In late April and early May of 2015 there was a significant occurrence of photosensitisation in sheep in the mid Murray Valley of NSW.
The incident seemed to be occur in all flocks within a few days and appeared to be primary photosensitisation without liver damage.
Despite the lack of other pathology up to 15% of some flocks died due to other conditions following the photosensitisation.
No consistent plant population was observed in paddocks with affected sheep and no consistent difference was observed between paddocks with affected and unaffected flocks.
Photosensitisation was diagnosed in sheep by Murray Local Land Services and private veterinarians on 8 holdings between Barooga and Corowa in the mid-Murray valley of NSW and within 40 km of the Murray River. Outside of this approximately 80 km by 40 km area no incidents were reported. Several of the incidents occurred on multiple sites under the same management up to 30 km apart, and anecdotal reports indicated that quite a few other flocks were affected.
Merino, crossbred and Awassi sheep were affected. Adult sheep , unweaned lambs and weaned lambs were all affected. The incidence in affected flock ranged from 50% to 100%. Almost all affected flocks had sheep in adjoining paddocks that had no sheep affected. In one case the affected and unaffected mobs had only been split 2 weeks prior to the event. In two separate cases a single sheep had gotten through a fence into the paddock where photosensitisation was occurring and were affected while the sheep in the adjoining paddock from which it had strayed were unaffected.
Clinically affected sheep had severe inflammation of the muzzle, eyelids, coronets, udder and inguinal area. This inflammation progressed rapidly to scabbing and ulceration and in two flocks there was secondary infection with Staphylococcus aureus causing multiple micro-abscesses of the subcutaneous tissues.
The involvement of the hooves was unusual and severe. Some affected sheep were severely lame, a few walking on their front legs only. On post-mortem examination these sheep had severe inflammation of the sensitive laminae of the hoof.
Another unusual feature of this incident was the relative lack of subcutaneous swelling of the ears although the skin of the ears appeared more fragile than usual.
In the Awassi sheep affected there was clear demarcation between affected white skin and unaffected pigmented skin.
Serum enzymology and histopathology from several of the affected flocks indicated that no liver pathology was involved with this incident.
The progress of the condition varied. In some flocks it progressed though inflammation, ulceration, sloughing and regeneration of the affected skin with little apparent long-term damage apart from some loss of body condition.
In 3 of the 8 flocks investigated 5-15% of the affected sheep died.
This appeared to be a result of the stage of the breeding cycle that the flock was at when the condition occurred.
In flocks of ewes on the point of lamb, some the ewes were diagnosed to have pregnancy toxaemia.
In flocks where the ewes were lambing some young lambs died of mismothering/exposure, presumably because their mothers would not let them suckle their inflamed teats.
In flocks with bigger lambs the lambs also suffered from photosensitization, presumably brought on because their mothers would not suckle them due to the pain in their teats and the lambs then ate the causative substance.
Some sheep died of Staph aureus pneumonia and septicaemia, presumably derived from the subcutaneous micro-abscesses. These deaths stopped following antibacterial therapy.
The area had several significant rainfall events during the summer of 2014-15 and 30-50mm of rain during the second half of April 2015. The April rain was followed by several mild (>20°C) sunny days and the photosensitisation incidents all seemed to occur over a few days a little over a week after the rain event.
There was no apparent difference in plant species between paddocks where 100% of the sheep were affected and adjoining paddocks where no sheep were affected. There also did not appear to be any correlation between the plant species present in paddocks on different properties where 100% of the sheep were affected. All paddocks contained a mix of summer-active native perennial grasses and winter-active introduced annual grasses and legumes. The winter-active annuals had germinated and were actively growing and the summer-active perennials had become active again.
This incident appears to have been a primary photosensitisation caused by rapidly growing plants following significant rain and mild weather.
The lack of any consistent plant species in affected paddock was quite puzzling. There was no single species that was common in all paddocks and the high incidence would imply that whatever caused the condition was quite common within a paddock.
Similarly the apparently random nature of the incidents with similar sheep in similar paddocks near to each other being affected differently is extremely puzzling.
The author has worked as a veterinarian within the region for over 40 years and cannot recall a similar incident.
The Murray Local Land Services animal health team is conducting further epidemiological investigations to further characterise the extent of this incident.