Producers are being urged to monitor stock grazing vetch and/or medic infested with cowpea aphid with cases of photosensitisation being seen for the third year in a row.
Fewer reports have been received this year than 2017; possibly a result of the poor season in the affected areas.
Last year it was estimated more than 25,000 sheep were affected by the condition, which was widespread on upper Eyre Peninsula, as well as reports from the Mid/Upper North and Yorke Peninsula.
Photosensitisation is inflammation of the skin, and occasionally conjunctiva and the cornea of the eye.
Sunlight activates photosensitising substances in these tissues, resulting in local damage. It differs from sunburn in that photosensitisation does not require prolonged exposure to sunlight to develop.
Signs include:
- Severe irritation, restlessness, rubbing and shaking of head and ears
- Seeking shade
- Swelling of affected areas including ears, eyelids, lips and nose. Swollen ears droop and may be a distinctive feature
- Discharge from the eyes, conjunctivitis, whitening of the cornea
- Sometimes lameness (photosensitisation can affect the coronary band)
- Dead and sloughing skin in the affected areas, with underlying tissues becoming inflamed and scabs forming over the inflammation. This is often first noticed at the tips of the ears, but can extend to the whole face, unpigmented areas of the body, and areas of bare skin like the udder, teats, vulva and upper surface of the tail.
Treatment of affected stock
Immediately remove affected livestock from the toxic paddock and provide shade.
Affected livestock will need protection from sunlight for at least seven days. If detected early, animals should recover within 4 to 5 days.
If skin lesions are more severe, recovery will take a little longer.
Use of anti-inflammatory and anti-histamine treatments can hasten recovery as well as antibiotics where the skin is cracked and susceptible to bacterial invasion.
During recovery, monitor affected stock for infection and flystrike. Seek veterinary advice for treatment of severely affected animals.
How is cowpea aphid implicated?
A common factor in most of the cases has been a heavy infestation of cowpea aphid on the grazed vetch and/or medics.
There is a suggestion that aphids may contain photodynamic fluorescent pigments that could cause phototoxic effects in stock, however this remains unclear.
Aphid control
There are several insecticides registered for use in vetch and medic in SA, which are likely to provide some control of cowpea aphid.
Be aware of any withholding periods associated with the insecticide used. Seek advice from your local agronomist.
Whilst the aphid is believed to be the cause of the issue, further work is planned to confirm this.
We are interested in speaking with producers that have had stock affected with the photosensitisation.
Sheep Connect SA is supported by Australian Wool Innovation, the SA Sheep Industry Fund and PIRSA.
- Details Ian McFarland on 0437 659 353 or email ian.mcfarland@sa.gov.au
