Cowpea aphid infestations linked to photosensitisation in thoroughbred horses


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Horse owners are urged to check pastures regularly and take necessary precautions to prevent Cowpea aphid outbreaks on their lucerne or lucerne mix pastures to reduce the likelihood of photosensitisation occurring in their horses

They may be tiny, but Cowpea aphids are a cause for concern among thoroughbred horse owners. The 2-5 mm long black bug infests older varieties of Lucerne pastures, and these infestations are linked to photosensitisation in horses.

In Autumn 2020, two thoroughbred horse studs in northern Victoria reported severe cases of photosensitisation after their horses had been grazing lucerne pastures. These horses required stabling and intensive treatment.

Although there is certainty around the link between Cowpea aphid infestation and photosensitisation in horses, the exact cause of the photosensitisation is unknown. Researchers are still working to determine if the accidental consumption of the aphid itself or the consumption of affected feed is the cause of the problem.

What is photosensitisation?

Photosensitisation causes the skin to become overly sensitive to sunlight, with this comes redness and swelling, followed by the development of lesions on the skin. Animals with areas of unpigmented skin, which is typically pink, are at higher risk of developing lesions. This usually occurs under white hair, on the muzzle and limbs.

Affected horses should be removed from infected pastures immediately and veterinary treatment may be required. The lesions are typically slow to heal and result in scarring.

Photosensitisation usually occurs on the limbs in patches of light skin and hair. Photo credit Emma Todd

What can I do to prevent photosensitisation as a result of Cowpea aphid infestations?

Often the best form of treatment is prevention. Most newer varieties of Lucerne are resistant to aphids, including Cowpea aphids – so for those still using older pastures, upgrading pastures should be considered.

Careful monitoring of pastures is recommended for both older and newer varieties. If Cowpea aphid infestations are identified, grazing horses should be removed from the pasture immediately. Cowpea aphids infest isolated areas of pasture, so it is important to regularly monitor different sites within a paddock.

Cowpea aphids on Lucerne. Photo credit Emma Todd

Where can I find more information?

To help breeders understand Cowpea aphids and associated photosensitisation, we have developed a factsheet – Photosensitisation in thoroughbred horses from Cowpea aphid infestations. This factsheet covers off on what to look for in terms of identifying infection and photosensitisation and where to go for help and advice. You can access the factsheet here.

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