing antibodies to bind to
them. If antibodies are present, the infected cells will fluoresce when stimulated by light at a
certain wavelength.
Fig 1. IFA
The IFA has a generally high degree of reliability, except for the occasional
presence of nonspecific fluorescence that obscures either a positive or negative result. Careful
handling of sera and interpretation of results are therefore very important. Several papers
reported up to 6 to 10% of these kinds of reactions. In areas of the world where ELISA test kits
are costly, IFA is an affordable, generally reliable test, with sensitivity approaching that of the ELISA
and specificity nearly that of the Western Blot. Fig 1 shows a positive IFA result for antibodies
to Ehrlichia.
Western Blot
The Western Blot (WB) uses antigen derived from whole virus particles. Antigens detached
from infected cells are separated and transferred to a membrane. The test sample is incubated
with these antigens, and FIV-specific antibodies bind to the different antigens. Anticat
immunoglobulin is used to react with the fixed antigens, forming dark blue bands wherever FIV
antibodies have bound. WB is considered to be the most reliable test for FIV antibodies because
antibodies to individual viral proteins are being detected and can be distinguished from nonviral
components of the sample. The test, however, requires technical expertise in interpretation.
Reactivity with two or three viral proteins usually is regarded as a positive result. The most commonly assayed proteins are p15 (the viral matrix protein), p24 (the capsid protein), and p13 and p55 (from capsid contents). There remains some controversy over which proteins the assay should include and how many should be found in order
to declare a positive result. Following (Fig.2) is an example of a Western Blot assay of three
samples tested for diagnosis of HIV.
Fig 2. Western Blot
Columns 1 and 2 are the positive and negative controls, respectively. Sample A, with no bands, shows a negative result. Sample C shows a positive result. (A positive result requires bands at
either p31 or p24 and at either gp160 or gp120,). Sample B, with only one band at a target
protein, represents an indeterminate result. Reactivity with only one FIV protein is considered
an equivocal result. A few samples, however, have high reactivity with all proteins on the blot.
These samples are considered equivocal. Most of these equivocal samples resolve upon
subsequent testing, but a few remain equivocal.
The WB has a specificity near 100%. This is the primary reason why the WB is the gold
standard for confirming positive results on the ELISA. Why not, then, simply test all cats
initially with a Western Blot? There are several practical reasons. One is that the test costs more
to do. Another is that it can’t be reduced to a kit for individual vets’ in-house use; WBs must be
sent out to a laboratory. But there is one more possible reason to perform the ELISA initially. It has been widely claimed that ELISA actually has a higher sensitivity (and specificity as well, according to the AAFP) than its more expensive counterpart; that is, while the WB is good at weeding out false positives, the ELISA, at the very least, will come up with fewer false negatives. Idexx competitor National Veterinary Laboratory vigorously disputes both claims, and says (citing unpublished data) that it has documented that its FIV WB is more specific and more than 100 times more sensitive than the Idexx FIV PetChek ELISA test.
Testing for Antigen
PCR
The PCR (polymerase chain reaction) can test directly for viral antigen. The PCR process involves adding “primers” engineered from known sequences of viral nucleic acid to a
prepared test sample. The mix is repeatedly heated and cooled to specified temperatures in the
presence of a polymerase enzyme to catalyze annealing of the primers with any FIV nucleic acid
of the “target” gene close enough in sequencing to allow binding. With each cycle (called
“amplification”) of heating, the double stranded viral DNA unwinds, with the single strands
seeking out complementary nucleotides to form new strands when cooling occurs. Discovery of a temperature-stable catalyzing polymerase called Taq (standing for Thermus aquaticus, the bacterial source of the polymerase) revolutionized PCR technology by making quantification in “real time” possible because new polymerase did not need to be added at each cycle of amplification. With
continuous measurement in “real time” following each cycle (hence Real-Time PCR) of bound
nucleotides on a logarithmic scale, software can verify the presence of the target gene in the
sample. Tagging with fluorescent dye is the usual vehicle for allowing measurement.
Fig 3. Icycler PCR Machine
Fig 4. PCR Trace of Serial Dilution
Fig 3 above shows a particular make of PCR machine with operator interface at the front and sample well immediately behind and above. The machine is connected at the rear to a computer, where software calculates and displays results. Fig 4 shows a data trace of serial 10-fold (1 log) dilutions of a sample plotted on a logarithmic scale. The horizontal orange line is a pre-set threshold at which “background noise” of nontarget nucleic acid (represented by squiggled lines at the base of the graph) no longer exists. Where transverse orange lines cover color-coded traces of individual dilutions exponential multiplying of target DNA or RNA is occurring. The
intersection of threshold and color-coded line is the so-called Ct value, representing the cycle at which exponential
growth is detectable. An actual assay for a target genetic sequence will be co-amplified with a known amount of a reference sample to allow normalizing for possible variation in the amount and quality of nucleic acid .
This process sounds foolproof, but a widely noted study published in 2005 found considerable discrepancies in the results of three different U.S. and Canadian laboratories on the same samples
taken from infected, vaccinated, and uninfected cats. Only one lab scored in the 90 percentile on
sensitivity (and one was in the 40 percentile range!), but that lab scored significantly lower on
specificity. Another lab scored perfectly on specificity, but much lower on sensitivity. Remarks one observer, “[These] FIV PCR tests result in a predictive accuracy that is essentially equivalent to flipping a coin. Differences in PCR design and quality of PCR execution or prevention of carry-over contamination may explain the markedly varied specificity and sensitivity of these FIV PCR tests that are clearly not suitable for diagnostic use.” The two most significant limitations in PCR technology have been these: (1) Viral
nucleic acid sequences too dissimilar from those that went into the engineering of the primers
may not bind with them. Because of the length of time it has existed and because of its
instability and rapid mutation, FIV exists as many strains and substrains with many variations in
nucleic acid sequencing. (2) Asymptomatic cats in the early stages of infection may have virus
that is absent or nearly so from peripheral blood taken for testing.
The best Real-Time PCRs are evolving into increasingly accurate instruments. Crucial to the design of a reliable PCR is the quality of the primer and probe (see below) design. Primers must be designed to amplify all strains. Accomplishing this means locating the most conserved FIV nucleotide sequences (i.e., those least subject to mutation) and having as few mismatched sequences as possible between the engineered primers and the target DNA. Two of the better U.S. labs currently performing Real-Time FIV PCRs, Idexx and Auburn University, have raised the bar for PCR accuracy through quantum advancement in design of probes and primers. Both claim the ability to accurately detect even very small numbers of virus (in the range of 10/ml) in blood specimens of known strains.
Idexx’s assay, introduced in June 2009, is an enhanced version of the Taqman technology formerly used by U.C. Davis. This PCR with Dual-Labeled Fluorogenic Probe quantifies proviral DNA, that is, DNA from the genes of infected cells. The Taqman process involves the use of
“probes” tagged at either end with a reporter and a quencher (i.e., inhibitor) dye. The probe anneals to the target viral gene prior to the introduction of the primers. The displacement of the Taqman probes by the primers while extending their binding under the catalyzing influence of the Taq fractures the probes and produces a fluorescent signal when the quencher dye moves out of proximity to the reporter dye. Idexx’s clinical validation studies
report 99.9% specificity, meaning that they have effectively eliminated false positives entirely.
Any cat found positive for FIV can be considered infected (not uninfected and not vaccinated-and-uninfected.) Unfortunately, sensitivity was found to be only 81%. This means that only 4 of 5 infected cats will actually register as positive; 1 of the 5 will be a false negative. So they do not recommend their PCR as a screening tool for FIV infections.
Auburn had, until recently, been running a Real-Time Reverse-Transcriptase PCR, in which, preliminary to PCR, any viral RNA present is converted to DNA with the same enzyme that FIV uses to convert its RNA to DNA prior to insertion into the host genome; this type of assay, normally more native to research than diagnostic use, actually measures the sum of viral RNA and DNA in samples. Results were reported as “mRNA,” or Messenger RNA, which is the nucleic acid that bridges the “transcription” gap between the DNA embedded in the genes of infected cells and the manufacture of proteins for
new virus. The simplified and enhanced assay currently used has eliminated the reverse-transcription step, and positive results are reported (as with Idexx) as viral DNA. Recently (March, 2010), an Auburn team claimed a very high degree of accuracy for their FIV PCR, which employs a form of fluorescence measurement called FRET Real-Time PCR for FIV. FRET stands for Fluorescence Resonance Energy Transfer, a method of quantification in which a dye tag bound to one end of the probe that binds to the target gene is excited by an external light source, then passes part of its excitation energy to the dye tag bound to the other end. The excited acceptor "fluorophore" emits light at a different wavelength, which can then be measured. Like Idexx, Auburn sets specificity at near 100%. A person associated with the program sets sensitivity in the 90+ percentile range, although at this time the figure is only an estimate
Antibody Tests Plus Antigen Tests.
It is an extremely hopeful development that PCR technology has evolved to the point where specificity is so high and where false positive results have been radically minimized. Now antibody testing, when properly conducted, is strong (i.e., with a high sensitivity and low rate of false negatives) where PCR may be weak, and PCR is strong (minimizing false positives) where antibody testing is weak. With the better PCR labs able to detect very small viral loads, the combined weight of antibody and antigen testing provides powerful predictive value for FIV infection.
4. Problem Situations
Kittens with mothers known or not known to be FIV+.
Antibody tests cannot distinguish between antibodies derived from an infected or vaccinated mother in utero and those produced in response to an actual infection. A negative test result on a
very young kitten would be reasonably reliable. A positive result would not. A recent study
showed that all kittens of vaccinated mothers had shed maternal antibodies by three (3) months and
tested accurately for natural infection. However, there is some feeling that a naturally infected
mother’s antibodies might require longer to clear. Therefore, regardless of a positive result in
earlier testing, a kitten should be retested at no less than six (6) months post partum. PCR testing
will not be thrown off by maternal antibodies, and can be used to test kittens prior to the six-month point. However, a negative PCR result lacks the reliability of a positive one.
Discordant and/or equivocal results.
A carefully developed FIV testing protocol requires that all discordant or equivocal results
should spur further testing, to continue until all tests are in agreement on a positive or negative finding. The protocol for resolving discordant results is diagramed at this link
Suppose, for instance, that an ELISA is positive and a Western Blot is negative? While it is true that a negative finding by Western Blot is usually regarded as definitive, the small possibility (1)
that the lesser sensitivity of the WB by comparison to the ELISA has resulted in a false negative,
or (2) that an error may have occurred in handling or interpreting a sample should be respected.
The same would be the case with an indeterminate WB, a not uncommon circumstance. Testing
should be repeated at intervals of three to four months according to the approved protocol until
ambiguities are resolved.
The possibly FIV-vaccinated cat
Cats vaccinated for FIV can produce antibodies for four years or more from their most recent immunization. Currently available antibody tests cannot discriminate FIV-vaccinated cats from FIV-infected cats. A recently developed discriminant ELISA has been found capable of doing what no
previous one has been able to: distinguish a sample containing antibodies spurred by killed-virus
vaccine from those spurred by living, infective virus. It does this because by zeroing in on the
formalin treatment used to fix the whole virus and synthetic transpeptide in the FIV vaccine.
Blood samples from vaccinated cats have lower reactivity to the formalin-treated products than
those from infected cats. This discriminant ELISA is projected to be more than 95% accurate in
distinguishing infected from vaccinated cats, although (curiously) it cannot discriminate
uninfected unvaccinated cats from infected unvaccinated cats. Therefore, this assay (in its
current form) could not replace, but could only supplement existing tests. Unfortunately, this
discriminant ELISA is not yet being manufactured for clinical use
In March, 2010 an Auburn team published results of a study on the basis of which they claim unequivocally to be able to distinguish FIV-vaccinated from FIV-infected cats. “In this study, we established a gag gene-based dual fluorescence emission FRET real-time PCR that amplifies single target copies of all known FIV strains and differentiates 5 FIV subtypes. . . . Collectively, these data unambiguously prove that blood samples from FIV-vaccinated cats remain negative by PCR assay of blood samples. Thus, positivity in the FIV PCR of whole-blood samples indicates FIV infection but not FIV vaccination.” These results suggest that the problem of distinguishing vaccinated from infected cats has been solved at the level of available technology. Unfortunately, large shelters, which must accurately make the distinction as a step in the survive-to-adopt process, may not be able to afford even reasonably priced PCR testing. The experimental discriminant ELISA therefore may remain the best hope for making the distinction on a cheap, reliable basis, provided that it can be made available as an on-site SNAP-type kit
The lost cat vaccinated for FIV
FIV-vaccinated cats are indistinguishable from naturally infected cats by all clinically available antibody tests. If a vaccinated cat should somehow end up in a municipal shelter where testing
for FIV and FeLV is routine, it could well be euthanized. It is a worthwhile investment to have
any FIV-vaccinated cat microchipped. All U.S. microchip registrars will record FIV-vaccination
status as part of the information available through accessing the chip. An alternative is tattooing
on the ear, e.g. “FIV VAX.” Owners should remember, as well, that the FIV vaccine is not 100%
effective. The good news in this case is that PCR (at least, it is claimed, the Idexx PCR) can
reliably distinguish a vaccinated but uninfected cat from a vaccinated and infected cat.
Therefore, if there is any doubt that a vaccinated cat has been exposed to FIV, PCR can establish
whether infection has occurred.
A positive result in an indoor cat.
Sometimes a cat taken to a vet with a condition frequently found to be secondary to FIV or FeLV infection will be FIV tested as a precaution. Any positive test should, of course, be confirmed. If
at some earlier stage of its life an indoor cat had access to the outdoors, the explanation will
usually be that the cat was infected by bite wound prior to becoming an indoor cat. If there are
other cats in the household, all cats should be tested since the positive-testing cat may have been infected by a housemate or may itself have infected housemates. If these explanations prove inapplicable, the result should not be discounted; the probability then exists that the cat was born
to an FIV+ mother and contracted the disease from her.
A possibly newly infected cat.
Although seroconversion to FIV+ status may occur within as little as two weeks, six to eight
weeks are the generally held standard. One source has reported seroconversion at nearly the one-year mark. Although it is recommended that cats bitten by another cat whose FIV status is either
known or unknown be taken to the vet, treated, and tested at that time, the results of such tests,
whether positive, negative, or equivocal, should always be confirmed by subsequent testing at or
beyond the eight-week mark
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References
August JR. Diagnostic Testing For Feline Retroviruses: Interpreting Unexpected Results. www.stlouisvma.org/downloads/FeLV-FIV%20jra.pdf
Barr MC, Pough MB, Jacobson RH, Scott FW. Comparison and interpretation of diagnostic tests for feline immunodeficiency virus. JAVMA, 1991; 199: 1377-1381.
Barr MC. FIV, FeLV, FIP: Interpretation and misinterpretation of Serological Test results. Seminars in Vet Med & Surg, 1996; 11(3): 144-153
Bienzle D, Reggeti F, Wen X, Little S, Hobson J, and Kruth S. The variability of serological and molecular diagnosis of feline immunodeficiency virus infection. Can Vet J. September 2004;
45(9): 753–757.
Hartmann K, Griessmayr P, Schulz B, Greene CE, Vidyashanka, Jarrett A, Egberink HF. Quality of different in-clinic test systems for feline immunodeficiency virus and feline
leukaemia virus infection. Journal of Feline Medicine and Surgery. 2007; 9(6):439-45.
Kusuhara H, Hohdatsu T, Seta T, Nemoto K, Motokawa K, Gemma T, Watanabe R, Huang C, Arai S, Koyama H. Serological differentiation of FIV-infected cats from dual-subtype feline
immunodeficiency virus vaccine (Fel-O-Vax FIV) inoculated cats. Vet Microbiol, Mar 10 2007 ;
120(3-4): 217-25.
Levy JK, Crawford PC, Kusuhara H, Motokawa K, Gemma T, Watanabe R, Arai S, Bienzle D, Hohdatsu T. Differentiation of Feline Immunodeficiency Virus Vaccination, Infection, or
Vaccination and Infection in Cats. J Vet Intern Med. 2008; 22:330–334.
Levy, JK. Innovation in Detection: Can PCR help with diagnosis in vaccinated cats? Idexx Learning Center Webinar, June 25, 2009.
MacDonald K, Levy JK, Tucker SJ, Crawford PC. Effects of passive transfer of immunity on results of diagnostic tests for antibodies against feline immunodeficiency virus in kittens born to
vaccinated queens. J Am Vet Med Assoc. November 15 2004; 225(10):1554-7.
Mortola E, Oliva G, Risso M, Pecoraro M, Venturini MC. Feline immunodeficiency virus infection: a comparative study of different diagnostic techniques. Arq. Bras. Med. Vet. Zootec.
2004; 56(1): 13-18.
Sand C, Englert T, Egberink H et al: Evaluation of a new in-clinic test system to detect feline immunodeficiency virus and feline leukemia virus infection, Vet Clin Pathol 39:210, 2009.
Sibille P, Avrameas A, Moraillon A, Richardson J, Sonigo P, Pancino G, Strosberg AD. Comparison of serological tests for the diagnosis of feline immunodeficiency virus infection of cats. Vet Microbiol. 1995 Jul;45(2-3):259-67.
Wang C, Johnson CM, Ahluwalia SK, Chowdhury E, Li Y, Gao D, Poudel A, Rahman KS, Kaltenboeck B. Dual-emission FRET real-time PCR differentiates feline immunodeficiency virus and separates infected from vaccinated cats. J Clin Microbiol. 2010 Mar 24 [Epub ahead of print].
Zuckerman EE. The Feline Immunodeficiency Virus (FIV) The FIV Test Patent Has Expired. National Veterinary laboratory Newsletter Spring 2009 Vol. 8, Number 2.
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