CASE NOTES


Two outbreaks of scabby mouth (orf infection) in small ruminants on the Central Tablelands of NSW

James Tamone (Final Year veterinary student, Charles Sturt University, Wagga) and Bruce Watt (Central Tablelands Local Land Services, Bathurst)

Posted Flock & Herd August 2017

INTRODUCTION

Scabby mouth (Orf, soremouth, contagious pustular dermatitis or contagious ecthyma) is a viral disease of sheep and goats caused by a Parapox virus endemic to Australia. It is responsible for a range of pox like diseases that produce proliferative crusting lesions around mucocutaneous junctions. It is highly contagious as well as having zoonotic potential and commonly causes lesions around the skin of the mouth, lips, nostrils and oral mucosa and uncommonly around the udder, vulva or feet. (Smith, 2014)

This is a report on two separate outbreaks of scabby mouth during the late spring and summer of 2016/17 in the Central Tablelands of NSW; one case in young sheep and one in adult does. Both properties had never seen an outbreak previously and therefore had not vaccinated their livestock. 

CASE 1

Property details

The property runs approximately 2000 fine wool breeding merinos in a rotational grazing system with primarily improved pastures of a subterranean clover (Trifolium subterraneum), lucerne (Medicago sativa), phalaris (Phalaris aquatica) and ryegrass (Lolium perenne) base. 

History

On Friday 11 November 2016 the producer noticed increased lameness in the mob of 1400 spring 2015 drop hoggets. Initially 4 sheep were noted to be non-weight bearing but upon closer inspection 15 to 20 lambs were affected; some were seen to have  lesions around the coronary band and extending further up the limbs while others showed only  oedema of the distal limbs. The worse affected lambs were found under trees and were not able to walk, in these cases multiple limbs were affected and secondary flystrike and infection was noted.

The mob was mustered on Saturday (12th of November) and checked thoroughly. Approximately 90% of the mob was noted to be affected and showing some signs of lameness. The worst 20-30% were drafted out as they were moved to the yards; these lambs drifted to the back of the mob and by cutting out these sheep they could be inspected more closely. The lesions varied from minor scabbing to extensive scabs of the lower limbs and many of these worse affected sheep suffered from secondary flystrike. All yarded lambs were treated with spinosad fly repellent (Extinosad Lice, Fly and Maggot Eliminator, spinosad 25 g/L, Elanco Animal Health) and cyromazine insect growth regulator (Vetrazin Liquid Sheep Blowfly Treatment, cyromazine 500g/L, Elanco Animal Health). The worst affected also received an additional antiseptic spray (Chlorimide Antiseptic Pump Spray, Cetrimide 8.0 g/L, N-Octyl bicycloheptene dicarboximide 5.0 g/L, Di-N-propylisocinchomeronate 2.5 g/L, Chloroxylenol 2.4 g/L, Orthophenylphenol 0.8 g/L, Pyrethrins 0.3 g/L, Troy ilium) before being returned to their original paddock. The remainder were turned out into a new paddock and were to be treated within the following days. No deaths had been noticed at this stage.

CLINICAL EXAMINATION 

The producer submitted photographs of affected lambs to the district veterinarian for diagnosis and discussion.  Lambs from one affected mob were examined in the paddock on the 23 November 2016. On the 23 December 2016, ten recovering cases were examined and samples collected to confirm the diagnosis. 

The producer described and documented (Figures 1, 2 and 3) red, proliferative, crusted lesions especially on the palmer/plantar surface of the hoof just proximal to the coronary band. In some cases, the entire distal limb was involved and the lesions were noted to extend proximally (Figure 1). Multiple limbs were affected in several of the worst cases and secondary blow fly strike was noted (Figure 2). Flystrike also noted along the ventral surface of the body in some cases where the affected feet had been in direct contact with the torso whilst the lambs lay in sternal recumbency. Lesions on the feet ranged from minor scabs around the coronary band and lower limb to severe swelling of the foot and proximally up the limb to the cannon bone, club foot appearance and severe ulceration. Many off the lambs also showed signs of minor crusting lesions around the muzzle and mouth (Figure 3). 

Figure 1. Resolving orf lesions as seen in many of the weaned lambs: displaying ulceration and hyperkeratosis in the distal limbs.
Figure 2. A more severely affected lamb with secondary fly strike. 
Figure 3. Orf lesions of the muzzle (photographs courtesy of the owner)

By the following day the treated lambs showed significant improvements due to reducing irritation by flies; less were noted to be stamping their feet or showing severe lameness. The rest of the affected mob was run through a foot bath in an attempt to treat those less affected. The foot bath contained the same mixture of spinosad and cyromazine used to treat the worst affected sheep previously. The foot bath solution was maintained at a minimum depth of 3 inches by regularly topping up the solution. 

Two weeks later there was significant improvement in the majority of the treated lambs however there was still a significant number of affected sheep in the mob and several lambs were euthanased on humane grounds as they were unable to walk.

The remainder of the affected lambs were treated with 2 doses of long acting oxytetracycline IM and silver sulfadiazine ointment was placed over the lesions around their mouths. Despite this treatment, flies remained an additional source of irritation in these lambs. The remainder of the treated mob responded well to treatment and only a few continued to show signs of lameness. Drier weather experienced late in 2016 and early in 2017 appeared to assist this recovery. In this mob a total of 13 sheep had died or were euthanased as a result of this outbreak.

Laboratory findings 

Laboratory results from samples collected on 23 December 2016 returned a negative culture of Dermatophilus while virology results were positive for the Orf Parapox virus; supporting the diagnosis of contagious pustular dermatitis or scabby mouth.

Revisit

In early January 2017 the grazier reported that the majority of the lambs had now fully recovered although he considered that the affected lambs had suffered a check in growth.  The group of 800 sale lambs had shown some signs of the disease however the producer identified this earlier and intervened, by foot bathing these lambs. No losses were noted. More severe cases in this mob also received a short acting oxytetracycline. This disease outbreak delayed the lambs finishing and the producer estimated that the lambs were not ready for sale until a month later than usual.

CASE 2.

Property details

The property runs 60 Boer does and 30 Dorper ewes on primarily improved pastures. In the previous spring the does yielded a 130% kidding rate.

History

In early January the producer first noticed a sudden outbreak of blisters and cracks around the lips of several of the Boer goat does. The worse affected of these were noted to have significant swelling around their lips, cheeks and heads. In total, approximately 50% of goats within the mob were noted to show some signs of lesions, however, none of the Dorper ewes displayed any symptoms even though they were within the same paddock. Additionally, at the time of the outbreak, kids reared at-foot did not display any signs of disease. 

Clinical examination

On initial exam 50% of the mob was noted to show signs of significant ulceration of the mucosal surfaces of their lips and the surrounding skin. There were also large proliferative, hyperkeratotic crusts in these areas and several small crusts were also present upon the ears of some of the does (Figure 4). There were no signs of lesions on the goats’ feet or udders and the goat kids appeared free from disease.

Figure 4: Severely affected doe showing proliferative crusting lesions (photograph courtesy of the livestock owner).

The producer moved the does, kids and ewes from their current paddock which had a high contamination of saffron thistle (Carthamus lanatus) to a cleaner paddock. She also used an iodine based spray to treat the affected does and to prevent secondary bacterial infection however noted minimal improvement. The kids were weaned a month earlier than usual in an attempt to prevent the spread of the disease to the kids.

Revisit

At the time of the revisit, on the 16th of January 2017, the does still showed signs of proliferative crusting lesions around their mouths, however the oedema had subsided considerably and the producer noted that the crusts had begun to heal (Figure 5). Samples of the proliferative crusts were collected at this visit for Dermatophilus culture and for Parapox virus electron microscopy identification. The producer also noticed a small number of lesions in the Dorper ewes however these appeared far less severe than those in the does.

Figure 5: Resolving lesions, 16th January, 2017 (photograph courtesy of the livestock owner).

Laboratory findings

Laboratory results returned a negative culture of Dermatophilus and virology results were positive for the Orf Parapox virus confirming the diagnosis of contagious pustular dermatitis or Scabby mouth.

DISCUSSION 

These two separate outbreaks highlighted the impact that scabby mouth can have on production. Although presenting quite differently in different species and classes of animals the cases shared features that were related to the disease introduction, current management procedures and environmental conditions. 

Firstly, the introduction of the disease to the naïve populations was suspected to be via purchase of breeding rams/bucks. In Case 1 a ram, which was suspected to be infected, was purchased 12 months prior but was not introduced directly to the mob affected. Due to the nature of the rotational grazing system the paddocks the ram had been in became contaminated prior to the lambs rotating onto that paddock. The Orf virus is capable of surviving months to years within the environment depending on conditions so contamination of paddock can cause disease long after infected sheep have been moved (Plant and Casburn, 2016). In Case 2 a ram and buck, neither of which showed signs of disease, were introduced directly to the mob recently so it was possible that they be the potential source. Although these were the only introduced animals for several years the virus’s prolonged survival means that another source of entry is possible. It is also possible that the virus was present on both properties but only caused an outbreak when suitable environmental conditions (in the case of the lambs an unusually wet spring leading to maceration of the skin above the feet and in the cases of the does, grazing of saffron thistles) occurred.

Due to a prolonged absence of the disease on both of these properties neither producer was employing vaccination. This, in addition to the absence of natural exposure, resulted in naïve populations in both flocks. Introduction of the virus with favourable conditions resulted in the high prevalence and severity of disease seen in these cases.  The only vaccine available in Australia is Scabigard (Zoetis Australia, 2016)  a live virus grown in tissue culture and administered via scratching of bare skin to inoculate (Nettleton et al. 1996). It has been suggested that the administration of live unattenuated virus to infected sheep may cause an additional challenge to the immune system, potentially delaying recovery. Since the outbreaks both producers plan to vaccinate all young stock at marking. It should be noted that due to the zoonotic nature of the virus and the use of a live unattenuated vaccine care should be taken when vaccinating stock.

While the stock were naïve, the season experienced in late spring and early summer was highly favourable for an outbreak to occur and explains the varying distribution of lesions noted in each case. Spread of the virus is through direct contact with infected animals or virus-contaminated fomites, and infection will establish only at sites where skin is traumatized (Aitken, 2008). At the time of the first outbreak (case 1) heavy and prolonged rainfall had resulted in softening and maceration of the lambs’ feet. This was coupled with increase in temperature resulting in rapid growth of feed and maturation of the pasture. Presumably wet, stalky  pastures damaged the softened feet of the lambs enabling introduction of the virus from the contaminated pasture. 

Case 2 occurred later during summer when the pastures had dried and the does were forced to forage more through the heavy saffron thistle. . Although running in the same paddock the ewes were much less affected than the goats presumably due to their different foraging habits.

There is currently no specific  treatment for the Orf virus and typically within 4 to 6 weeks infected animals recover (Plant and Casburn, 2016). Treatments are therefore commonly focused on limiting secondary infections that result from the disruption to the skin barriers due to ulceration and crusting. In each of these cases different treatments were selected. In Case 1 the main source of secondary infection was fly strike with bacterial infection presumably also present. A multimodal treatment of topical fly repellent, fly strike treatment, topic silver sulfadiazine and parenteral antibiotics was used and found to be effective in most cases, the exception being in those lambs so severely affected that they were unable to walk. Only topical iodine was used in Case 2 and this was deemed to be of limited efficacy. This treatment protocol is supported by Dar et al. (2015) who found that the number of sheep and goats who fully recover and the time taken for recovery was significantly better in those treated with both topical and parenteral treatments. They also concluded that topical application of antiseptics alone was not effective in treating Scabby mouth. It is also clear from the second mob to become affected in Case 1 that early identification and initiation of treatment can significantly improve recovery and reduce losses.

CONCLUSION 

It is clear that a combination of prolonged environmental survival of the virus, naïve animals, lack of vaccination and ideal environmental and climactic conditions contributed to the outbreaks described in this report. It is recommended that both producers continue to vaccinate all young stock at marking, at least while the challenge of thistles and wet pastures continues. . 

REFERENCES 

  1. Aitken, I. (2008). Diseases of sheep: John Wiley & Sons
  2. Dar, K. H., Tufani, N., Dar, S. H., Hafiz, A., & Naikoo, M. D. (2015). Comparative therapeutic management of contagious ecthyma in small ruminants. Intas Polivet, 16(2), 431-435 
  3. Nettleton, P., Brebner, J., Pow, I., Gilray, J., Bell, G., & Reid, H. (1996). Tissue culture-propagated orf virus vaccine protects lambs from orf virus challenge. Veterinary Record, 138, 184-186
  4. Plant, J. W., & Casburn, G. (2016). Sheep health - scabby mouth. Primefact. Retrieved from www.dpi.nsw.gov.au
  5. Smith, B. P. (2014). Large animal internal medicine: Elsevier Health Sciences
  6. Zoetis Australia. (2016). Scabigard®. Retrieved from www,zoetis.com.au

 


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