CASE NOTES
Failure of Innoculation (Scabigard, Zoetis) to Prevent Scabby Mouth (Orf) in Australian White Lambs
Bruce Watt, Central Tablelands Local Land Services, Bathurst and Kelly Graham, Zoetis
Posted Flock and Herd June 2024
Introduction
Scabby mouth (orf, contagious ecthyma or contagious pustular dermatitis), is a disease caused by a parapoxvirus and affects sheep, goats and occasionally people. Infection occurs through damaged skin, most commonly to the lips or feet. The virus is highly resistant and can survive in the environment for more than one year. Scabby mouth occurs most frequently in young lambs but is occasionally seen in older sheep. Lambs grazing thistle-infested pastures or coarse stubbles are particularly susceptible to infection, as are lambs that are being supplementary fed in shared feeders. There is no specific treatment for scabby mouth so when farms are known to carry the disease it is best managed by vaccination. A live vaccine is administered by skin scratch on a protected site, usually in the axilla inside the foreleg, producing a mild and controlled form of the disease (Scabigard®, Zoetis).
History
In June/July 2023, a Central Tablelands livestock producer marked 600 May/June-drop Australian White (AW) and Dorper lambs. At marking the lambs were inoculated against scabby mouth (using Scabigard, Zoetis). In mid-November the owner noticed that lambs in one mob of 110 mixed-sex AW lambs had symptoms of scabby mouth. These lambs were running in a paddock infested with saffron thistles (Carthamus lanatus) that had been sprayed with a broadleaf herbicide (Lontrel/MCPA). Lambs in the other mobs, running in paddocks less infested with saffron thistles, were not affected.
The owner reported that in the mob of 110 affected lambs, 40% were mildly affected while 10% were severely affected.
Clinical findings
A sample of affected lambs was examined on 27 November 2023. Of the eight well-grown 35-45 kg lambs in the yard (Image 1), five were severely affected with swollen, crusty, bleeding lesions on the upper and lower lips and skin around the nares. No lesions were noted elsewhere on the body.
Image 1. The sample of eight lambs including five severely affected lambs
Image 2. Closeup of a severely affected lamb. This lamb was sampled for virus detection.
Laboratory findings
On 27 November 2023, samples of scab material from the lips of one severely affected lamb (Image 2) were submitted to EMAI Menangle with the aim of confirming orf infection. The samples were reported as positive for orf using negative staining electron microscopy.
FOLLOW UP
The owner was highly experienced both as a sheep owner and lamb marking contractor. He was aware of good vaccination technique and adhered to vaccine storage requirements. He had not seen this problem before.
Discussion
The cause of this vaccine failure, (assuming that the vaccine was stored and used correctly), was likely due to a combination of heavy virus contamination in the paddock and a heavy thistle infestation causing damage to the skin and mucous membranes. In situations such as this one, scabby mouth can still occur in correctly vaccinated flocks, but the resultant lesions should be less severe, impact fewer sheep and resolve more readily than those affecting unvaccinated flocks.
Given the viral contamination on this property, continued vaccination is recommended to minimise any economic impact of this disease in the future.
Zoetis Guide to the Implementing an Effective Scabby Mouth Vaccination Program
Scabigard is a modified live vaccine and therefore needs to be handled and stored correctly (according to the label) to ensure the vaccine remains efficacious. Any vaccine not used on the day of vaccination should be discarded and dips, disinfectants or other chemicals should not be applied to the scratched area (or on the applicator).
While it may not always be practical, a sample of vaccinated sheep or lambs should be examined 10-14 days after vaccination. There should be a continuous line of scabs visible along the site of the scratch made on the skin. This lesion is indicative of successful vaccination and referred to as a 'take'. Failure to show evidence of a 'take' may indicate problems with vaccination technique or handling of the vaccine, resulting in loss of potency. Re-vaccination of animals should be considered where a 'take' is not evident.
The following checklist provides a guide to an effective Scabigard vaccination program
- ALWAYS READ THE LABEL BEFORE USE, and ensure you handle and store the vaccine appropriately.
- On farms known to have scabby mouth, vaccinate all lambs every year at the first opportunity, generally at lamb marking/tailing/docking. Lambs can be vaccinated within the first week of life.
- Vaccinate sheep on the bare skin preferably in the armpit area inside the front leg or on the side of the brisket. The scratch should form a line (approximately 5cm). Press firmly enough to scratch the skin to ensure vaccine 'take' but not firmly enough to draw blood (bleeding may 'wash' the vaccine away from the site).
- Check for 'takes' from a sample of the flock, 10-14 days after vaccination, to ensure immunity has developed.
- If not previously vaccinated, whole flock vaccination (not just lambs) should be considered, as sheep vaccinated with a live scabby mouth vaccine may act as a source of infection for an unvaccinated mob.
- Vaccination should be completed at least 6-8 weeks before the commencement of shearing to give the scabs enough time to form and fall off, thereby not placing the shearers at potential risk, as the virus can cause a similar skin disease in humans (as it does in sheep).
- Vaccination of ewes should be completed at least 6-8 weeks prior to the commencement of lambing to ensure young lambs are not placed at risk of disease.
- The vaccine does not provide life-long protection. It will provide 12 months of immunity and then it is the contaminated on-farm environment that continues to boost flock immunity. Therefore, if sheep are moved to 'clean' paddocks 12 months or more post vaccination, they may be susceptible to reinfection.
- When introducing sheep onto a property that is known to have scabby mouth, it is recommended that they are all vaccinated prior to being put to pasture.