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Greg Curran, Veterinary Officer, DPI, Broken Hill

Posted Flock & Herd March 2012


While testing for ovine brucellosis in two ram flocks in Far Northwestern NSW in December 2011, a high prevalence of back lesions was observed. The lesions varied from severe to trace, with some rams not affected. The lesions involved loss of wool and skin, with some crusting. These changes varied in extent and severity. Good regeneration of wool had occurred, although shorter than unaffected areas. A small number of rams had wounds that had not healed, or had broad scar tissue.


The rams in both flocks had been seen several times in previous brucellosis sampling, with no evidence of these lesions. Both flocks were shorn and treated in November, 3 to 4 weeks before examination.

One flock was 4th generation South African Meat Merino (SAMM); the other was a mix of horned Merino and SAMMs. One flock (A) consisted of a 2yo group, and an older group (3 years old up). The other flock (B) were all 3 years old up. The two flocks were owned by the same family. One landholding is to the west of Milparinka, and the other is about 150km south.


The rams in both flocks were reported to have been sedated with acepromazine (1ml) before shearing and backlined with Extinosad Pour-On after shearing. The rams were in good condition, and weighed about (80 to 100) kg. I understand that the rams were dosed to the heaviest in the group (~100kg).

It was reported that both ram groups in flock A were shorn in the latter part of the morning; both groups were backlined shortly after shearing, and the older rams then remained in open yards, while younger rams were put under shade until the late afternoon. Flock B rams were reported to have been shorn earlier in the day, but backlined in late afternoon.


Each ram was assessed visually, and graded as 4 (Severe), 3 (Moderate), 2 (Low), 1 Trace, 0 (No lesion). Images of examples of each grade are given below, apart from 'Trace'.

In flocks A and B, rectal temperatures were taken to test whether body temperature played any part in development of the back lesions. A digital thermometer was used, with a 30 second recording period, ending with an audible beep. With flock B, rectal temperatures had been taken with the same thermometer in October, as part of another project considering effect of high ambient temperatures on reproductive performance.

Testes were palpated to assess size and any changes in consistency. Rams examined had good size bar those with flystrike. A portion had slight softness, rather than the firm resilience expected with rams in good condition shortly before joining. This softness suggested some form of damage in the testes related to the effects of back lesions, distress, or hyperthermia, or a combination of these.

Image of shorn sheep with healing back lesions
Severe Grade 4: scarring; extensive fleece loss (breadth and width)

Note some evidence of trails running laterally down from central area.

Image of shorn sheep with healing back lesions
Moderate Grade 3: no or minor scarring; extensive
Image of shorn sheep with healed back lesions
Low Grade 2: no scarring; limited extent

No image

Trace Grade 1: no scarring; one or two lesions less than 2cm
Image of shorn sheep back
Nil Grade 0: no lesions

Other lesions observed:

Healing sunburn of perineum

Image of shorn sheep rear
This sheep had no back lesions, suggesting that the perineal lesion was primarily
due to the effects of solar radiation
Image of shorn sheep with healed back lesion
Healed flystrike over rump; no lesions elsewhere

The nature of the wool loss area correlated well with changes expected with flystrike but could represent minor changes related to the treatment. Strike healing at shearing

Image of shorn sheep with healed back  lesions
Healed flystrike plus back lesions

Strike occurred before shearing; probably had not healed at shearing

These lesions are sometimes seen in other western NSW flocks. See image 1.

Image of shorn sheep with sloughed skin on back
Image 1 (Courtesy Charlotte Cavanagh)

Nature and distribution of lesions

The distribution of the lesions on individual sheep and across both mobs provides some indication of their pathogenesis:

Extinosad Pour-On is usually applied in two relatively broad, opposing and separate arcs, conjoined at start (behind the head) and end (at the rump), either side of the spine along the thorax and lumbar region. How the pour-on was applied on each ram might vary between rams

Key features of lesion distribution were primarily longitudinal, and secondarily lateral; lesions could be taken as rounded patches or thinner linear areas; the linear areas had an elongated triangular form when lateral, and extending down the sides, with a dorsal base; lesions tended to be more severe along the midline.

The lesions were not consistent with primary dermatophilosis, in my opinion, but it is possible that some secondary dermatophilosis spread from primary lesions: there were patches of wool that could be lifted readily, with a length expected of normal growth since shearing, but whose base had relatively dense white flocculant material. Images of severe and moderate lesions above show small areas of wool lifted while examining the lesion

In two sheep that appeared to have healed flystrike lesions at time of shearing, treatment did not adversely affect the healed struck area, but did affect adjacent areas. Treatment worsened and appeared to extend damaged skin in one sheep with what appeared to be recent or active extensive flystrike at time of shearing

Two sheep in flock B had severe broad midline lesions which had failed to heal. They had signs of fly worry but no flystrike.

Wool had grown in most but not all affected areas, but was less dense and shorter than normal fleece, indicating these areas took some time to heal.

Some areas had not healed and had no wool, or sparser fibres. Some areas had dark red scabs that appeared to be healing, but without wool/hair.

Rams were seen with resolving swellings of their perineum. This lesion was not evaluated in all sheep but was seen often. This lesion appeared to be sunburn, and not related to the chemical. It was seen in rams with no back lesions. The swollen perineum was at a distance from any back lesions, suggesting that it was not due to chemical being misplaced on the perineum

The broad flatter conformation of the backs of some rams may have contributed to extent and severity of lesions. This conformation is more common in meat breeds, than the rounded or ridged-back conformation of Merino rams. This broader back exposes a larger area to direct sun, with consequent heating of this area in the same way as bare ground, whereas skin or fleece on sides or at an angle to direct sun tends to deflect radiation, with less absorption


The lesion grading of rams in Flocks A and B is given in Tables 1 and 2. In the older Flock A rams, 56% had moderate or severe lesions, compared with 8% in the Flock A maidens. More maidens (54%) were not affected than older rams (25%). Flock B had moderate or severe lesions in 30%, with 29% having no lesions, similar to flock A.

Table of back lesion severity
Table 1. Severity of lesions in older and maiden rams in Flock A and B
Table of severity of lesions and chemical application
Table 2. Severity of lesions by time chemical applied and solar exposure


Initially, it appeared that the lesions could be related to several potential risk factors. These are listed below with epidemiological assessments

The lesions developed after all animals were shorn. Removing most but not all the fleece will increasing exposure to sun. Shearing alone is not known to cause similar skin lesions

All rams were sedated. Any dosage effect might produce more severe lesions in smaller rams. The rams were not weighed but a visual comparison did not show any clear differences in severity between smaller and larger rams. There was a longer period between (sedation/shearing) and Extinosad Pour-On in flock B than flock A, which might reduce their effects and interaction in flock B. Side effects of acepromazine are photosensitivity, excitation, aggression, convulsion, muscle spasm, and hypotension.

All sheep were treated with Extinosad Pour-On. Extinosad Pour-On uses propylene glycol (20%), trimethylnonanol polyethylene glycol (10%), and water (>60%) to carry its spinosads. These might produce irritation and consequent inflammation, or alter susceptibility of skin and fleece to sun, a humidity at skin level is reduced. Extinosad Pour-On is capable of minor skin irritation of people. It is possible that the Extinosad Pour-On caused a chemical burn, from being denatured, or from a faulty batch. Both were considered unlikely, as this should lead to lesions being seen at similar frequencies, extent and severity in similar sheep, largely independent of other external factors. Extinosad Pour-On alone is not known to cause skin lesions in sheep at shearing.

The lesions could indicate infrared heating of the skin, or effects of ultraviolet (UV) radiation on the skin (sunburn, immunosuppression). Exposure to sun alone is not known to cause these skin lesions in such a high proportion of recently shorn sheep. Exposure to sun after application of Extinosad Pour-On varied between mobs. In flock A, the young rams held in shade after treatment had lower prevalence and severity of skin lesions than the mob held in open sun. Sheep in flock B were sedated and shorn in the morning, but treated later in the afternoon, and then held in open sun for a shorter period than flock A. These differences are consistent with the intermediate prevalence and severity of flock B. Temperatures and solar radiation ranged from low to high during November 2011 (see Table 3).

Older rams had similar prevalence of the condition in flocks A and B. Maiden rams had much lower prevalence. The age difference may have been confounded with whether rams were in direct sun or shade after pour-on application.


Flock A's climate during November and December was inferred from Fowlers Gap AWS, 20 km away; and flock B's was related to Tibooburra Airport, 45km away. Data was taken from Bureau of Meteorology website.

Flock A was examined on 6th December 2011 (Fowlers Gap maximum 29oC; equal to Tibooburra on 27 October); flock B on 16th December (Tibooburra maximum 37oC).

Flocks A and B and adjoining areas had higher than usual rainfall in November, with Fowlers Gap recording 54mm (median 11), and Tibooburra 139mm (median 8.4). Higher rainfall in November would increase humidity, exacerbating heat stress from high ambient temperature.

Table of temperatures and solar radiation
Table 3. Maximum temperatures and solar radiation near flocks A, B


Rectal temperature records were analysed to see whether a ram's ability to handle heat had any effect on whether a ram was affected or not; and the severity of any lesion. This analysis unexpectedly revealed there were such relationships between body temperature, prevalence and severity in both flocks.

Body temperatures are summarised in Table 4. They ranged from normal to hyperthermic (>40oC). In flock A, maiden rams had statistically significantly higher body temperatures than older rams, a difference in means of 0.45oC (ttest p: 0.0001).

Table of body temperatures
Table 4 - October & December body temperatures in Flock A & B rams (ToC)

Mean body temperatures in December for flock A were lower (39.48oC) than for flock B (40.35oC), a relatively large difference of 0.87oC, due in part to the higher ambient temperature at flock B (37oC compared to 29oC).

Maximum ambient temperature for flock B in October was the same as flock A in December (29oC). Despite similar ambient temperatures, the older flock B rams in October had higher mean body temperatures (39.55oC) than older flock A rams in December (39.20oC).

Flock B ram body temperatures

In Figure 1, the distribution of flock B body temperatures in October is compared with severity of lesions seen in December. Rams that subsequently developed severe lesions had higher body temperatures in October than those with less severe lesions. Visually, October temperatures (a month before shearing) can be related to severity of lesion after shearing.

Chart of back lesion severity
Figure 1.

Sheep with (moderate, severe) lesions had statistically significantly higher October temperatures (+0.29oC) than those with lesions graded as (nil, trace or low) severity

Sheep with (moderate, severe) lesions had statistically significantly higher December temperatures (+0.30oC) than those with lesions graded as (nil, trace or low) severity:

Rams with moderate and severe lesions had a slightly greater change in temperature between October and December [+0.98oC; (95% CI: 0.76 to 1.21)] than rams with less severe or no lesion [+0.82oC: (95% CI: 0.68 to 0.95)], a difference of +0.16oC.

December body temperatures for each severity of back lesion are given in Table 5.

Table of body temperatures
Table 5 Body temperatures by severity of lesion at Flock B in December

Regression analysis found a small but statistically significant trend between 'Severity of lesion' and increasing October body temperature (probability of 0.029)

These observations indicated that October body temperature (before shearing) was a risk factor for severity of lesions that developed later at and after shearing. Having moderate or severe lesions increased a ram's December body temperature by 0.16oC or more than rams with low, trace or no lesion. If a low outlier was excluded as indicating a reading error, this increase was 0.30oC

There was remarkable consistency in body temperature rank between October and December measurements (signrank test: probability 0.0000). See Figure 2.It was expected that a ram's temperature would be influenced more by the many factors that could drive a single temperature at any point in time: how it was handled, position in race, whether with mates or not, how it reacted to changing circumstances, length of time it was restrained, the time of day temperature was taken, the ambient temperature at any time, the time it had spent in direct sun or shade, as well as the back lesions, for example.

This consistency indicated that relative body temperature was characteristic for each animal.

The mean change in body temperature was 0.83oC (95% CI: 0.70 to 0.95), ranging from (-0.50 to +2.0). Body temperature difference was normally distributed, so although there could be some measurement error for some animals, most were representative.

Table of body temperatures
Figure 2.

Flock A ram body temperatures

With Flock A, rams with lesions had higher body temperatures than non-affected rams, in both age groups. See Figure 3.

Chart of lesion numbers
Figure 3.

As noted above, the maiden rams had a lower prevalence (46% vs 75%), and fewer severe lesions than older rams (See Table 1). This is believed to reflect the maiden rams having been put in shade after shearing and Extinosad Pour-On, whereas older rams remained in direct sun. The possibility that older rams may have been more susceptible was considered, but discounted as this would be counter to the key finding that higher body temperatures predisposed to higher prevalence: older rams had generally lower body temperatures than maidens.

From regression analysis of body temperature considering both age group and whether affected, the model showed that having any back lesion raised body temperature by 0.18oC (p: 0.11 - borderline) while being an older ram reduced body temperature by 0.50oC (p: 0.000). The model had reasonable explanatory power (R2: 0.35)


The back lesions were not simple burns due to exposure of the skin to the chemicals in the product applied.

It was considered that the back lesions resulted from a combination of:

A. Being shorn reducing insulation from solar radiation (heat and UV). Lowering humidity in fleece adjoining skin, increasing heat absorption

B. Being sedated causing a form of photosensitisation and reducing rams ability to thermoregulate

C. Application of a chemical to treat sheep lice (Extinosad Pour-on)

D. Exposure to direct sun after shearing and pour-on application. Longer periods in direct sun immediately after pour-on increased incidence and severity of the lesions. Being shaded immediately after pour-on reduced incidence and severity but did not prevent lesions. It was likely that exposure to direct sun the following day or days had a pathological effect.

E. The relatively higher solar radiation and ambient temperatures in this arid area, with its general lack of shade led to increased pathology of the skin from UV radiation and heating. The slightly higher radiation of flock B compared with flock A to the south may have led to very severe lesions in 2 rams, and may have hampered healing.

F. Each ram's core body temperature and ability to thermoregulate altered pathology, with higher body temperature increasing severity and incidence. It is important to note that some rams with low to normal temperatures in October developed lesions (10 of 18 rams with temperatures less than 39.3 developed lesions, 2 classed as moderate or severe). The pathogenic effects of heat are known to affect some animals at relatively low body temperatures and levels of exercise (Hubbard 1977).

Other key conclusions were that solar radiation and heat can be pathogenic for skin in wool sheep in certain situations. Higher body temperatures may contribute to the pathogenicity of solar radiation and heat on skin. Individual rams varied in their susceptibility to the back lesions in terms of their body temperature. A small proportion with low body temperatures were affected, with a higher proportion affected among those with high body temperatures.

Inflammation from the active skin lesions reduced thermotolerance, increasing body temperature in moderately high ambient temperatures (Lim 2006, Lim 2007).

Relative body temperature was characteristic for each ram, with ranking similar on separate occasions when ambient temperatures differed: 'cooler' rams remained cooler when temperatures rose; 'hotter' rams remained hotter when days were hotter.

The difference in rectal temperatures measured in flock A between October and December are believed to be due to higher ambient and environmental temperatures; shearing removing insulating wool (Thwaites 1976); presence of the back lesions.

Higher than normal rainfall during November may have contributed to the lesions, and may have led to mild dermatophilosis in some sheep. Extreme Bureau of Meteorology ambient temperatures were not required for rams to be hyperthermic (body temperature >= 40oC- Lovatt 2010). In flock B, 5 rams (8%) were hyperthermic on 27 October 2011 when maximum ambient temperature was 33oC nearby at Tibooburra; while 51 rams (84%) were hyperthermic on 16 December 2011 when maximum ambient temperature was 37oC at Tibooburra. However no ram was seen to open-mouth breath, which is considered to be a sign of heat stress (Stockman). All rams appeared healthy.

More rams are likely to become severely hyperthermic as ambient temperatures moved beyond 37oC recorded at Tibooburra near flock B on 16 December.

Inflammation when the lesions first developed may have led to pyrexia, creating the palpable softness of testes. This softness could indicate reduced sperm production and fertility.

Age was not important, compared with risk factors, if at all. Breed differences were not studied but did not appear important.

With evidence of increasing temperatures now and in the future, the chance of these lesions occurring may increase. Tibooburra's temperatures are extreme now. Tibooburra has averaged 74 days each year with maximum temperature over 35oC, compared with Broken Hill's average 36 days, both with over 90 years' records. Tibooburra's ambient Stevenson box temperatures ranging from 40 to 45 are common, leading to ground and surface temperatures of 55 to 70oC (unpublished work). Tibooburra has been identified by Australian Bureau of Meteorology as a site with clear evidence of increasing temperatures.


Several options were available, including avoid sedating with acepromazine then using Extinosad Pour-On. If this combination of sedative and product was used again in warm season months, do so on dusk, and keep sheep in shade for at least 36 hours. Ensure the chemical is stored as recommended (below 30oC).

Changing time of shearing (to cooler months) is not possible generally, due to consequential disadvantageous effects on productivity, health, marketing or management of sheep, but could be considered.

Given the significance of body temperature, and their repeatability, another option to prevent development of these lesions would be to select rams with low body temperature when purchased.

To my knowledge, checking rectal temperatures is not part of ram selection in the Merino industry, so may not be acceptable. This approach would not be justified in terms of preventing the back lesions alone. However, testing body temperatures when selecting rams for purchase (or sale) potentially might offer other benefits of considerable value.

Consider the following:

One conjecture was that if rams were selected for thermotolerance, they may be able to handle heat better in an area with extreme temperatures. If thermotolerance in sheep is heritable as with cattle, using thermotolerant rams would gradually increase thermotolerance in ewes, improving ewe reproductive performance.

Improving the flock's reproductive performance was the central purpose for examining the flock's rams. The investigation of ram back lesions could indirectly and potentially improve lambmarking %. Work to understand the significance and role of high temperatures on sheep reproduction is underway.


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