DL: Johne's disease eradication plan on a dairy unit

Question 1

List different testing methods for Johne’s

Answer 1

• bulk milk tank
• blood Ab
• milk Ab
• faecal culture
• faecal PCR

Question 2

Subclinical CS - Johne’s

Answer 2

milk yield, fertility decreases, poor weight gain, increased SCC, more lameness

Question 3

Clinical disease - Johne’s

Answer 3

not typically seen before age of 2 yo as cell-mediated immunity works until the age of stress with starting milking herd/ calving etc .(2 yo)  failure in cell-mediated immunity  development of subclinical signs. Usually age 3-5 for clinical disease. Oedema (d/t loss of protein – leaky gut) – submandibular, sometimes round brisket too, late stage disease typically.

Question 4

Mode of transmission - Johne’s

Answer 4

colostrum, milk, faeces (control = think hygiene). Within 1st 4 weeks of life with contact with one of those three methods. Adults very rarely infected.

Question 5

Advantages/ disadvantages of bulk milk tank Ab

Answer 5

ADVANTAGES: only know Johne’s is in herd, don’t know how many or which cows, consider dilution factor (e.g. 300 samples to 1 positive), sensitivity low (30%) so a negative result doesn’t really tell you anything
DISADVANTAGES: screening – easy to do – quick – cost £4 (any Ab test if an ELISA)), some subclinical will show some Ab levels (variant levels)

Question 6

Outline individual milk Ab

Answer 6

DAIRY COWS
Advantage – profile for individual animal
Disadvantage - wouldn’t detect subclinical disease in non-milking cows

Question 7

Outline individual Abs blood

Answer 7

BEEF COWS
Advantage – profile for individual animal, some people argue this is slightly more sensitive than the milk sample.
Disadvantage – at 2 yo, quite possible for a false negative d/t low Ab levels.

Question 8

Is faecal culture or PCR the best method to detect Johne’s?

Answer 8

culture the absolute best of these, £30 each depending on lab. For faecal culture, the animal needs to be shedding in the faeces to get a positive result. Takes 3 months to get a result from culture. PCR is very quick (next day). Either could be used strategically if you have a positive Ab result (blood or milk) but farmer unwilling to cull – will tell you if cow is shedding.

Question 9

Specificity of Johne’s tests

Answer 9

Specificity for all tests 99% (i.e. you can trust a positive)

Question 10

Sensitivity for Johne’s tests

Answer 10

Sensitivity – variable for all of the tests – faecal culture best (40% sensitivity)

Question 11

What method is commonly employed to detect Johne’s is present on a farm?

Answer 11

Tend to do a 30 cow screen (irrespective of herd size) to confirm Johne’s is present on the farm. (individual milk/blood Ab).

Question 12

T/F: you can never say with 100% certainty a cow is Johne’s free

Answer 12

True d/t low sensitivity. You can say the more negatives an animal has and the older an animal is (w/o overt CS) the more certain you can be of the negative results being true. But there are always exceptions.

Question 13

Methods of Johne’s control

Answer 13

• ID infected cows – ensure faeces/ colostrum/ milk doesn’t come into contact with the calves (0-4 weeks) this is snatch calving
• Cull positive
• Calve in isolation.
• HERDWISE – by NMR group. Program to determine level of disease on a farm. Traffic light system/ RED: cull prior to next calving. No colostrum/milk to be used for calves. YELLOW - good hygiene at calving. Cull only if a few high risk cows. No colostrum/milk for calves.

Question 14

How does a TB test interfere with MAP test?

Answer 14

Leave 6 weeks between the two (ideally 3 months - more evidence for this)

Question 15

What is snatch calving?

Answer 15

ensure calf doesn’t come into contact with colostrum/faeces etc.