email received Feb 8 2006
Johnes disease and TB reactorsI have done some investigation into Johne's as several dairy farms in our region have been affected.I even had blood samples done on our cattle because I wanted to make sure there was nothing bad sneaking around. We came up with some positive results (blood), culture of faeces always negative and never had a clinical case.Retesting of blood samples (3 different labs) came back with 3 different results ( negative, positive, inconclusive) and as the cattle are still alive and healthy I am, as usual, more than suspicious when it comes to labs.Johne's is a very tricky thing; a positive (Elisa) blood test doesn't say anything except that the animal had contact with Mycobacterium parat. ( or, any other Mycobacterium as has been proven recently) , the "gold standard" for confirmation is culture of the bacterium in the faeces of cattle affected.This culture takes up to 6 months so by the time there is confirmation, a clinically affected animal hasa) shed loads of bacteria thus infecting other animals, especially youngstockb) is dead because of progressing "starvation" ( they are scouring themselves to death)I was wondering about all these positive reactors for TB in the context of a relatively high prevalence of antibodies for some sort of Mycobacterium and the number of cattle negative for TB after post mortem.My experience with laboratory diagnosis and the vaccine trial gave me some more ideas about the reliability of tests.( High doses of Dexamethason prior to a skin test very often give a negative result )
This email folowed the exchange below
8th February 2006 ~ Answer received about Johnes disease and TB reactors
Warmwell is grateful for this comment in response to the question posed below
"A friend recently had 10 IR's (Inconclusive Reactors) to Tb test and we both asked the same question, "Could mycobacterium paratuberculosis have skewed the test?" as they had had a bull positive for Johnes in recent weeks. (This was a restocked herd after FMD, hence the bought in cattle)( For details of the project aiming to investigate the biology of M. bovis in soil and host (badgers, cattle) excreta, and to evaluate the attributable risk of bTB breakdown from different environmental sources on UK farms, see this Warwick University webpage).
The answer, both from their SVS area and ours, was that an exposure to Johnes as recent as theirs was, was known to throw up inconclusive reactors - but rarely Reactors. (If reactors then possibly clinical Johnes in the making? - we don't know.) All the IR cattle subsequently went clear on retest. The bull positive for Johnes was slaughtered.
In a Defra funded study, Prof. Liz Wellington at Warwick has refined PCR to pick up Johnes in cattle dung. I wonder if they'll ever use it? In 40 years of cattle farming, I have never seen a clinical case of Johnes. I understand it's pretty unforgettable. (Cattle back ends just melt away, emaciate with constant scouring. Transmission is mainly to a calf at birth through direct contact I understand.There is no treatment and no cure)"
7th/8th February 2006 ~ "Could naturally occurring Mycobacterium paratuberculosis affect the reliability of the TB test?"
A farmer writes,
" I was thinking about TB and what came to my mind last night was : a couple of years ago there were trials with a vaccine against Johne's disease in cattle. The owners of vaccinated cattle had to sign an agreement that the cattle couldn't be exported because no intracutanous TB test could be carried out on vaccinated cattle. This test would show up positive.Does any veterinary expert have an answer to this interesting question?"
My question is : would a natural occurring infection with Mycobacterium paratuberculosis affect the reliability of the test as well ? I know from the ongoing survey that there are all kinds of Mycobacterium around, and in the case of testing cattle for Johne's disease (blood test) a lot of false positive due to cross-reaction occur..."