NO VISIBLE LESIONS ON POST-MORTEM
If an animal produces a positive response to a test for TB (a.k.a. a test ?reactor?) and then shows no visible lesions (NVL) on Post Mortem, your first response may be to think that the test was wrong, assume that it was a false positive result and that your animal was unnecessarily culled.
However, we need to realize that there are other explanations.
If the test is very good at detecting early infection there might not yet have been time for lesions to develop. In that case, it's a brilliant test, because it could then be a tool for removing animals before they are infectious, allowing us to prevent the spread of infection.
If the test detects an immune response to infection, rather than infection itself, this is still very useful: Once exposed to the tb bacterium an animal could pose a serious risk if it remains undetected. Although a proportion of animals which are exposed to infection may become immune, there's no way of knowing which ones remain carriers with latent Tb infection. At least knowing which animals have been exposed, gives us a way to halt infection by not giving them the chance to start spreading it. In a herd where TB infection has been identified, by culture of M. bovis, the presumption has to be that all animals may have been exposed to the bacterium. The control of many infectious animal diseases, including bovine TB, is predicated on the early detection and removal of infected animals before they become infectious to other animals, allowing us to break the cycle of infection. Remember that our herds and neighbouring livestock and wildlife are at risk, and as this is a zoonosis, so are we.
The only way to find out whether the Gamma Interferon or Rapid Stat Pak test throw up real false positives, is to test a large number of animals which are not believed to have been exposed to TB, and see how many react to the test. If the BAS/BCL funded trial of the gamma interferon and Rapid Stat Pak tests didn't go ahead, we will never know how good the tests are; we absolutely can not infer anything about their specificity (i.e. whether they really throw up false positives) from using them only in infected herds.
Culture of NVL's
This is potentially a way of telling whether the animal truly is infected, but, if there are no lesions to see at post mortem examination, we are struggling to know what exactly to culture from.
In NVL cases, VLA can attempt to grow M. bovis from a pool of grossly normal lymph nodes, but this is a slow-growing bacterium that may be found in very low numbers in the initial stages of TB. Therefore, it is not surprising that only about 9% of NVL alpacas (1 in 11) do yield TB bacteria on culture at VLA. This doesn't mean the other 91% didn't have TB - it just means that it wasn't possible to culture it... This is akin to finding a 'needle in a haystack' and means that, when no gross lesions of TB can be found at post-mortem, culture of M. bovis is very inefficient. The problem is with finding the right bit of tissue to culture from, rather than that the bugs weren't there.
The Rapid StatPak test is much more effective in identifying VL alpacas than the skin test, which misses most of them. These are the animals most likely to be infecting other members of the herd and perpetuating the TB breakdown. Even so, it is unlikely that a single round of Rapid StatPak blood testing will detect every infected animal that may be present in an infected herd, hence the need to develop and validate the new camelid gamma interferon test.
One thing we are certain of is that to come out of TB restriction after a culture-confirmed TB outbreak having used only the skin test risks leaving behind undisclosed infected animals in your herd. This creates the possibility of greatly spreading the disease throughout the industry by way of sales, matings, shows etc. The data collated by the Veterinary Laboratories Agency on the Rapid Stat Pak test used in TB-infected herds are proving this already.
Gina Bromage MA, Vet MB, DVM, MRCVS.
31st July 2010
Side Effect Following The Skin Test.
I urge everyone to watch the video (below) that is involved in TB Skin testing on camelids.
If you know of anyone who isn't in the TB Support Group who is going through herd testing or considering having the skin test done privately please forward them this document and video
If you know of any vets who are involved with TB testing in camelids please forward it to them.
I have also sent it to B.A.S. ,B.C.L., B.L.S. and B.V.C.S. and hope they put it on their respective websites.
In March this year I added a question on the monthly data forms asking if any of the group had alpacas or llamas which suffered any side effects following the skin test. I was astonished with the response and at how many had witnessed the same experience that I had. .
I have just received the attached video sent to us by a member of the group that shows the dramatic reaction of one alpaca following her skin test jabs in Dec 09.
Please note when you watch the video the owner means the skin test jab when he states injection.
1. From those in touch with the Camelid TB Support Group we have 22 alpacas which suffered this same dramatic reaction. Remember not all herds under restriction are in touch with us so I strongly believe there may be many more that have experienced this.
2. Prior to the skin test jab ALL 22 had no outward signs or symptoms
3. Within hours of the test 22 suffered this dramatic reaction - not all were necessarily immediately after but all of them were the same day as the test.
4. From this 22 ? 4 died or were euthanized before the skin test was read (within 72 hours)
The remaining 18 including the alpaca in the video appeared to fully recover the next day.
These remaining 18 then went on to either fail a blood test ? or later were euthanized
due to showing clinical signs or died.
5. All 22 had Tb lesions on Post mortem examination only 3 of the 22 failed the skin test.
Therefore in these cases this side effect is more accurate than the skin test result itself. It is therefore extremely important.
6. Before you jump to the wrong conclusion and think the skin test jab has given these animals Tb, this is not the case. ALL these alpacas were found on post mortem to have advanced Tb lesions and for some unknown reason had experienced this dramatic reaction. Many leading vets have already suggested anaphylactic shock.
7. Those owners that carried out the Chembio Rapid Stat Pak test or Gamma test within 10 - 30 days following the skin test - the blood tests picked up ALL the animals that had this dramatic reaction. Owners who didn't have blood test done had to endure either the alpaca dying or developing severe clinical signs which required euthanasia on welfare grounds..
Therefore, please monitor your herd very closely following the skin test procedure. If you hadn't checked them until the next day you could have easily missed the fact that one or more of your herd had suffered this side effect because most of them appear to have fully recovered the next day.
If you experience this in your herd I strongly recommend you remove them from the rest of the group together with a companion to an isolation paddock or into your watch group with no nose to nose contact with your remaining herd or neighbouring livestock and do not return to your herd.
I urge all Vets or AHos who are conducting TB skin testing in camelids to make the owners aware of this side effect and strongly advise the owner to monitor the herd very closely.
I had one alpaca have this side effect on my first skin test in Jan 2009 and again on the next skin test 90 days later ? and both times fully recovered by the next day. He then failed the Chembio Rapid following the second round of skin tests. Both my vet and AHO were unaware of any side effects following the skin test and they both thought it must be stress. At the time having only lost one alpaca before this to TB I didn't make any connection nor was I aware of any other alpaca owner who had experienced this.
If I had received this email back in January 2009 not only would I have understood why my alpaca had this reaction but I would have certainly adopted a totally different approach. I hope this helps you do the same. It's a classic example of 'If I knew then what I know now'
Once again - the gathering of data such as this is vital and it is helping others make important decisions and we have to thank the people in the Tb Support group who religiously send in their monthly data to us - and to the member of the group who sent us this video.
For those reading this and not in touch with the Tb Support Group who have had camelids or their clients camelids with the same experience or perhaps other side effects please contact either myself or Dr Gina Bromage and let us know. All information will be treated in the strictest confidence. We also welcome input and comments from industry professionals.
We are hoping to have this 'Side Effect' data written up as a paper for wider circulation in the veterinary press.
Dianne Summers firstname.lastname@example.org 01209 822422
Gina Bromage MA, Vet MB, DVM, MRCVS email@example.com
Camelid TB Support Group
July 31st 2010