Does virus testing really identify viruses

[nhman]

OK, so heres the story:
Just bought a wonderful and expensive orchid (Eria gigantea 'Waterfield') from a grower that I've had excellent luck with previously. ( I have probably over 25 orchids from this grower) I always virus test ANY orchid before letting it loose into my collection, using Agdia testing strips. This one plant tested positive for a virus on three separate tests.
So, I called the company.
I was told- appropriately - to send the plant back for a full refund, but then he stated that there were NO virus tests, no lab tests that "worked". Period!!
I find that this is difficult for my scientifically trained brain to accept. Why wouldn't they "work" at IDing a virus?
It IS rare that I get a positive test, I should note. I also don't smoke and wash my hands with an alcohol based prep between plants (tobacco smoke has been felt to be a possible disease vector to orchids and so has poor plant biosecurity)
Anyone have any thoughts to support that this concept is true (virus testing doesn't work), if not, I will continue to test any and all plants that cross my doorstep for virus.
I'm interested in what the experience and thoughts of this Board may be. Thanks in advance.

[camille1585]

Virus tests do work! I'm doing a master's degree in plant pathology, and have done plenty of virus experiments. I did ELISA assays to detect virus in tobacco plants we inoculated. Sometimes plants with no symptoms showed up as positive for virus in the assay. As part of our practical we then isolated virus from those asymptomatic plants, and looked at the virus particles at 120 000 magnification. So the tests DO work.

I don't doubt the test strips either. It's the same principle as the ELISA assay, except that it takes a few minutes instead of 2 days. Strips just tell you YES/NO for virus I think, and with ELISA you can quantify the exact amount of virus in the plant. Ever professional plant breeders use those strip tests sometimes to double check plants (to avoid having to send just a few samples to the lab)

So if you have 3 separate, positive, virus tests, I believe them.

Which virus by the way?

[nhman]

The strips showed repeatedly positive for CymMV (Cymbidium mosaic virus) with a positive Control line at 30 minutes.
So they SHOULD be appropriate by the data from Agdia.
Thanks for your info!!

[ChrisFL]

Out the vendor! Who was it?

[nhman]

Ha'iku Maui Orchids was the vendor.
As I noted, I have purchased many plants from this vendor that have arrived well packed, healthy and tested virus negative with the same Agdia testing methods.
It WAS an unusual response from them, though, to say the least. At least they DID NOT hesitate in offering to take the plant back for a refund (altho this IS a hassle factor!), but the response will make me hesitate in purchasing from them again.
My basic purpose in posting this was to stimulated to flow of information, thoughts, and opinions as to the reasonableness and validity of viral testing GENERALLY - not in specific diagnosis, treatment,etc (therefore posting under the Pests/Diseases section) nor to rain on a vendor that I have previously had excellent luck with.

[ChrisFL]

The part that concerns me is that they want the plant back.

[camille1585]

I've been reading up on these test strips, and apparently they are 100% reliable, showing no differences with ELISA tests on the same plants. One thing I found out though is that the tests don't work well if you don't use them within a year.

The only way the test could be wrong is if it gives you false negatives, which obviously isn't the case here. It could happen that a plant you test is recently infected and does not have enough virus particles for the detection antibodies in the test strip to pick up on.

If the vendor is willing to take the plant back, all is good, other than the hassle and cost of sending it back!

I wonder what others have to say about this. I do know stuff about viruses detection, but like I said, I'm just a student, so this stuff is still relatively new to me.

[Don Perusse]

Thanks for the info and thanks for this thread. I don't have any info to share that would help but you have helped me. I didn't know about the virus testing strips. Do you get them on line? I was given some divisions in the past I thought had a virus but did keep them isolated until signs were clear one way or another. These strips would certainly help with ID before introduction to your collection. Thanks.

[Andrew]

Virus tests do work most of the time but they're not 100% accurate. As Camille said they are ELISA (Enzyme-linked immunosorbent assay) based tests. The assay in principle is fine but it relies on a few assumptions. Firstly it assumes that the virus is present in the tissue and that the tissue is expressing the viral antigen in detectable quantities. This is probably a major reason for false negatives. ELISAs also work on the assumption that the antibody only binds to the target antigen. This is a factor that is overlooked not only in orchid virus tests but in a lot of immunochemical and immunohistochemical techniques. Anyone who is familiar with Western blotting knows that commercial antibodies often do not bind solely to the intended antigen and sometimes the antigen specificity can even change between batches of the same antibody. I suspect specificty problems are responsible for a lot of the false positives. I would suspect that labs doing virus testing on orchids are unlikely to do a lot of in house qualty control of the integrity of their assays so you are relying on whoever is making the antibody to ensure that it is truly specific. The problem is further complicated by the fact that not all infected plants show symtoms so determining whether you or the assay is wrong is not easy. For any plant of concern you should ideally submit a few samples for analysis and play the averages.

[Blueszz]

Talking about false positives (this topic came up in the chat last week)...
How often do false positives and false negatives occur in these (different) tests when you look for virusis in plants? There must be data available about this.

(A little back ground, why I ask this... ---> OT)

Years ago I had a dog with Leishmaniosis and learned a bit about ELISA. False positives and false negatives where possible. One reason of the fasle possitives was a cross reaction with antibodies from a tick disease, Ehrlichiosis. That is why I got the advise to test the dog several times for both! (Turned out to be positive for both).
Also, when testing humans for Boreliosis (Lyme) they advise to test several times as false negatives are possible.