Cotton Fireweed has not previously, to my knowledge, been definitely incriminated in stock losses, but it has been associated with cattle mortalities on four properties in the Upper Hunter and Merriwa Pastures Protection districts between October, 1964, and February, 1965. Although the plant has been known on these properties for some years it has apparently made very prolific growth only recently in response to top-dressing with super, and/or gypsum, a characteristic of this plant. Local conditions, such as heavy stocking rates over a dry winter and scarcity of spring feed, were partly responsible for forcing the cattle on to this otherwise unpalatable plant.
Cotton Fireweed has been known for some ten years to contain pyrrolizidine alkaloids (Culvenor, 1955), some of which are well known for their specific hepatotoxic action, and which are also found, for example, in Heliotrope, Paterson's Curse, Ragwort and Crotalaria species.
Affected cattle have varied from six-months-old calves to aged cows, and all were bred on the properties concerned. On one property over 52 of 400 head were lost, including a number of aged cows, while on another property 12 yearling cattle died over a period of two months. On a third property 16 six- to 24-months-old animals died out of a total of 1400. While the latter two mortalities are not perhaps spectacular, they did occur within quite a limited period. Sheep are apparently not affected, or no reports have been received, but we have heard a report of photosensitivity in horses in the district which could be related.
Clinical observations, gleaned from field reports, have included the following comments. There may be loss of condition in affected animals over two weeks to two months, with or without a chronic scour, animals were sometimes noticed to be tucked up and to show low exercise tolerance, unsteadiness of gait and partial blindness. At least one animal was found prostrate with opisthotonos and blind in one eye, while others were merely found dead. Suggested diagnoses early in the piece ranged from anthrax and enterotoxaemia to encephalitis and pneumonia, and it is interesting to compare this situation with the report on Heliotrope poisoning in cattle in 1960 by Bull, Scott-Rogers, Keast and Dick (1961), where initially seven of 61 introduced steers were found dead on a property. Anthrax was suspected but not confirmed at V.R.S., but vaccination and quarantine measures were nevertheless carried out. Twenty-four of the 61 steers eventually died, and it was found that cattle might die within a month or two of starting to graze the plant. Inco-ordination of gait and scouring were observed in affected animals.
To go back to Cotton Fireweed poisoning, post-mortem findings included varying amounts, up to two gallons, of clear fluid in the abdominal cavity, with massive, varying to slight, oedema of the mesentery, lymph nodes and alimentary tract in general. Livers generally appeared dark with rounded edges and were very firm and tough to cut, but were not noticeably reduced in size. Jaundice was not reported. In several cases subserous petechial and ecchymotic haemorrhages were present, particularly over the spleen, kidneys, and gastro-intestinal tract, and were sometimes also seen in the thoracic cavity and heart.
The diagnosis was based on the histopathological findings in the livers from these animals, which consisted of varying degrees of megalocytosis, in some cases affecting the whole lobule throughout the liver, with degrees of perilobular and intralobular fibrosis and collagen formation, and degrees of fibrous tissue obliteration of the central and sub-lobular hepatic veins. In some cases the remaining parenchyma consisted of islands of megalocytes embedded in proliferating fibrous tissue and bile ductules. In other cases fibrosis of the central vein was the most prominent feature, megalocytosis being minimal or barely apparent. This is reminiscent of liver changes seen in Jamaicans subjected voluntarily to prolonged exposure to "bush teas" prepared from infusions of Crofalaria spp. (Mclean et al., 1963).
These liver changes, in particular the megalocytosis, are well recognised as the pathognomonic manifestation of subacute to chronic pyrrolizidine alkaloid hepatosis, and circumstantial evidence pointed to the involvement of Cotton Fireweed with its known content of pyrrolizidine alkaloids.
The only other constant finding was severe vacuolation throughout the white fibre tracts of the brain, which was seen in one or more animals from each property. There is apparently no demyelination, and no accumulated substance is demonstrable in the vacuoles in routine haemotoxylin and eosin stained sections or frozen sections of fixed brain subjected to various staining procedures (Carne, pers. com., 1965). Frozen sections of unfixed brain will be examined when available. The significance and pathogenesis of these changes is not understood, and although signs of central nervous system disturbance are recorded in ragwort poisoning and other conditions where liver dysfunction is a feature, no lesions have yet been described. The central nervous system signs could be ascribed to the accumulation of ammonia in the blood-stream as a result of liver dysfunction and failure to form urea.
The liver dysfunction and in particular the failure to form urea (Bull, 1961) raises the possibility of increased susceptibility of animals exposed to pyrrolizidine alkaloids, to poisoning from urea supplements.
Control of Cotton Fireweed poisoning is difficult as the plant is widespread on properties, responds to top-dressing and is not easily eradicated by the usual methods such as hormone spraying and cultivation.
A trial has been set up on the worst-affected property to investigate the possible protective effects in cattle of cobalt administration. The trial is suggested by Gregory's work which indicates the formation of a non-hepatotoxic derivative of pyrrolizidine alkaloids in vitro in rumen liquor in the presence of adequate vitamin B., (Gregory, 1963). The trial consists of a group of 25 yearling cattle, each with two cobalt bullets, running together with a control group of 25 comparable untreated yearlings on heavily infested country. These will be observed for clinical evidence of hepatosis and will finally be slaughtered and examined for evidence of liver changes.
The field observations were made by C. F. Cargill, Veterinary Inspector, Upper Hunter and Merriwa P.P. Districts, and B. W. Bootes, District Veterinary Officer (C).