snap tests

[Groominator]

the parvo thread reminded me - I overheard my vet discussing the snap test with another vet. I discussed it briefly with my vet too, later. And I'm wondering what you guys know on the subject. I didn't take immunology yet so i'm not quite certain that I'm understanding correctly.

Basically the sitution was this - a homeless puppy that belonged to the clinic (up for adoption at the time) began to show symptoms of what looked like parvo, namely nonstop bloody eruptions from both ends.
vet 1 says: 'its parvo. i work in a shelter and i see parvo all the time, definitely parvo'
vet 2: 'let me get a snap test'
vet 1: 'don't waste your money, it'll show as positive because the puppy has been vaccinated for parvo before'
vet 2: 'I want to run the test anyway'
vet 1: 'don't bother. if this was a client's dog you should do it for liability purposes, but since this is "our" dog we might as well skip it since we know it'll show up positive and treat symptomatically to stabilize the dog.'
(the story sadly ends with a deceased puppy, he did not respond to treatment and faded away. I have never spent so much time disinfecting stuff in my life. They never ran the test.)

So what I'm wondering is whether it is true that an animal that is vaccinated against a particular virus will show up as positive for that virus on a snap test. Because it looks for antibodies, yes? Because if that's the case i can understand what being detrimental in some cases - lets say you find a cat that was vaccinated for feline leukemia. And you do a snap test and it shows up as positive and the cat is euthanized. My vet was telling me that this was not the case but wasn't too clear on explaining why (english isn't his first language). I'm sure I'm oversimplifying it in my head.

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[RockStarKDub]

It depends, sometimes snap tests are testing for antibodies in which case sometimes vaccination will give a false positive. Sometimes, though, they are testing for the antigen directly.

 

[david594]

So what I'm wondering is whether it is true that an animal that is vaccinated against a particular virus will show up as positive for that virus on a snap test. Because it looks for antibodies, yes? Because if that's the case i can understand what being detrimental in some cases - lets say you find a cat that was vaccinated for feline leukemia. And you do a snap test and it shows up as positive and the cat is euthanized. My vet was telling me that this was not the case but wasn't too clear on explaining why (english isn't his first language). I'm sure I'm oversimplifying it in my head.

Its actually FIV(antibody test) that will give you a false positive due to vaccination instead of FeLV(antigen test) on a SNAP test.

I've read similar things indicating that you could get false positives on a parvo snap test shortly after using certain types of parvo vaccines. Modified live where the ones that seem to come to mind for causing this.

There is one reference here to false positives due to vaccines here: http://www.sheltermedicine.com/portal/is_panleukopenia.shtml#vaccination

 

[HopefulAg]

As stated, if it's an antibody test (testing what the white blood cells produce) then it could show up as a positive if vaccinated. If it's an antigen test (testing for what the pathogen produces), then it should only be positive if the pathogen is present. But sometimes it requires a certain threshold depending on how sensitive it is.

 

[No Imagination]

I love wikipedia and the Immune system.

After Hopeful said "WBC" producing Ab, I said to myself, he means B-cells (no offense, just thinking to myself), then I think, any other cells capable of making Ab besides B's? Thats were I came across this jewel:

Mammalian B-cells are made in the Bone Marrow. Rabbits are an exception; their B cells develop in the appendix-sacculus rotundus.

I love exceptions to the rules.

Anyone know why rabbits differ from other mammals in the production of B-cells? Related to avian Bursa by any chance? Wiki doesn't say, and thats all the energy i have to put into it at the moment.

 

[HopefulAg]

Aren't B-cells considered WBCs? I thought B and T cells both fell under that category.

 

[No Imagination]

Aren't B-cells considered WBCs? I thought B and T cells both fell under that category.

Oh yeah, I wasn't criticizing you amd your 100% right. WBC constitute a ton of immune cells, B's, T's, macros, almost all of them. B's are the only ones that do Ab's. I knew what you meant - I was just rambling as to why I went to wiki to look something up and thought the rabbit thing was really cool.

 

[sumstorm]

If you read the insert on the snap tests, it actually cites several studies that performed snap tests on pups/dogs after X days /weeks of vaccination, and found that fost positives did not occur. This would actually be a pretty easy study to conduct, since your only looking for false positives. Could probably, if you had the funds for the snap tests, arrange it with HS's that do adoptions where they provide a few sets of vaccines first (or the entire series in puppies.)

I did read the studies when I found them on the insert, but I don't have any notes on what they were or where to find them again.

 

[InfiniVet]

I love wikipedia and the Immune system....Anyone know why rabbits differ from other mammals in the production of B-cells? Related to avian Bursa by any chance? Wiki doesn't say, and thats all the energy i have to put into it at the moment.

Don't get me wrong, my diploma will read "Courtesy of Wikipedia" on the bottom right hand corner - but, its not the end all say all. =)
From what I gather, the lymphatic tissue thats in the gut of all mammals is very important for immunity (actually this brings us full circle to Parvo coz thats where Parvo likes to hunt & kill), but there's been some studies on rabbits to further understand the immune system. I think that's really the limit of it, and it may/may not end up in textbooks.

From my understanding of immunology rabbits do primarily produce the bulk of their B cells from the marrow.

The avian bursa thing is that's how B cells got their name, because thats where they first found them. T cells were first found in the Thymus.

 

[JustCats]

Since this is something I'm passionate about, I'd like to chime in. In spite of the fact that the FELV SNAP tests do test for antigen and not antibody, any positive test should be confirmed by an IFA at least a month later. FELV SNAP tests actually have a positive predictive value of 73.5 to 90%--and the IDEXX SNAP is at 73.5%--for a number of reasons, one of which is that the test detects primary viremia and some of these cats will not become persistently infected. I have personally seen this happen several times--a positive SNAP followed by a negative SNAP and/or IFA a month later. It upsets me a great deal that vets tell people a cat has FELV and may recommend euthanizing without even mentioning a confirmatory test. (Statistics are from this Journal of Feline Medicine and Surgery article).

Here's a link that expalins it well in layman's terms.
This is similar from Cornell.

IDEXX says their parvo test is 100% accurate. Incidentally, you can use the canine parvo to test for feline panleukopenia (feline parvo) as well.

Its actually FIV(antibody test) that will give you a false positive due to vaccination instead of FeLV(antigen test) on a SNAP test.

 

[No Imagination]

Infiniti,

I know and understand what your saying about Wiki. What I should say I love about it so much (besides getting a general overview) is the interconnectivity of it.

I will go there, and start reading, see something interesting click a link, and continue reading. After an hour, I could have started with Immunology and ended up reading about physics or something else.

Like you said, everything (not just wiki) needs to be taken with a grain of salt.

I am def. interested about the rabbit situation - going to dive into it on Monday I think. thanks for the heads up about the bone marrow bit.

 

[david594]

Since this is something I'm passionate about, I'd like to chime in. In spite of the fact that the FELV SNAP tests do test for antigen and not antibody, any positive test should be confirmed by an IFA at least a month later. FELV SNAP tests actually have a positive predictive value of 73.5 to 90%--and the IDEXX SNAP is at 73.5%--for a number of reasons, one of which is that the test detects primary viremia and some of these cats will not become persistently infected. I have personally seen this happen several times--a positive SNAP followed by a negative SNAP and/or IFA a month later. It upsets me a great deal that vets tell people a cat has FELV and may recommend euthanizing without even mentioning a confirmatory test.

That does not make the initial FeLV positive a false positive though. It is a actual positive, just no guarntee it will lead to persistant infection.

I had read one thing in I think a phifzer vaccine booklet recently that the incidence of FeLV exposure resulting in persistant infection reduces as a cat gets older.(their point being all kittens should be FeLV vaccinated)

 

[CatVet2Be]

IDEXX says their parvo test is 100% accurate. Incidentally, you can use the canine parvo to test for feline panleukopenia (feline parvo) as well.

That's actually very cool, I had no idea.

What about doing FELV tests too early on kittens? If I remember right, this can lead to false positives because they may still have maternal antibodies but no infection. We usually tell people to wait until kittens are ~9 weeks or older in order to do the FELV test because of this.

 

[david594]

That's actually very cool, I had no idea.

What about doing FELV tests too early on kittens? If I remember right, this can lead to false positives because they may still have maternal antibodies but no infection. We usually tell people to wait until kittens are ~9 weeks or older in order to do the FELV test because of this.

FeLV is the antigen test. FIV is the antibody test. So its possible to see a false positive on the FIV test due to maternal antibodies at a very young age. On very young kittens its also possible that you will see a false negative on the FeLV test.

So general thoughts when testing a very young kitten with a combo test:

FIV (-) => pretty reliable
FIV (+) => iffy, could be maternal antibodies causing false positive, retest when older
FeLV (-) => iffy, could be false negative, retest when older
FeLV (+) => 🙁 pretty reliable, sorry kitten

Or atleast thats the way I have learned it. Where I work we recommend testing after 16 weeks to avoid the false positives and negatives.

 

[JustCats]

Agreed except for the FELV-. I can't imagine a circumstance where an FELV SNAP shouldn't provide a false negative in young kittens since it does detect primary viremia. Can you please clarify?

Also, one anecdote: I did once have a litter of 4 kittens that tested FELV+ at about 8 weeks of age. On retest at 16 weeks, two remained positive and two were negative. So even some kittens can overcome primary viremia. It's really best to wait until 16 weeks of age to test or retest. (The two postive kittens lived to three years of age before they died.)

FeLV is the antigen test. FIV is the antibody test. So its possible to see a false positive on the FIV test due to maternal antibodies at a very young age. On very young kittens its also possible that you will see a false negative on the FeLV test.

So general thoughts when testing a very young kitten with a combo test:

FIV (-) => pretty reliable
FIV (+) => iffy, could be maternal antibodies causing false positive, retest when older
FeLV (-) => iffy, could be false negative, retest when older
FeLV (+) => 🙁 pretty reliable, sorry kitten

Or atleast thats the way I have learned it. Where I work we recommend testing after 16 weeks to avoid the false positives and negatives.

 

[JustCats]

Okay. Your contention is that it's not a false positive because the test was not inaccurate--it did detect virus.

How about this:
A positive FELV SNAP test does not mean that a cat will actually develop FELV so a positive FELV SNAP should be confirmed with an IFA.

That does not make the initial FeLV positive a false positive though. It is a actual positive, just no guarntee it will lead to persistant infection.

I had read one thing in I think a phifzer vaccine booklet recently that the incidence of FeLV exposure resulting in persistant infection reduces as a cat gets older.(their point being all kittens should be FeLV vaccinated)

 

[david594]

Okay. Your contention is that it's not a false positive because the test was not inaccurate--it did detect virus.

How about this:
A positive FELV SNAP test does not mean that a cat will actually develop FELV so a positive FELV SNAP should be confirmed with an IFA.

Yup, thats exactly what I meant. Its semantics I know. I also work for a vet who tells clients when they are lyme positive on a snap test that it could be a "false positive" and that is why we really need to do a C6 instead of actually explaining it all to them.

Regarding FeLV false negatives:

How accurate is the test?
The blood test itself is quite accurate, but not perfect. Because cats can be transiently infected, it is possible that a cat will initially test positive for FeLV, and then recover and test negative at a later date. In most healthy cat populations FeLV is quite uncommon, and this leads to an increase in the relative number of false positive results. At minimum, all positive ELISA tests should be repeated to ensure that correct technique was used; if whole blood was used for the initial test, it should be repeated on serum. (See below for additional information on testing.)

The blood test may also falsely identify recently infected cats as negative. To be absolutely certain, cats must be tested 1-3 months after their last known exposure. False negatives are more common when samples other than blood (e.g. saliva, tears) are used and when multiple samples are pooled.