Farm Skills: Ovine Johne's disease

Question 1

What clinical sign of Johnes is seen in cattle but NOT in sheep? Why is this?

Answer 1

• Cattle with Johnes show profuse watery diarrhoea but sheep do not
• This is probably because sheep can resorb a high quantity of water in the distal colon
• If diarrhoea is seen in sheep with Johnes, it is intermittent and mainly associated with concurrent diseases

Question 2

Clinical signs of Johnes in sheep

Answer 2

• Severe weight loss
• Submandibular oedema due to hypoproteinaemia
• Wool break
• Normal or increased appetite
• Reduction in general immunity (which might predispose to bacterial infection, high parasite burden etc.)

Question 3

How does susceptibility to MAP change over a sheep’s lifetime?

Answer 3

Susceptibility decreases with age - sheep are most susceptible in the first few months of life

Question 4

When do sheep show clinical signs of Johnes?

Answer 4

Clinical signs are seen in sheep >2 y.o.
* Usually appear following stressful events e.g. lambing, lactation, nutritional deficiency or transport

Question 5

What is the main route of infection for Ovine Johnes?

Answer 5

Main route of infection: faeco-oral

Other ways sheep can become infected:
* Mammary secretions
* Some evidence for intrauterine and venereal transmission but this is less important

Question 6

Ovine Johne’s presents as ill-thrift in adult animals. What other differentials do you have for this?

Answer 6

• Nutritional deficiency
• Parasitic gastroenteritis (PGE)
• Chronic liver fluke
• Periodontitis
• Ovine Pulmonary Adenocarcinoma (OPA)
• Maedi Visna
• Caseous Lymphadenitis (CLA)
• Intestinal adenocarcinoma
• Other chronic infection e.g. pneumonia, mastitis

Question 7

Which strains of Johne’s affect sheep and cattle?

Answer 7

• Sheep are primarily infected by S strains (Type I and Type III)
• Cattle are usually infected by C strains (Type II) but may be affected by sheep strains
• Cross-infection occurs experimentally but its importance in practice is under debate

Question 8

Which strains of Johne’s affect goats?

Answer 8

Goats are easily infected with both C and S strains

Question 9

What are the possible outcomes when an animal becomes infected with MAP? What influences this?

Answer 9

Depends on pathogenicity of MAP strain, dose and frequency of exposure, and host immune response and susceptibility.

Options:
* Animal may develop resistance to infection after MAP exposure
* Animal may become infected, and shed bacteria asymptomatically
* Animal may become infected and develop clinical disease

Question 10

True/false: controlling Fasciola hepatica may have implications for Johne’s control.

Answer 10

True
* Research ongoing, but fascioliasis / PGE may dyregulate protective immunity against mycobacterial infection
* It has been suggested that co-infection of MAP with Fasciola hepatica could accelerate development of Johne’s

Question 11

True/false: it is safe for humans to eat and drink milk from animals infected with Johne’s.

Answer 11

False
* Meat and milk from animals infected with Johne’s is usually contaminated. Methods such as pasteurisation can reduce bacterial load but not eliminate it entirely.
* There is not yet a causative link between Crohn’s and infected animal products but it is advisable to reduce human exposure to MAP

Question 12

True/false: low albumin is seen with Johne’s and diagnosis can be made on the basis of this.

Answer 12

False
* Low albumin is seen consistently with Johne’s even before clinical signs
* However this finding is not specific to OJD alone

Question 13

What are the advantages and disadvantages of serum ELISA for OJD testing?

Answer 13

Serum ELISA
✅ Low cost
✅ Highly specific (94-99%) - we can trust that a positive is a truly infected animal
❌ Low sensitivity (14-21%) - we cannot trust a negative result as being rruly negative

ELISA can be performed on blood (most commonly) or colostrum.

Question 14

What is considered the gold standard test for OJD? What are the advantages and disadvantages?

Answer 14

Gold standard = Faecal culture
✅ Can identify infected animals and those shedding the highest doses of MAP
❌ Very expensive
❌ Long turnaround time; sheep strains even harder and slower to grow than cattle
❌ Possible false positives due to passive shedding of MAP (esp on highly contaminated farm, or where super shedders are present)

Question 15

What are the advantages and disadvantages of faecal PCR for detection of OJD?

Answer 15

✅ Higher sensitivity (30-60%) than ELISA in subclinical cases
✅ More rapid and sensitive than faecal culture
❌ High cost
❌ Potential false +ves due to environmental mycobacteria

Question 16

How could you reduce cost of OJD testing?

Answer 16

Pool samples. Recommendations depend on prevalence in flock and individual shedding but roughly:
* At least 6-7 faecal pools per flock (where prevalence is >5%)
* At least 50 animals per pool

Question 17

True/false: all sheep infected with OJD will show gross lesions on PM exam.

Answer 17

False
* Some infected animals may have no gross lesions whereas others may be immediately apparent.
* In sheep (compared to cattle) it is common for subclinical cases to not show gross or histological pathological lesions which makes diagnosis even harder

Question 18

What PM findings might you see in a sheep with OJD?

Answer 18

• Thickening, corrugation and reddening of small intestinal mucosa
• Enlarged mesenteric LNs and draining lymphatic ducts
  These are usually more severe in terminal ileum and ileo-caecal valve.
• Orange-yellow pigmentation of intestinal mucosa - seen in minority of cases
• Serous atrophy of fat
• Submandibular oedema
• Ascites
• Hydropericardium

Question 19

Answer 19

Question 20

Answer 20

Section of jejunum showing diffuse and severe thickening of intestinal mucosa

Question 21

Answer 21

Thickened and easily visible lymphatic vessels on the intestinal serosa

Question 22

Answer 22

Severe serous atrophy (gelatinous appearance) of mesenteric fat

Question 23

You have performed a PM exam and suspect OJD. What samples do you collect to diagnose this?

Answer 23

• Blood sample for serology
• Tissue from terminal ileum and a mesenteric lymph node for histology
• Faecal sample for culture and PCR

Question 24

A farmer wants to totally eradicate Johne’s from his flock. What measures could you advise?

Answer 24

The following has been done in Australia:
* Destock and rest pasture for 18 months (2 summers)
* Restock from disease-free source

There is a high economic cost associated with this and can be difficult because we don’t know true prevalence. Control measures may be more appropriate than eradication.

Question 25

Describe control measures for OJD

Answer 25

• Identification of affected animals, isolate them and do not breed from them e.g. BCS score and test ewes before they are put to ram, or use routine pooled faecal samples to assess prevalence
• Reduce faeco-oral transmission by general cleaning of farm, design of feed and water troughs, avoid feeding pooled colostrum, do not spread manure on fields grazed by youngstock, limit other diseases e.g. PGE
• Biosecurity: quarantine and testing of new animals to prevent entry of OJD ± entry of more pathogenic OJD strains than already present
• Vaccination: there is an inactivated vaccine licensed in small ruminants which prevents clinical signs and reduces mortality, but does not prevent infection and may not reduce bacterial shedding. This vaccine can interfere with TB testing.

Question 26

How does Johne’s vaccine impact TB testing?

Answer 26

• Chance of false positive reaction at the SICCT test up to 12 months post-vaccine
• Other tests e.g. interferon gamma test may not have this issye