CASE NOTES


High prevalence mild dermatophilosis (lumpy wool) in unweaned merino lambs following a wet spring

Will Berry (Final year veterinary student, Charles Sturt University, Wagga Wagga NSW) and Bruce Watt, Central Tablelands Local Land Services, Bathurst, NSW

Posted Flock & Herd August 2017

INTRODUCTION

Dermatophilosis in sheep (lumpy wool) is a dermatitis caused by the bacterium Dermatophilus congolensis. Dermatophilosis affects a range of animal species including horses, cattle, sheep, goats and humans. Chronically infected sheep with minor active lesions (often on the face and ears) are the main source of infection. The spread of infection occurs when the fleece becomes wet for an extended period. Outbreaks of dermatophilosis therefore tend to be sporadic and are associated with prolonged wet weather as was the case during the 2016 winter and spring in the central tablelands of NSW. 

Dermatophilosis is of economic significance as it results in losses through lowered production of wool, decreased wool value, culling losses, treatment costs and difficulty shearing (Bateup & Edwards, 1990; Edwards, 1985). It can also predispose sheep to secondary skin infections, fleece rot and flystrike, which may cause deaths (Gherardi, Monzu, Sutherland, Johnson, & Robertson, 1981).

In October 2016, two cases of high prevalence dermatophilosis in unweaned merino lambs were investigated. The clinical manifestations were the same for both properties and were consistent with dermatophilosis but laboratory confirmation was only achieved in the second case. 

While the prevalence was high, most cases were mild and were resolving at the time of examination. The owners of the first flock (Property A) elected to treat most of the affected lambs to reduce the risk of flystrike, impaired production and wool damage. The owners of the second flock (Property B) only treated a small proportion of the lambs as they believed the infection was lifting from the skin. 

The diagnosis, management and treatment of ovine dermatophilosis are discussed, as well as the management and outcome of the two cases.

HISTORY 

Property A had 3700 August-born merino lambs from ewes lambing in ten mobs. At lamb marking (8-12 weeks old) the owner noticed 50- 90 percent of lambs had crusts on the dorsum consistent with dermatophilosis, while the ewes did not appear affected. The Local Land Services (LLS) was contacted and arranged to visit two weeks later when the ewes and lambs were brought back in for ewe crutching. 

Property B had 730 September/October-born merino lambs split into three mobs based on early-late lambing. A high proportion of lambs were noted with crusts on the dorsum prior to lamb marking. The LLS was contacted and arranged a visit to coincide with marking. 

CLINICAL FINDINGS

Property A: In the mob examined approximately 80 percent of the lambs had crusts on the nose, ears and dorsum consistent with dermatophilosis (Figure 1). Most of the lesions appeared to be lifting and resolving (Figure 2). However, the owner decided to treat any lambs that were severely affected -  2600 lambs were treated with 1 ml of intramuscular long acting oxytetracycline. This property also had a smaller mob of British breed crossbred lambs of the same age, in which 10-15 percent were affected with similar lesions. It was decided not to treat these lambs.

Property B: At lamb marking in the first mob of 152 lambs, approximately 30 percent had lesions consistent with dermatophilosis and 13 percent were considered active. The lambs with active lesions were treated with a single intramuscular injection of procaine penicillin/benzathine penicillin (Benacillin Long Acting Penicillin Injection, Procaine Penicillin 150 mg/ml, Benzathine Penicillin 150 mg/ml and Procaine Hydrochloride 20 mg/ml, Troy Laboratories). In the second mob of 202 lambs approximately 15 percent had lesions and four percent were considered active and were treated. In the third mob of 376 lambs, approximately 15 percent had lesions and none were treated as they all appeared to be lifting from the skin and healing.

Figure 1. Dermatophilosis lesions on dorsum
Figure 2: Resolving Dermatophilosis lesions lifting from the skin
Figure 3: Dermatophilosis lesions on dorsum
Figure 4: Dermatophilosis lesions over dorsum, ears, and face

LABORATORY FINDINGS 

On Property A at ewe crutching (25 October 2016) samples of crusts were collected from a number of lambs and sent for culture.  No organisms consistent with D. congolensis were detected on smear examination or culture.

On Property B samples of scabs were collected on 18 October 2016 and sent for bacteriology. Organisms consistent with D. congolensis were detected on smear examination. 

DISEASE OUTCOME

On 27 January 2017 the owner of Property A reported that in almost all cases the scabs had lifted from the skin. Losses were negligible although a few lambs died of body strike, and perhaps the dermatophilosis made them vulnerable to secondary body strike. The lambs were successfully shorn in January, three months earlier than usual, in part because of the threat of grass seeds and in part to remove D. congolensis-affected wool.  

On 11 January 2017 the owner of Property B commented that by weaning in late December the scabs had lifted from the skin in all cases examined, whether treated or untreated. In total 30-40 lambs were treated. 

DISCUSSION 

Dermatophilosis occurs in all areas of the world, but is more commonly seen in high rainfall and tropical areas. In Australia the disease is sporadic, and most commonly seen in the high and medium rainfall areas (Edwards et al., 1985; Radostits et al., 2006). Animals at all ages can be susceptible, but because previously exposed animals develop immunity younger animals tend to be more often affected (Norris, Colditz, & Dixon, 2008). Edwards et al. (1985) found that medium and strong wool merinos have a higher prevalence of dermatophilosis than fine wool merinos, suggesting that susceptibility increases as fibre diameter increases. It is reported anecdotally and in non-peer-reviewed literature that merinos are more susceptible than British breeds, however no study has been completed to confirm this observation. 

Diagnosis is usually based on clinical appearance of lesions coinciding with a wetting event. The lesions tend to be distributed over the dorsal midline, and spread laterally and ventrally (Radostits et al., 2006). For laboratory confirmation, impression smears from the moist side of active lesions can be taken. A Gram or Giemsia stain will reveal gram positive cocci, which divide both horizontally and longitudinally to produce filamentous branches with double rows. 

In consultation with their private veterinarian, Property A decided to treat affected lambs with a single injection of oxytetracycline. The owners were concerned about decreased growth rates in the affected lambs. Treating at this time was partly a practical decision as bringing large numbers of ewes and lambs through the yards is labour intensive. 

Property B was advised by their private veterinarian to treat affected lambs with long-acting penicillin. However, once the owners noticed that most lesions appeared to be resolving spontaneously they decided against mass treatment. 

A number of antibiotics have been evaluated for their efficacy in treating dermatophilosis. Scrivener and Vizard (1995) compared the efficacy of a single dose of either erythromycin or penicillin combined with streptomycin. The study did not show any benefit of either erythromycin or penicillin/streptomycin for treatment of ovine dermatophilosis. Many of the lesions in the control group resolved spontaneously and the sheep were able to be shorn without treatment. It was recommended that sheep not be treated prior to shearing, but any sheep that cannot be shorn at the time of shearing be treated and shorn at a later date or culled. 

Jordan and Venning (1995) compared the efficacy of a single injection of long-acting oxytetracycline (20mg/kg IM) to  a lincomycin/spectinomycin combination. They identified 307 merino sheep with active dermatophilosis lesions, many of which were chronic.  The sheep were evaluated 40 days after treatment and immediately after shearing, which occurred 61 days after treatment. It was found that more lesions resolved in sheep treated with either product than in those with no treatment. At day 40 in sheep treated with oxytetracycline, 62.4 percent of animals had all lesions resolved compared to 50 percent and 20.4 percent for lincomycin/spectinomycin and the control group respectively. At shearing 40 percent of untreated sheep were incompletely shorn due to active lesions, compared to 9.8 percent of those treated with oxytetracycline. 

Awad, Nadra-Elwgoud, and El-Sayed (2008) conducted a small trial evaluating the efficacy of two doses of long-acting oxytetracycline compared to a single dose in sheep. Five sheep were administered two doses of long-acting oxytetracycline one day apart, and all lesions resolved after two weeks. In sheep administered a single dose, in four out of five all lesions were resolved in two weeks, and one had lesions that persisted beyond four weeks. In the control group of three, no lesions resolved after four weeks. The sample size of this study is unlikely to produce significant results. The efficacy of penicillin alone has not been reported in the literature for the treatment of ovine dermatophilosis.

Bactericidal dips have been used for treatment in the past. Le Riche (1967) evaluated a number of different dipping fluids including zinc sulphate and found that none of the products had any therapeutic activity against established lesions. Topical treatments are now not recommended for the treatment of dermatophilosis in sheep. 

Long-acting oxytetracycline is now the treatment of choice, as lincomycin/streptomycin is not registered for use in sheep in Australia. A recommendation for the treatment of adult sheep affected with active dermatophilosis lesions is a single intramuscular injection of long-acting oxytetracycline at 20 mg/kg at least 40 days prior to shearing. There is little specific literature on the treatment of young lambs with dermatophilosis. Treatment may be most beneficial during outbreaks when environmental conditions result in high prevalence within flocks. 

Manipulation of the risk factors associated with dermatophilosis is an effective method of prevention and control. A logical approach is to avoid risk factors where possible. Isolation of affected sheep particularly during wet periods is likely to reduce transmission but is often impractical. Yarding of wet sheep should be avoided as this has been demonstrated to increase transmission. Close yarding immediately after shearing or dipping should also be avoided. If animals are being dipped for lice control, D. congolensis affected sheep should be done last and the addition of 0.5-1% zinc sulphate to the dipping solution can prevent the transmission of D. congolensis (Le Riche, 1967). 

Where dipping has been identified as a significant means of transmission, pour-on lice control may be considered. Culling of chronically affected sheep has been used as a method of control. It may be beneficial as it reduces potential sources of infection for other sheep. It may also be a useful control method as susceptibility to dermatophilosis may be heritable. 

Norris et al. (2008) reported that heritability of resistance to dermatophilosis is estimated as less than 0.14, but the heritability of severity of infection ranged from 0.25 to 0.42. This finding suggests that the identification of gene markers may be possible in the future to aid in the selection of resistant sheep. Masters, Ellis and Groth (1997) found that sheep that develop chronic dermatophilosis are more likely to have a change in their CD3-gamma protein compared to those that develop self-limiting infections. A gene marker has been identified in cattle that is strongly correlated with susceptibility to dermatophilosis, and culling based on this marker reduced prevalence of disease from 75 to two percent in a study over five years (Maillard et al., 2003).

Vaccines against D.congolensis have been investigated with poor results. Ellis et al. (1987) demonstrated that repeat inoculations with D. congolensis resulted in faster resolution of lesions. It was suggested that IgA mediated humoral immunity may be largely responsible.  Sutherland, Ellis and Edwards (1991) evaluated a live and killed vaccine, with no difference between the vaccinated and control groups. They postulated that antigenic variation between the vaccine and field isolates of D.congolensis may have been a problem. Due to the sporadic nature of the disease  vaccination is unlikely to be a feasible control option in most flocks  including in these flocks. 

REFERENCES

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