Why are SRS Merino sheep naturally resistant to fly strike?

James Watts

The answer is simple.

The sheep are hard to wet to skin level and able to dry quickly.

It is the quick drying of the skin that is crucial. The SRS Merino is  plain bodied with a smooth (wrinkle free) skin surface (Figure 1 right). The skin does not remain wet and does not develop “skin scald”, a weeping dermatitis, which attracts blowflies.

Figure 1: Merino ewes, recently shorn. The plain bodied Merino (right) does not have skin wrinkle. The Merino sheep (left) has wrinkly skin which extends all over the animal’s body

Figure 1.  Merino ewes, recently shorn. The plain bodied Merino (right) does not have skin wrinkle. The Merino sheep (left) has wrinkly skin which extends all over the animal’s body.

When the ewe urinates, as it does every few hours, it is the drops of urine that drain away at the end of the process that may fall on to the wool and skin of the breech region.  These drops quickly evaporate if there are no skin wrinkles present.

If there are wrinkles present (Figure 1 left), the breech region becomes sodden with urine, the skin becomes severely scalded, and the blowflies are attracted to these sites.  The blowflies lay eggs in these sodden patches of wool, the maggots hatch within 24 hours and invade the skin. None of this happens if the sheep is plain bodied.

Another source of repeated wetting of sheep is rain.

If heavy and prolonged rain occurs, the skin of the plain bodied sheep is either not wet, or dries quickly. “Skin scalding” does not occur (Figure 2A).

But when the skin of the wrinkly sheep is saturated by rain, free moisture is held at skin level at the base of the wrinkles, and the skin becomes thickened and inflamed within several days. A weeping dermatitis known as fleece rot (figure 2B and Figure 3) develops

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Figure 2. Vertical skin sections of: A. normal, dry skin with intact wax layer (wl), thin epidermis (e) and non-inflamed dermis (d), and B. fleece-rot lesion, showing a microabscess (ma) in the hyperkeratotic epidermis (e) and disrupted wax layer (wl); (x 120 magnification)

Figure 2. Vertical skin sections of: A. normal, dry skin with intact wax layer (wl), thin epidermis (e) and non-inflamed dermis (d), and B. fleece-rot lesion, showing a microabscess (ma) in the hyperkeratotic epidermis (e) and disrupted wax layer (wl); (x 120 magnification)

Fleece rot is a weeping dermatitis. Seropurulent exudate accumulates in the fleece (Figure 3) and opportunist bacteria, notably Psedomonas aeruginosa, multiply extensively. The odours from the fleece rot lesion attract blowflies to strike the sheep

Fleece rot is the major predisposing cause of body strike in sheep. The dermatitic exudate is seen as the brownish, cakey material in the fleece. The green colouration is produced by the opportunist pathogen, Pseudomonas areuginosa.

Figure 3. Fleece rot is the major predisposing cause of body strike in sheep. The dermatitic exudate is seen as the brownish, cakey material in the fleece. The green colouration is produced by the opportunist pathogen, Pseudomonas areuginosa.

Why is body strike, and not breech strike, the main type of fly strike ?

Because most Merino sheep in Australia have been mulesed since the 1960s, breech strike is seen infrequently. Mulesing removes breech skin wrinkles (with shears) so that the skin, once healed, is difficult to wet and dries quickly. Probably the only time breech strike becomes a problem in mulesed sheep is when sheep scour and faecal soiling occurs.

If mulesing stops, breech strike will automatically become the No. 1 problem. But only if the sheep are wrinkly. If the sheep are plain bodied, nothing happens, no fly strike of any kind.

Body strike outbreaks are confined to very wet periods during the warm months of the year. However, the incidence of body strike in young Merino sheep that are wrinkly, can be very high and happen quickly, and many sheep may die.

The unspoken message here is that woolgrowers who breed wrinkly sheep, and insist that mulesing is essential otherwise lots of sheep will die from fly strike (breech strike), have still bred a type of  sheep where lots will die from body strike.  

Further reading:

  • Bull, L.B. (1931). Some observations on dermatitis of the folds in the breech of sheep, and its possible relationship to blowfly strike. Aust. Vet. J., 7, 143-148.
  • Belschner, H.G. (1937). Studies on the sheep blowfly problem; !!. Observations on fleece –rot and body strike in sheep, particularly in regard to their incidence, type of sheep susceptible and economic importance. Sci. Bull. Dept. Agric. N.S.W. No. 54, 61-95.
  • Burrell, D.H., Merritt, G.C., Watts, J.E. and Walker, K.H. (1982). The role of Pseudomonas aeruginosa in pathogenesis of fleece-rot and the effect of immunisation. Aust. Vet. J. 58, 34-5.
  • Hayman, R.H. (1953). Studies in fleece-rot of sheep. Aust. J. Agric. Res. 4, 430-468.
  • Hollis, D.E., Chapman, R.E. and Hemsley, J.A. (1982). Effects of experimentally induced fleece-rot on the structure of the skin of Merino sheep. Aust. J. Biol. Sci. 35, 545-56.
  • Merritt, G.C., and Watts, J.E. (1978). The changes in protein concentration and bacteria of fleece and skin during the development of fleece-rot and body strike in sheep. Aust. Vet. J. 54, 517-20.