During July and August in 2006 I investigated two cases of significant mortality in sheep, following good rain which fell after a protracted dry spell, near Copeton dam, Inverell, NSW. Sheep of all classes died rapidly as they were mustered and handled during shearing. In the first case 100 of 1800 sheep died. There was also a history of spontaneous leg fractures associated with minimal handling and restraint. Some sheep were homebred and others had been purchased some weeks previously.
The country is quite hilly and rough with sandy, “trap rock” soil and abundant rock fern (Cheilanthes tenuifolia), mature natives and speargrass. The soil, described as a granite loam type was similar on the both properties. The soil on one property was reported to be low in copper and potassium.
Feed quality and quantity was poor on large areas of both properties. The sheep had been regularly drenched. Sheep on one property were not supplementary fed and on the other were given barley and calcium blocks prior to shearing.
Some sheep were seen with a fever, epistaxis and a rapid respiratory rate after handling. No sheep showed signs of diarrhea. The owner reported that sheep collapsed in shearing shed and that some recovered if left alone. One owner also reported blood from the eyes.
A lamb in poor body condition was post-mortemed. The carcase was anaemic and the lungs appeared consolidated and “oozed brownish blood.” Red stalks consistent with rock fern were found throughout the rumen.
Pathology findings included mild anaemia, blood in the urine and elevated ALP, AST, CK, urea, and creatinine. Anthrax and TSE were excluded.
Very poor nutrition probably contributed to the disease syndrome described above. The photograph (below) of a typical paddock in this region shows there were few nutritious plant species in the pasture. This could account for the spontaneous leg fractures (as could copper deficiency).
Sheep seem to be much more resistant to the radiomimetic effects of ptaquiloside than cattle. In these two cases, rock fern was found in the stomachs of all sheep autopsied. The exercise intolerance, haemorrhagic disease and mild anaemia reported is consistent with the disease as described in sheep (Beckett, 1984).
Histology was suggestive but not conclusive of rock fern poisoning. Anthrax was excluded on direct examination of peripheral blood smears.
Owners were first advised to move the sheep to a paddock with no or minimal fern. If this was impractical an alternative suggestion was to graze fern infested paddocks for ten days and then graze fern free paddocks for a minimum of twenty one days. Supplementary feeding with cottonseed meal was also recommended. Losses were minimized when ewes were immediately put onto oats. Owners were also advised to minimize the stress of exercise and handling by only penning a few sheep at a time and where possible delaying handling until the sheep recovered with the help of improved nutrition and removal from fern infested paddocks.
Beckett RJ (1984) The effects of rock fern on sheep and cattle. PhD Thesis, University of Sydney