A Literature Review and Investigation of Staphylococcal Necrotic Dermatitis in Sheep

INTRODUCTION Skin lesions, particularly those that occur on the face of sheep present certain diagnostic challenges because of some macro-scopic similarities between conditions that have widely divergent aetiologies. Incomplete descriptions, or limited investigations , may lead to further confusion. The earliest reference to the disease of sheep now known as staphylococcal dermatitis (SD) was apparently made in Scotland in 1966 20,42. It was recorded as 'periorbital eczema' and even 'facial eczema', although the latter term is now regarded as inappropriate as it is used for photosensitisation caused by Pithomyces chartarum 16,43. In 1951 a condition was described in America that bore several similarities to the disease in Britain 12. Another disease in the USA with some similarities to SD became known as lip and leg ulceration, and was later known as ulcerative dermatosis (UD). This disease was 1st described in 1910 and subse


INTRODUCTION
Skin lesions, particularly those that occur on the face of sheep present certain diagnostic challenges because of some macroscopic similarities between conditions that have widely divergent aetiologies.Incomplete descriptions, or limited investigations, may lead to further confusion.
The earliest reference to the disease of sheep now known as staphylococcal dermatitis (SD) was apparently made in Scotland in 1966 20,42 .It was recorded as 'periorbital eczema' and even 'facial eczema', although the latter term is now regarded as inappropriate as it is used for photosensitisation caused by Pithomyces chartarum 16,43 .In 1951 a condition was described in America that bore several similarities to the disease in Britain 12 .Another disease in the USA with some similarities to SD became known as lip and leg ulceration, and was later known as ulcerative dermatosis (UD).This disease was 1st described in 1910 and subse-quently in 1941 37,39 .It was later ascribed to infection by a virus similar to the orf virus 37,38 but because of the macroscopic similarities of lesions compared with those of SD, some confusion may have occurred.SD has also been reported in Ireland 24 , New Zealand 43 and France 31 , and staphylococcal folliculitis, not associated with necrotic dermatitis, in several other countries 14,43 .Most of the investigations that led to the elucidation of the cause, epidemiology and control measures for SD were conducted in Britain, and the identification of dermonecrotic strains of Staphylococcus aureus led them to change the name from the more restrictive and inaccurate periorbital eczema to SD.
An intensively investigated outbreak of a disease of sheep in South Africa is described that corresponds with the essential features of the condition termed staphylococcal dermatitis in sheep 21 .

Lesion size and number
Most reports indicate a size up to 4 cm across 11,20,21,32 and some larger 6,12,32 , up to 15 cm, but in many reports size is not recorded.The number of lesions per sheep appears to be variable and although this is not specified in the reports, it can be inferred by the references to multiple sites that multiple lesions were often seen.

Differential diagnosis
There are many diseases that could complicate making a definitive diagnosis of SD, particularly in a clinical situation where diagnosis is based on macroscopic lesions, signalment and epidemiology.These diseases comprise at least the following: ulcerative dermatosis 19,23,[36][37][38] , photosensitisation 2,16 , orf 2,18,19,29 , lumpy wool or dermatophilosis 2,19,27 , sheep pox 17 , papillomatosis, peste des petits ruminants 30 , abscesses caused by Corynebacterium pseudotuberculosis 1,2 , Arcanobacterium pyogenes 2,5 , Pseudomonas aeruginosa 43 , or Actinobacillus lignieresii, 2,15 Mannheimia (Pasteurella) haemolytica 10 and foot-andmouth disease 35 .Ectoparasites include blackfly lesions (Simulium sp.) 2 , ringworm 2,27 , trombiculid mites 25 , leishmaniosis 40 and sheep scab 2 .Irritant substances like the sap of Euphorbia species and plant awns 16 may also produce lesions that cause confusion.With lesions on the lower leg, diseases like foot rot and foot abscess 2,7,43 as well as strawberry foot rot and foot scald would have to be considered.The reported association of UD with genitals may require consideration of diseases like posthitis 2,9 , ulcerative balanoposthitis and vulvitis, and urolithiasis 2 .It is only by careful examination of the lesions, the history and epidemiology that a satisfactory clinical diagnosis can be made.It may be necessary to confirm or rule out SD by bacterial culture in cases of doubt.

History
The outbreak occurred on the farm Border Brook, southwest of Pretoria, Gauteng.The farmer had bought about 150 Letelle (Merino-type) ewes from a farmer in the Smithfield district, Free State Province between 6 and 15 months prior to the outbreak.On enquiry, that farmer reported that he had never experienced any similar cases or outbreaks.The current owner, however, believed that a single case had occurred the previous year in his own flock.The sheep were sent out to graze on Eragrostis curvula hay stubble during the day.Within some of the windrows not completely harvested during baling, a number of thistle plants were observed which, together with a few weeds, comprised the only green plants available at that time of year.At night the sheep were housed in close confinement as a precaution against theft.Here they were given access to commercial sheep pellets in feed troughs, as well as a block lick and hay bales.Normal vaccinations and anthelmintic treatments had been given but not immediately prior to the outbreak.The outbreak was reported to the private practitioner on 12 May 2002 (autumn-early winter) and referred to the authors on 21 May for further investigation.

Examination and recording
All 147 sheep present on the farm were examined for skin lesions on the woolfree areas of the face, udder, legs and perineum.These were classified according to the size, location, severity and stage.Photographs were taken and sketches of lesions made.Age was recorded according to the number of incisor teeth erupted.All data recorded were subsequently analysed for patterns of disease occurrence according to age, lesion distribution, lesion size and lesion type.

Histopathology
Histopathology was undertaken by a veterinary pathologist on 3 typical lesions collected in formalin for examination.

Specimen collection
Swab samples were taken from the edges of active lesions of 7 sheep using commercially available sterile cotton wool swabs.The swabs were cut and placed into cryovials containing 0.5-1 m brain heart infusion broth as transport medium for bacteria, and Hayflick's broth for mycoplasma isolation.Wound scabs and crusts were collected for virus particle detection by means of electron microscopy from 7 sheep.All specimens were transported to the laboratory in a coolbox containing frozen ice packs.

Microbiological examination
For bacterial culture, specimens were inoculated onto Columbia blood agar (Oxoid) with 5-6 % horse blood and MacConkey agar (Oxoid).Two blood agar plates were inoculated with each specimen and incubated respectively in an atmosphere of 5 % CO2 and anaerobically at 37 °C.All cultures were incubated for 48-72 hours.The bacterial isolates were identified using standard methods 28 .
Specimens were inoculated onto Hayflick's agar and into Hayflick's broth 13 and incubated in a moist chamber with 5 % CO2 at 37 °C to detect mycoplasms.The cultures were incubated for a minimum of 7 days and considered negative if there was no growth after 10 days of incubation.
No attempt was made to isolate viruses in cell cultures.Standard procedures for transmission electron microscopy were used to detect viral particles in the wound scabs.
The in vitro antimicrobial susceptibility of the bacterial isolates was established according to the method described by Bauer and Kirby 3 .

Lesions
Initially the farmer reported 19 of the 147 sheep to be affected (13 %).Of these, the lesions of 4 were judged to be severe (>20 mm), 9 moderate (10-20 mm) and 6 mild (5-10 mm).Closer examination of all sheep showed that lesions were much more widespread in the flock (57 sheep or 39 %), although most of the additional cases were minor.The size, distribution, type and age group occurrence of the lesions are summarised in Tables 1-3.
Figures 1-4 illustrate the position, shape and size of the lesions found on the upper lip, where the great majority of lesions occurred, from mild to severe.The early cases showed full-depth necrosis of the affected skin, which was sharply demarcated from the surrounding healthy skin.At the junction, the healthy skin was inflamed for a few millimetres.Removal of the necrotic, scabby layer revealed the presence of pale yellow thick pus and a rough, bleeding, granular ulcer.Within a week of initial examination most lesions showed significant improvement with scarring but no regrowth of hair.Nearly all the sheep subsequently recovered completely, except for the worst cases, which had permanent scarring and disfigurement.The mildest cases were of very short duration and within 2 weeks only the most careful examination could reveal a previous infection.Two of the worst cases appeared to extend by millimetres into the nares.Most lesions appeared to be only on the right side of the upper lip.Foot lesions recorded at the coronary junction appeared to be traumatic rather than infectious but are included for the sake of completeness.The vulvar lesions recorded did not appear to be contemporaneous with the facial lesions in that they were all chronic, scarred lesions that may have been caused by ticks in previous seasons.From the time that the 1st signs were noted by the farmer the disease appeared to have spread rapidly, but between the 1st and 2nd examinations 1 week apart, only 1 possible new case was recorded.After this, no further problems were encountered, including during the subsequent 2 seasons.
The farmer had initially treated the affected cases with an antiseptic chlorhexidine (Hibitane, Dismed) spray and later a tetracycline (Terramycin, Pfizer) spray, but after 3 days the animals appeared worse.The private practitioner recommended injectable florfenicol (Nuflor, Schering Plough) for all cases and this was associated with sustained improvement.There were no rams present on the farm and none of the ewes were pregnant or lactating during the outbreak.
Altogether 57 sheep (39 %) had lesions, of which 43 (75 %) had only unilateral lip lesions and 3 (5 %) had lesions on both sides.Six (11 %) had multiple lesions, and all of these had lip lesions.Two sheep had only bilateral lip lesions with vulvar lesions, 1 had a unilateral lip lesion with a leg lesion, 1 had a unilateral lip lesion with a vulvar lesion and 1 a unilateral lip lesion with both leg and vulvar lesions.There were 7 sheep with only vulvar lesions and 2 with only leg lesions.Altogether there were 10 vulvar lesions (6 %) and 4 leg lesions (3 %) compared with 46 with only lip lesions (80 %).A surprising total of 36 (55 %) of the 57 sheep with lesions had lip lesions on the right side, but just 16 (24 %) on the left.The total number of leg lesions in the flock (4) represented just 3 % of the total flock but 7 % of the affected group.Similarly, the total number of vulvar lesions (10) represented 6 % of the flock but 16 % of affected sheep.

Microbiological examination
Electron microscopic investigation did not reveal any viral particles.
All specimens yielded pure cultures of a beta-haemolytic, greyish, smooth colony that microscopically appeared as Grampositive cocci.The organisms were cata-  lase-positive, coagulase-positive and DNase-positive.Further confirmation of these isolates as Staphylococcus aureus was achieved using the biochemical tests described by Quinn et al. 28 .No mycoplasmas were isolated.The growth inhibition zone sizes and their interpretation for each drug tested on 3 selected isolates are shown in Table 4.

Histopathology
Skin biopsies revealed full-depth necrosis of the epithelium with severe ulceration and acute tissue necrosis.There was prominent oedema, rhabdomyolysis and acute neutrophilic infiltration in some parts of the subcutis.A pathological anatomical diagnosis of severe multiflocal necrotic dermatitis accompanied by exudation and ulceration was made.The lesions were found to be consistent with a dermotoxic insult.

DISCUSSION
This investigation into an outbreak of necrotic dermatitis in a sheep flock revealed key features consistent in all material aspects with previous reports of the disease now generally referred to as ovine staphylococcal dermatitis.In fact, the great majority of findings confirm those documented by previous investigators.It is therefore strange, given the ubiquity of S. aureus 22,41 and the frequency of encountering the predisposing factors, that this disease has not been properly documented before in South Africa.Informal discussions with veterinarians revealed that it has probably occurred over a long time, but infrequently or only affecting a few sheep, or in a mild form.The great range of potential confusers that are listed under the differential diagnoses clearly indicates that misdiagnoses may easily occur and are excusable unless all factors, lesions and tests are carefully considered.Since the outcome of SD is usually complete healing and no mortality, an incorrect diagnosis can easily go unnoticed and unchallenged.Superficial examination in this outbreak only showed 19 clearly affected sheep, while a detailed and careful inspection uncovered another 38 sheep with milder lesions that in all probability would have gone undetected had there been no severe cases that triggered the full investigation.One factor that should not be overlooked is that S. aureus isolates can vary markedly in their dermotoxicity and therefore only particular isolates will cause significant disease.Although SD on the basis of current knowledge cannot be regarded as a significant threat to the local sheep industry, it is important that it is clearly differentiated from other skin conditions so that appropriate measures can be taken to control an outbreak.
Recommendations to the farmer comprised twice-weekly examinations, separation of affected and healthy sheep, wound treatment, antibiotic injections, removal of old feed pellets, disinfection of hands and feed troughs and checking and repair of any sharp edges in the troughs.
One intriguing and possibly instructive aspect of this investigation was the marked preponderance of lip lesions on the right side of the face, compared with the left.It is unlikely, given the numbers involved, that this 2:1 ratio was mere  chance.Rather, it points to a causative factor that created more skin damage on one side of the muzzle than the other.This could be associated with the thistles in the paddock, if the sheep were consistently grazing more from 1 side.A more likely explanation is that sheep were feeding at the troughs more from 1 side than the other and that skin damage and contamination, when present, was more frequently on the right.Unfortunately this could not be confirmed by examination of the troughs or observation of the sheep.The range of size and severity of the lesions and the large number of unaffected sheep may point to differences in susceptibility between the animals.Since the same range of predisposing factors previously reported were found in this outbreak, eliminating or ameliorating them may assist with preventing or containing outbreaks.

Fig. 1 :
Fig. 1: Position, shape and size of lip lesions in a mild case of SD.Fig. 2: The appearance of a more severe case of SD.

230 0038 -Fig. 3 :Fig. 4 :
Fig. 3: An advanced case of SD.Fig. 4: One of the most severely affected cases of SD, with destruction of the upper lip.

Table 1 :
Proportion of age groups affected.

Table 2 :
Lesion size on lips.

Table 4 : Zone size and interpretation criteria of antibiograms for selected isolates. Isolate E2 Isolate E3 Isolate E7 Drug tested Zone size (mm) Interpretation Zone size (mm) Interpretation Zone size (mm) Interpretation
*R = resistant,**S = susceptible.