Alpaca and TB testing

RE: Alpaca and TB testing

Anthony Todd — Sat, 25 Sep 2021 16:28:30 GMT

[quote userid="3094" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233199#233199"]Eho would have thunk it??[/quote]

Nah, I knew all along!

RE: Alpaca and TB testing

Julian Earl — Sat, 25 Sep 2021 15:50:44 GMT

wELL THEN. Alpacasmay be the danger to use all by carrying TB silently but on the other hand,or hoof they could also be a solution to our much bigger problem right now?? '''

http://digitaleditions.telegraph.co.uk/data/734/reader/reader.html?social#!preferred/0/package/734/pub/734/page/24/article/222332

Eho would have thunk it??

RE: Alpaca and TB testing

Anthony Todd — Sat, 25 Sep 2021 15:29:25 GMT

Have the females with him been tested yet? What happened to them?

RE: Alpaca and TB testing

Bob Russell — Thu, 23 Sep 2021 12:44:33 GMT

[quote userid="5904" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233131#233131"]So does anyone, at all, believe the demise [so not to dramatise the facts at all] really helps and promotes the eradication of TB in all it's ramifications of species, ages, histories, antigenic contact, etc. etc.[/quote]

Yes. Absolutely.

RE: Alpaca and TB testing

Arlo Guthrie — Thu, 23 Sep 2021 10:45:44 GMT

I'm not sure how this is relevant to Nick Blayney's quote!

[quote userid="5904" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233131#233131"]The Authorities missed a good, and very rare opportunity to show TB in all the ramifications to the great uneducated and mentally unwashed public and missed a great PR opportunity, IMHO.[/quote]

Hmm ... PR being my specialist subject ...

Was an opportunity missed? I'm not sure. As I understand it from the DEFRA website, it will take some months before it can be said conclusively that the animal had TB. So it was always going to be killed on the basis of something less than 100% certainty. And it was a cutesy alpaca. And it was called Geronimo. So I am not convinced there was ever an opportunity. It was always about mitigating a PR disaster.

If it were me advising the government, though, I probably would have done something more dramatic than the patient Defra blog. I might have wheeled out a symptomless but TB positive cow. I would have said: "Have any one of you complaining about this test campaigned for any one of the thousands of cattle that have been 'dragged screaming' to their deaths, and rightly so because of the devastating consequences of this disease. No? Of course you haven't. They're just cows. And they're not called Geronimo. They're just called A.N.Other-Cow.

Or something like that.

[quote userid="4046" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233150#233150"]This woman has got a lot to answer for.[/quote]

Indeed. Makes my teeth grind uncontrollably whenever I see another TV appearance.

RE: Alpaca and TB testing

Nick Blayney — Thu, 23 Sep 2021 10:41:09 GMT

The recent covid coverage in our scientifically illiterate press (and nation) reveals why there was little chance of a sensible discussion about this serious issue. I used to say, until it swept across my practice in the early 2000's that the control of tb was the greatest acheivement of the veterinary profession. Not so easy to say it now! This woman has got a lot to answer for. Miind you, camelid owners were more like pet owners in their outlook than farmers. I'll say not more for fear of breaching the high standards of politeness of this esteemed list!

RE: Alpaca and TB testing

Iain Richards — Thu, 23 Sep 2021 10:34:47 GMT

[quote userid="5904" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233131#233131"]

So does anyone, at all, believe the demise [so not to dramatise the facts at all] really helps and promotes the eradication of TB in all it's ramifications of species, ages, histories, antigenic contact, etc. etc.

The Authorities missed a good, and very rare opportunity to show TB in all the ramifications to the great uneducated and mentally unwashed public and missed a great PR opportunity, IMHO.

The take home message will only be of a poor creature being dragged to slaughter on a very long rope 'cos some sort of test for some rare disease which doesn't occur here anyway was positive like a breath test in many minds....

No logic, no patient explanation, no Xrays of children's TB lungs, no stats, just an image of a,,,,,,,,,,defenceless , Etc.!!

[/quote]

Yes.

After the first test, the animal had a high probabilty of having bTB
After the second test, the animal had a high probabilty of having bTB

After the PM, the animal had a high probability of having bTB

It was logical. The DEFRA blog (and comments on here) are the patient explanations.

RE: Alpaca and TB testing

Anthony Todd — Wed, 22 Sep 2021 15:21:41 GMT

[quote userid="4046" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233068#233068"]reassuring to hear that this concept escapes others as it does me: sPecificity = chance of false Positive, seNsetivity = chance of false Negative. The SICCT is maybe 75 - 90 percent sensitive but 98% specific: so: a negative might be a missed animal but a positive is probably an infected animal. Am I right? I recall a SPVS sailtraining course where we did some of this stuff (an extremely tax efficient way of enjoyable cpd). I have Alan Leyland to thank for my understanding (or not) of this concept.[/quote]

The Authorities missed a good, and very rare opportunity to show TB in all the ramifications to the great uneducated and mentally unwashed public and missed a great PR opportunity, IMHO.

No logic, no patient explanation, no Xrays of children's TB lungs, no stats, just an image of a,,,,,,,,,,defenceless , Etc.!!

RE: Alpaca and TB testing

Nick Blayney — Sat, 18 Sep 2021 19:25:24 GMT

reassuring to hear that this concept escapes others as it does me: sPecificity = chance of false Positive, seNsetivity = chance of false Negative. The SICCT is maybe 75 - 90 percent sensitive but 98% specific: so: a negative might be a missed animal but a positive is probably an infected animal. Am I right? I recall a SPVS sailtraining course where we did some of this stuff (an extremely tax efficient way of enjoyable cpd). I have Alan Leyland to thank for my understanding (or not) of this concept.

RE: Alpaca and TB testing

Richard Carter — Sat, 18 Sep 2021 14:27:36 GMT

I remember as: specificity has P so test of how many positive results believed ie true positive: sensitivity with N so how many negatives are really negative

RE: Alpaca and TB testing

Iain Richards — Fri, 17 Sep 2021 20:06:14 GMT

[quote userid="13609" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233058#233058"]

Oops. :-)

I've tried to cram it in my head so often, you'd really think I'd get it right without looking it up again every time. Yet, I also should know I'd better look it up.

[/quote]

It's an absolute bugger! And not helped by worms that sound very similar

RE: Alpaca and TB testing

Andreas Ege — Fri, 17 Sep 2021 15:29:47 GMT

[quote userid="4367" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/232988#232988"]Yes to most of that, except 98% Specificity is 2% false positive, 40% Sensitivity is 60% false negative. (I'll stand corrected because Sp/Sn always my my head spin[/quote]

Oops. :-)

I've tried to cram it in my head so often, you'd really think I'd get it right without looking it up again every time. Yet, I also should know I'd better look it up.

Thanks to you and Beats to correct.

RE: Alpaca and TB testing

Julian Earl — Mon, 13 Sep 2021 19:57:27 GMT

eVEN MRI would not be all that helpfil for a likely microscopic lesion. Not sure thst would be worth the time, effort and expense? No, some gross lesion(s) followed by microscopy/culture is the only sure way of absolutely proving a positive I think! HTH?

RE: Alpaca and TB testing

Julian Earl — Sat, 11 Sep 2021 18:53:42 GMT

[quote userid="4046" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233010#233010"]Ah,the spitting. However much you try to avoid it they get you in the end.[/quote]

Only quoted to post his in response: Thank you for making me laugh Nick! I hope that you and yours are well? fellow Liverpool graduate! 1980...IIRC? Yes??

RE: Alpaca and TB testing

Nick Blayney — Sat, 11 Sep 2021 18:10:32 GMT

Ah,the spitting. However much you try to avoid it they get you in the end. I speak from bitter experience!

RE: Alpaca and TB testing

Mike Dale — Sat, 11 Sep 2021 17:48:37 GMT

Anthony Todd said:

Arlo! "disagree" to posts should not be allowed without reasons [IMHO]

Tony, will you please stop complaining about disagree. It cannot be changed. It is what it is. You are the only person who complains. And you complain regularly. Please stop.

Thanks Arlo

This thread now resembles some of those appearing on my local football team supporters chat page!

I think my original question has been very adequately debated now and I thank everyone (and I mean everyone) who has contributed. I have learned a great deal. It will be interesting to see the final lab results and reports from the PM investigation if they are indeed made fully public. I am sure it is in everyone's interest for that to happen and equally sure that it will promote continued disagreement.

Bottom line - a question from my astute wife to those who think the alpaca should have been spared. "Given the doubt surrounding the animal's TB status, would you be happy for it to spit in your grandchild's face?"

That's my last word on the subject.

RE: Alpaca and TB testing

Julian Earl — Sat, 11 Sep 2021 14:49:01 GMT

Sorry Anthony,

I have done it again butyou asked for that one did you not? If I agree with Arlo below do I have Do I have justify that as well? In your opinion that is?? TIA!

RE: Alpaca and TB testing

Arlo Guthrie — Sat, 11 Sep 2021 12:49:12 GMT

[quote userid="5904" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/233002#233002"]Arlo! "disagree" to posts should not be allowed without reasons [IMHO] [/quote]

Tony, will you please stop complaining about disagree. It cannot be changed. It is what it is. You are the only person who complains. And you complain regularly. Please stop.

RE: Alpaca and TB testing

Anthony Todd — Sat, 11 Sep 2021 12:02:48 GMT

[quote userid="3094" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/232999#232999"]Sorry Anthony, you must have expected such aresponse as mine to your disagreepost above? Ithink that a disagree-post, even the anonymous ones can be usedul. [/quote]

Not if the denigrated post was "multipoint". which, I think, mine was, and particularly when I gave, I think, quite a few reasonable support arguments in the post.

Arlo! "disagree" to posts should not be allowed without reasons [IMHO] With the exception, obviously, of posts with only one definite point or view.

The greatest value of vetsurgeon.org should be the dissemination of useful correct, and/or corrected, information..

RE: Alpaca and TB testing

Julian Earl — Sat, 11 Sep 2021 09:19:37 GMT

Sorry Anthony, you must have expected such aresponse as mine to your disagreepost above? Ithink that a disagree-post, even the anonymous ones can be usedul. They mightmakesomeone think again/reconsider their opininion etceven if it has limited detauls so far? It might sop someone, eg me, not you of copurse, from becoming smug ove a position held in the debate?

I have the convemient excuse, asit were that I do openly adit my brain does not work thesame way anymore, ever since I fractured my skull with the multiple and life-threatening haemorrhages sustained on the 8th August 2012. [ ]Okay I am now no longer a youngster, apparently? but following that trauma, I aged 30 years overnight or at least within a few weeks! Just one of the reasons why I had to take early retitement despite my evident youth!!! Apart from not being allowed to drive because of the risks of seizures and/or just blacking out, Idid keep some sense of responsibility and knew that working, on farms in particular would not have been safe for me, and neither SA- work, nor LA-work would have been safe for my patients either I suspect?

So bullet bitten and retirement it was. I stay ion this excellent site to try to pass on experiences, keep in touch with colleagues, and yep, even count it as CPD, whether necessary or not?

Thak you for reading,my thread hi-jack is over for now!! Probably!!

RE: Alpaca and TB testing

Iain Richards — Fri, 10 Sep 2021 20:47:05 GMT

[quote userid="12930" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/232990#232990"]a) is not a reason to cull an animal against an owners wishes. If it was, there would be a mass cull occurring on rabbits and guinea pigs in the UK presently[/quote]

Fair comment. Better not to own them in the first place....

Your Se/Sp explanation is excellent and I will steal the "cow goes moo"

RE: Alpaca and TB testing

Beats — Fri, 10 Sep 2021 19:41:51 GMT

[quote userid="4367" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/232988#232988"]a) welfare of a herd animal b) leaving an animal to potentially develop life threatening lesions isn't humane c) it isn't logical to allow it to potentially infect other animals/humans. [/quote]

I would drop (a) and (b).

(a) is not a reason to cull an animal against an owners wishes. If it was, there would be a mass cull occurring on rabbits and guinea pigs in the UK presently. I think the better welfare by being dead argument only works here where the definition of welfare has been made very narrow and artificial. Cat's like to roam, but a cull of indoor cats on that basis would not be rational.

(b) when read alone, I think you'll agree, is not a reason to cull an animal "because it might/will get sick later".

Tagging on these additional points weakens and distracts from the actual reasoned point.

RE: Alpaca and TB testing

Beats — Fri, 10 Sep 2021 19:30:34 GMT

Perhaps a tangent on diagnostic tests and misleading stats would be better than continuing here. For clarity, I am talking in the general and not related to any individual animal or test type etc. I am referring to TB testing with intradermal testing in cattle at points.

I think a mistype above has caused you some confusion, Anthony..

Sensitivity = the % of actual-disease-positives that test-positive.

Specificity = the % of actual-disease-negatives that test-negative.

98% specificity of a diagnostic test means 2% false POSITIVES.

The number of false NEGATIVES is related to the SENSITIVITY.

A test with a low sensitivity (say TB intradermal skin test in cattle), will miss quite a few individual cases. The herd sensitivity may be higher than the individual sensitivity.

A test with a high sensitivity is useful for ruling a disease out (there will be few false negatives, so if negative can largely be believed to be so.

A test with a high specificity is useful for ruling a disease in (there will be few false positives, so a positive can largely be believed to be so.

The intradermal skin test in cattle has (to my recollection) an extremely high claimed specificity way above 98% (I have heard claims of 99.99%, albeit I personally think that is a bit of a ludicrous claim in field conditions). Its sensitivity, on the other hand, is maybe 70-80% (someone can correct this figure, as I'm not sure) at best on the individual test basis?

The sensitivity and specificity of a test are both properties of the test, and based on a cut-off point between what is negative and what is positive. Moving that cut-off point will change the sensitivity and specificity (when one goes up the other will go down and vice versa).

It is quite possible to have a TB test in cattle with a near 100% sensitivity. [Does it moo? would be such a test]. There will, however be a very high false positive rate and very low specificity. Equally, the sensitivity of the intradermal skin test in cattle can be made higher (potentially) by reducing the cut-off point, but at the expense of the specificity.

The sensitivity and specificity are properties of the test, based on decisions on the cut-off point. They do not tell us the likelihood that an individual testing positive or negative has or has not the disease in question.

For that we need to consider the prevalence of disease in the population being tested (how likely we thought it was that the animal being tested might have had the disease in the first place). The positive predictive value and negative predictive value will vary depending on the prevalence of disease in the population being tested. Consider a population of experimental cows which have been deliberately infected with TB. They are all known to have the disease. All positives will be true positives (100% of positives are accurate); all negatives are false negatives (0% of negatives are accurate). The same test in a population that is known to be TB-free will yield the opposite expectations of positive and negative results; while most results will be negative, those that are positive will be false positives (a 100% rate of false positives) in this population.

If we wished, we could even use the magnitude of quantitative data in diagnostic tests (where the magnitude of the quantitative results is correlated with the likelihood of disease). Sometimes we see this in tests as a move from a dichotomous yes vs no result to three choices of yes, indeterminate or no. We see this more so with the application of likelihood ratios, considering the pre-test probability of disease first, then the post-test probability of disease having taken into account the magnitude of the test result.

What we should NOT suggest is that a x% specificity means that there is a x% chance that an individual animal testing positive is a true positive. That is simply bad maths. An x% specificity simply means that in a population with no disease (100-x)% will test positive anyway. We need to consider the prevalence of disease, or the pre-test likelihood that the animal had the disease to make an estimation of the % chance that an individual animal testing positive is a true positive. It is fine to state that a test has a % specificity, but that may not be the answer to the question that is being asked, which is more likely to be along the lines of "What is the probability that an animal testing positive is indeed diseased". As a rule, bad maths and misleading statistics do not inspire confidence in those one is trying to educate - it instead makes them feel that there is an agenda rather than accurate statements of fact. I am not criticising anyone here, but instead the poor use of misleading statistics generally seen from government and others looking to be trusted.

RE: Alpaca and TB testing

Iain Richards — Fri, 10 Sep 2021 19:01:29 GMT

Because the skin sensitivity is rubbish, which is why the enferplex (and other tests) were developed. Or, he's picked up TB since arriving in a TB hotspot (gloucester)

[quote userid="13609" url="~/001/veterinary-clinical/farm-animal/f/discussions/29950/alpaca-and-tb-testing/232974#232974"]

The simplest answer to that is, that a 98% specifity still means 2% false negatives, i.e. might have been one of 2 out of a 100 genuine positives showing up as negative.

If I remember right, very early and very late stages have a much lesser specifity.

Having said that, I think the test is only validated for cattle, so specifity and sensitivity in other species might be differen.

Finally, it could be technical issues, e.g. poor testing quality.

[/quote]

Yes to most of that, except 98% Specificity is 2% false positive, 40% Sensitivity is 60% false negative. (I'll stand corrected because Sp/Sn always my my head spin (Spin and Snout I think, Sensitive test when Negative rules OUT the disease’, SPIN for, ‘Specific test when Positive rules IN the disease)

Repeated skin test in cattle can lead to anergy - a reduced response, so inconclusive in cattles should be just culled as many go on to be reactors. The data on anergy in camelids is less certain, but probably the same.

Probably. That's the key Anthony. Probably. We cannot treat TB that well in people, we struggle in monogasttrics, it is very inneffective in fore-gut fermenters and the course is so long, it's questionable re antibiotic reistance. So treatment is not an option. Permanent isoaltion is also not an option a) welfare of a herd animal b) leaving an animal to potentially develop life threatening lesions isn't humane c) it isn't logical to allow it to potentially infect other animals/humans.

If you've read the papers I advised, you know that all mycobactial infections are charactereised by latency and recudesece when the immune system is compromised - HIV or malnutrition (UK Victorian slums).

Given that, on the basis of the blood tests, Geronimo was, on the balance of probabilities, infected with M.bovis, the only reasonable course of action is euthanasia. It might have been a wrong decision, but it was the correct decision for all the reasons above. Further testing is irrelevant, BECAUSE he's had two positive tests.

Get me a perfect test and you may have a point. But there isn't, and, becuase of M bovis evolution, I doubt there will be.

RE: Alpaca and TB testing

Anthony Todd — Fri, 10 Sep 2021 17:49:20 GMT

i never think an anonymous "disagree" helps anyone. Far better to demolish comments or arguments so everyone learns, I think?

Indicates a lack of contra-argument or either a personal or intellectual problem perhaps?