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email received Feb 8 2006

Johnes disease and TB reactors

I 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 has
a) shed  loads of bacteria thus infecting other animals, especially youngstock
b) 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

7th/8th February 2006 ~ "Could naturally occurring Mycobacterium paratuberculosis affect the reliability of the TB test?"