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This article was published in 1993
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Johnes Disease in Sheep: Transmission Between Sheep-Cattle?
SJ Whittaker, District Veterinarian, Albury/Holbrook
It's not my intention to give a complete rundown on Johne's disease in sheep. The subject has been well covered at previous Conferences (most recently by Chris Dent at the '87. Conference), and in various scientific papers by just about every Veterinary Research Officer that has worked at the Orange R.V.L.
Rather I wanted to briefly run through an incident that I investigated in the Albury region in the autumn of 1992. I found this case particularly interesting from a number of points of view:
1. The outbreak would appear to be the first confirmed instance of Johne's disease in sheep in New South Wales outside of the Central Western area. The only other reported instance in sheep that I can find elsewhere in Australia is that it was strongly suspected (if not proven) in one ex 6 sheep that were running at Bairnsdale RVL in 1980.
2. The apparent relatively high prevalence of J.D. within the affected flock/mob concerned. - The prevalence in the affected mob was of the order of 5% and was possibly as high as 10%. Average flock prevalence in Central Western New South Wales infected flocks is quoted as between 1% and 5% but overseas experience has shown this figure can be much higher e.g. as high as 40% has been recorded in some Icelandic flocks.
3. The ramifications of interstate movement restrictions which were placed on cattle ex a property where at the time J.D. had been confirmed in sheep only.
4. The possibility of a link between Johne's disease in sheep and Johne's disease in cattle. J.D. was subsequently confirmed in an in-contact heifer ex the same property.
Case Report
The outbreak occurred on an Albury property of about 1000 hectares which runs approx. 3000 sheep and 200 cattle.
History
Though well respected farmers in the community perhaps some indication of the managerial efficiency of the property involved can be gleaned when I tell you that on my first visit to the property to investigate a problem of ewe mortality (some 2 years prior to the Johne's disease incident) I uncovered total worm levels in excess of 100,000 and a drench programme that was restricted to routine administrations of a mixture of nicotine sulphate and bluestone!
During subsequent visits the owner often mentioned sheep continually fading away with some developing terminal scours and all eventually dying. With my earlier experiences in mind I tended to pay little heed to the matter until I was confronted with a history of 120 ex 1050 three to five year old Merino wethers (on a solid drenching programme this time!!) that had died over a period of 12 months. Many others in the mob looked as though they were also heading the same way.
Clinical Signs
The owner had isolated a mob of approx. 40 affected sheep (ex the 1050) which were in poor condition, had very little body fat and some were scouring quite profusely. Drenching with Ivomec had made no difference to the course of the problem.
Post-mortem
Ostertagia were in evidence but the internal parasitism level generally seemed to be not severe.
The only other significant findings were confined to the terminal small intestine and the colon which appeared thickened and whose associated lymph nodes were enlarged and juicy.
Laboratory Results
- Total worm counts - 1300 Ostertagia; 2000 Nematodirus.
- Faecal egg counts x 10 from the mob averaged 330 eggs per gram.
- Histopathology confirmed severe chronic diffuse granulomatous colitis; and severe chronic lymphangitis and
vasculitis.
Conclusion
The R.V.L. had no doubt the findings were typical of Johne's disease in sheep.
FURTHER INVESTIGATION WORK
During the course of the next few months I had occasion to autopsy a further 6 sheep which were selected at random from the mob of 40 clinicals, from which a wide range of samples were collected for research purposes. The clinical, autopsy and pathology findings were similar in virtually all cases.
Post-mortems
- The terminal ileum was thickened with obvious mucosal ridging.
- The caecum and large bowel were occasionally similarly involved.
- Cording and vascularisation of the serosal vessels were frequently apparent.
- Regional lymph nodes were enlarged and juicy.
Histopathology (in all 6 cases)
- Severe chronic generalised granulomatous enteritis, typhlitis and colitis.
- Severe sub-acute focal necrotising lymphadenitis
Bacteriology
- Masses of mycobacteria on ZN stains were obvious.
- All attempts at culture were unsuccessful.
Serology
- All were positive to the Johne's Disease GDPT.
EPIDEMIOLOGY
1. The flock prevalence of illthrift/death syndrome varied from year to year, but at its worst had approached 5%. The mortality rate due to Johne's Disease in this particular mob of wethers is difficult to determine because some deaths could obviously have been due to other causes such as internal parasitism and nutritional stresses. However if the 120 that had already died plus the 40 'clinicals' which seemed apparent in the mob, all in fact had Johne's disease, the prevalence within the mob of 1,050 would have exceeded 10%!
2. The flock was basically a closed flock - with the last introductions being a mob of 600 one year old ewes in 1984. It was in this mob of ewes that the owner first began to notice the illthrift, scouring, death syndrome - approx. 1-2 years after their purchase. Though these sheep had been purchased locally, traceback information to the original owner failed to throw any further light on the source of the problem. The traceback owner could not recall any similar problems in his own flock and had never purchased stock from the known endemic area of N.S.W. Further traceback work on earlier purchases into the known infected flock also failed to reveal any contact with the Central West of N.S.W.
3. Following the appearance of the syndrome in the index mob of 600 ewes in 1985/86, the problem subsequently appeared in all mobs on the property.
4. Age Affected - Principally older sheep, but on occasions sheep as young as 2-3 years of age would seen to have been affected with Johne's disease.
5. Soil Acidity - The soils on the property are quite acid with a pH. of approx. 4.5.
6. Contact with Other Species - Cattle graze the same paddocks as the sheep. There are no goats on the property.
MOVEMENT RESTRICTIONS
Confirmation of the disease in sheep restricted cattle ex this property moving into Queensland feedlots. The Queensland Schedule 1 clearly states that cattle may not be introduced into Queensland if they are derived from a herd or property in which JD has been known or suspected during the previous 5 year period.
I began checking the literature for evidence of JD transmission between sheep and cattle and vice versa. It seemed to me that while the possibility of interspecies transmission could not be absolutely ruled out (from the field perspective) the prospect was highly unlikely (and a little akin to the role cattle play in the transmission of virulent footrot!). I was convinced that I had a reasonable case to put to the D.P.I. when 'Murphy's law' intervened.
Bovine Case Report
Some months after the initial confirmation of JD in sheep the owner drew my attention to a recumbent 3 year old Jersey X heifer running on the same property that was in very poor condition and had been scouring for some time. The heifer was destroyed and was subsequently shown to be serologically ELISA positive to Johne's disease. Histopathology also confirmed typical findings of Johne's disease. From an epidemiological point of view it may appear that as the heifer had access to paddocks that the affected sheep had grazed, some form of cross-species transmission may have taken place. However further investigations confirmed that the heifer in question had been purchased as a calf ex a known endemic area for JD in Victoria. The heifer had also recently undergone severe nutritional and pregnancy stresses. It is considered most likely that the heifer was infected as a neonate prior to purchase and that the subsequent stresses mentioned above precipitated an early breakdown.
The incident however prompted me to look further at existing evidence of transmission of M. paratuberculosis between the species.
Evidence of Differences Between Strains of M. paratuberculosis
1. Epidemiological - this is basically only circumstantial geographical evidence e.g. in New Zealand De Lisle notes that JD in sheep is common in certain areas of New Zealand while common in cattle in other parts of the country and there seems to be little or no indication of cross over between sheep and cattle in either of these areas. To date the story seems to be similar in the Central West of NSW. However in Iceland there is evidence that a sheep strain has spread to cattle - albeit that the strain was considered to be of relatively low virulence to cattle.
2. Experimental Infections - As far back as 1953 under experimental conditions Taylor had shown that so called sheep adapted strains from Iceland and Scotland were both capable of causing clinical Johne's disease in cattle. Similarly in 1968 Kluge et al. in the American Journal of Veterinary Research report that bovine isolates could be used to experimentally produce JD in sheep. However it needs to be kept in mind that in the majority of such trial work very large doses of mycobacteria were used.
3. Bacteriological Evidence - Cultural procedures that are highly successful in isolating M. paratuberculosis in Cattle have commonly failed with sheep.
4. DNA Technology - In 1991 in N.Z. Collins and De Lisle reported using two different techniques (DNA restriction endonuclease analysis and hybridisation) to examine both sheep and bovine isolates from New Zealand, Australia, Canada and the U.S.A. Both methods clearly demonstrated that there are two major groups of bacteria. All the cattle strains fell into one group while all the sheep strains fell into another with the exception of some Canadian sheep isolates which seemed to belong to the cattle group. Therefore it would appear that the distinct division of isolates into sheep and cattle groups may be more indicative of a host preference for M. paratuberculosis rather than an absolute host specificity. Nevertheless this host preference is an important barrier against spread of the disease between cattle and sheep in mixed animal farming operations.
I would like to conclude with a direct quote from Seaman and Cooke's paper in 1991:
'Of great concern is the parallel between JD in New South Wales and that reported in New Zealand. In New Zealand JD in sheep was first recognised in Canterbury in 1952. 9 years after the first diagnosis there were 30 infected flocks, all confined to the Canterbury area. 25 years later (in 1986) 759 infected flocks had been identified right throughout the country. In New Zealand Johne's Disease has become the single most common laboratory diagnosis of illthrift and/or diarrhoea in sheep 2 years and over, and comprising 23% of diagnoses, well ahead of internal parasitism at 10%).'
References:
1. Collins et al. 1991 Surveillance 17 (4) p.24
2. Seaman and Cook 1991 Aust. Sheep Veterinary Society Proceedings P.135-139
3. De Lisle 1986 Proceedings Sheep & Beef Cattle Society (NZVA) 16 : P.186-190
4. Taylor 1953 Journal of Comp. Pathology 63 p.368-73
5. Kluge et al. 1968 American Journal Veterinary Research 29 p.953-62
6. Dent 1987. Proceedings Veterinary Inspectors Conference p.138-143