Scabby mouth or Orf is a common disease of Australian sheep. Weaned lambs are most often affected. In my experience rarely causes deaths but can be sometimes cause a severe setback in lambs especially those already hampered by under nutrition. It can also hinder the sale or shearing of sheep. On properties where the disease is a problem, lambs can be vaccinated at lamb marking. On properties that vaccinate lambs, subsequent disease is rare.
When a mixed age mob of 1030 Merino ewes was mustered for drafting prior to shearing on 13 June 2008, it was noticed that a proportion of the mob had lesions resembling scabby mouth. This property has routinely vaccinated lambs at marking for at least 30 years. The owner has however noticed the occasional lamb affected with scabby mouth over this time. The mob had been running on a perennial grass based pasture that also had some Scotch thistles present. The ewes had been foraging under the thistles.
Several ewes were examined and found to have lesions typical of scabby mouth. Some were affected on the lips and chin while many were only affected on the commissures of the mouth. The owner subsequently reported that 375 of 682 (55%) two-year-old ewes had lesions and 24 of 308 (8%) three-year-old ewes were affected. None of the 69 older ewes had lesions.
“Surely if none of the older ewes were affected this suggests a vaccination problem? Either the owner has used out of date product, has had a new operator not using it properly, or has stored it incorrectly and it has lost its potency. Those options would certainly be worth investigating.”
Lee Cook (Veterinary Officer, Biological and Chemical Risk Management, NSW DPI, Orange).
“I don't recall seeing it in vaccinated sheep; I would have thought the lamb vaccination would result in plenty of virus to stimulate a challenge in the ewes. Many croppers have not vaccinated for many years as they have effectively eliminated thistles, they sometimes come unstuck with:
1. self feeders
2. stalky rolls of hay
3. in spectacular fashion with late spring rain on lucerne pastures when there is a strong growth of sub clover, the lambs feet are in a hot humid environment and the interdigital skin is scarified by the mature lucerne plants as they walk through it. The feet lesions are spectacular and the prevalence high and merino weaners in these conditions fall in a heap. After this they usually resume vaccination as even if this only happens 1/10 years it is so devastating vaccination is cheap compared with the havoc of scabby mouth in the interdigital space and coronets.”
A G (Tony) Morton (DV Wagga RLPB)
“I see severe outbreaks in vaccinated ewes from time-to-time (say once every 2-3 years) and sporadic cases in vaccinated flocks quite regularly. I attribute this to two main factors:-
1 poor vaccination technique/inappropriate handling of live vaccine (very few check for vaccine takes).
2 lack of natural exposure to stimulate ongoing immunity (my understanding was that effective immunity after scratching lasts about 2-3 years).
I regard the disease as endemic/latent in most flocks here and expression is closely related to thistle control. I have been able to wean many flocks off long-term vaccination programs where thistle control is good.
John Evers (DV Young RLPB)
As Lee, Tony and John have indicated there are three possibilities for this outbreak of scabby mouth. They are failure of vaccination technique, failure of the product or lapsed immunity. It is sometimes recommended that producers should check the success of vaccination by inspecting vaccinated lambs one or two weeks post vaccination. However this is often impractical as many Australian Merino producers (including the producer in this case) also mules lambs at marking. It is inadvisable to handle mulesed lambs for about a month to allow healing. Incorrect vaccine storage has led to outbreaks of scabby mouth. However, the new product Scabigard needs only to be refrigerated not frozen so has less stringent storage requirements. Immunity to scabby mouth is not lifelong but should be effective for two years (Radostits, et al, 2007).
This producer is a meticulous manager and has a long-term very experienced farm employee. While operator error and vaccine failure remain possible (and have not been tested) in my opinion the most likely explanation for this outbreak was a lapse in immunity under the challenge of grazing in a paddock with some thistles. This conclusion is open to challenge as none of the older ewes was affected. This may simply reflect their small number and previous history.