Scabby Mouth (Contagious Pustular Dermatitis, Orf) is a highly contagious parapox viral disease of sheep that causes crusted pustules around the mouth, eyes, legs, and occasionally poll and udders, of sheep and goats. It is often an innocuous condition cycling endemically at low levels in sheep flocks. Occasionally it presents as a spectacular and extensive outbreak.
In March 2013 Scabby Mouth spread through 1500 crossbred lambs shortly after they were introduced into an on-farm feedlot. Around 50% of lambs in all five pens were affected with lesions at the height of the outbreak. There were a low number of mortalities. The outbreak resolved over four weeks.
A producer experienced in lot feeding lambs reported extensive Scabby Mouth in this year's intake of 1500 lambs.
The producer lot feeds lambs each year and has done so for 10 years. He breeds lambs to finish them on a feedlot ration and also opportunistically buys in lambs. He has noticed the occasional Scabby Mouth lesion in the flock but has not had a problem in the feedlot before.
At weaning the tail of the mob were put straight into the feedlot on pellets and straw. Approximately 3 weeks later their cohorts and some bought in lambs were put onto the feedlot on pellets and straw, having spent a few weeks at pasture first. They were around 35kg live weight.
The producer noticed a few lamb deaths with rapid autolysis in the first week after all lambs were in the feedlot. He suspected pulpy kidney/enterotoxaemia and he yarded and treated them all with a Clostridial vaccination.
The deaths with rapid autolysis stopped, but he lost a few lambs, mostly lighter ones, over the next seven to ten days.
Ten to fourteen days after the Clostridial vaccination he noticed that the lambs were not doing well. He yarded them intending to give them another drench. (The lambs had been drenched previously with a long acting moxidectin drench in early January.)
At this time he noticed scabs and swelling around the mouth and eyes in about one in five lambs in a mob of 300. After drenching that mob with moxidectin he stopped further handling of the lambs, leaving the remaining pens undrenched, and contacted the Hume LHPA.
On the 20th March 2013 the property was visited and the lambs inspected in the feedlot. There was a range of body size in the lambs and body condition also varied from good to poor. One lamb had died overnight but the carcass was unavailable for examination due to predation.
Many lambs were bright and feeding well, but some had marked visible crusting proliferative lesions around the muzzles and eyes.
Many affected lambs were depressed and reluctant to move. One was observed to place its swollen and crusted muzzle under running water at the water trough. Several approached water but hesitated to drink. Some rubbed their muzzles with lesions against the concrete water troughs.
A few lambs had pasty scours. Two lambs of the 1500 were heard coughing during the 30 minute inspection.
One weaker lamb, in a pen that had not been drenched, was euthanased and autopsied. There was no evidence of nematode parasites in the ingesta or in (or on) the abomasal lining, the intestine and caecum. There was no reddening of the intestinal lining. The manure was soft, a pasty beige colour with no particular odour.
The lungs were normal with no evidence of parasitism or pneumonia.
On the basis of this examination it was decided not to drench the remaining lambs at this time. Supportive care of the worst lambs was initiated but drafting and mustering the lambs was to be avoided.
The property was revisited a week later (two weeks after the outbreak started) and the lambs were generally much improved in all but one pen. The crusted lesions were no longer as swollen around the face and lambs were generally more active.
The worst affected pen had been the one handled and drenched a week previously. This pen had more light weight lambs, a few still looking depressed, and some coughing lambs.
One lamb in good condition was found dead in this pen and was autopsied.
There was generalised inflammation of the subcutaneous tissue, gastrointestinal tract and kidney. There were lesions consistent with pneumonia in the right lung (fibrinous adhesions between lung and pleural surface, consolidated dark red and purple localised areas and purulent material in the right pleural space (pyothorax)). There were Scabby Mouth lesions around the mouth. The cause of death was determined to be pneumonia and pyothorax.
The producer had noticed the occasional small crusting pustule on his sheep and recognised the Scabby Mouth lesions, but had not had an outbreak in his lot feeding operation before.
The parapox virus can survive in scabs in the right environmental conditions for at least 12 months, if not years.1
Scabby Mouth is recognised to cause debilitation but is not usually associated with causing deaths.1 This is consistent with the findings in this case - high morbidity but few mortalities. The limited autopsies that were performed indicate that death was associated with other conditions such as pneumonia or failure to adapt to grain feeding. The viral infection may have had an additive effect on the lambs' viability and predisposed the animals to succumb to these conditions.
In a previous local outbreak in 400 Merino lambs in November 2010 mortalities also occurred. Groups of five to six lambs were found dead under trees. Deaths had occurred over several days. Again there were only limited autopsy opportunities. The lamb autopsied in that case had pleuropneumonia as well as scabby mouth lesions.
It is possible that the introduction of purchased lambs brought onto the farm may have had a role in the development of the current outbreak. Introducing lambs with an unknown status to a flock thought to have endemic Scabby Mouth may have given an opportunity for the virus to multiply in naive sheep.
There was no history of recent introductions in the flock that was affected in 2010. The cyclic appearance of infection in flocks with endemic disease is demonstrated in this case.
The straw of the feed lot ration could have also contributed to the development of the disease by creating skin abrasions to allow virus entry and establishment of lesions. The water troughs may have been another focal point for spread as lambs were observed rubbing their mouths on the concrete.
In the 2010 outbreak young sheep were grazing paddocks with rough vegetation such as rushes and thistles that could scratch skin, and in wet conditions that promoted skin maceration and fragility.
Yarding the lambs for vaccination seems to have been a key event prior to the rapid spread of the virus, and presumably close contact in the race with infected lambs and contaminated facilities accelerated the opportunity for spread. The mortalities in the 2010 outbreak also followed a yarding event.
This outbreak did run its course in 3-4 weeks with most recovered lambs clinically well by the end of week three.
After four weeks, the lambs in the feedlot were again growing well, but the producer had lost 20, culled 57 out to pasture, and had delayed finishing the lambs. The producer estimated that the outbreak had cost him $10 per head due to the culled and dead lambs lost from production, and due to the extra feed now needed to finish the remaining lambs.
Now that the yards have been extensively contaminated the owner will commence an annual vaccination for Scabby Mouth in lambs well prior to feedlot entry to prevent a recurrence.
www.dpi.nsw.gov.au Retreived 26th June 2013