A number of species of Vicia (vetch) are naturalised in Australia, of which the pasture species, Vicia villosa spp dasycarpa (woolly pod vetch), Vicia benghalensis (popany vetch, purple vetch) and Vicia satvia (common vetch) are known to cause toxicoses in cattle (McKenzie 2012, Harper et al 1993, Peet and Gardner 1986, Peake et al 2015). Numerous cases have been reported overseas (Enneking 1995, Panciera et al 1992, Oruc et al 2011). Australian cases have been reported from the North Coast (Harper et al 1993), Riverina (Peake et al 2015) and North West (B McKinnon, T Irwin and S Slattery, personal communication) regions of NSW and also from Esperance, Western Australia (Peet and Gardner 1986).
This report describes a case of vetch toxicosis in six Angus and Murray Grey cows in the Tamworth district of North West New South Wales (NSW). The cows were grazing a mixed pasture where woolly pod vetch (V villosa spp dasycarpa) was the predominant species, constituting around 90% of the pasture sward. The cattle were placed on the pasture as the producer was unaware of any issues with grazing vetch.
Two separate mobs of 38 and 43 (81 in total) mixed age (four to six years) Angus and Murray Grey cows and calves grazed two separate pastures of predominantly woolly pod vetch from early September to late October 2017. The pastures (Figure 1) were located on two separate properties in close geographic proximity, sown at the same time (April) and grazed at the same time. There was no history of these animals grazing vetch in previous years.
At the time of pregnancy testing by a private practitioner in mid-October, seven cows (three in one mob, four in the other) were noticed to have a generalised dermatitis and mild puritis (morbidity rate of 8.6%). Treatment with an antiparasitic preparation was undertaken. The case was referred as suspected vetch toxicity to the District Veterinarian.
The affected cattle were initially in good condition (BCS 3-4). The owner of the cattle noted that the onset of signs coincided with pod formation and seed setting in the plant. The first sign noticed was mild dermatitis that progressed to affect a greater proportion of the head, neck and body (Figures 2-4). Concurrently the cattle lost weight, developed loose stools and became lethargic, followed by recumbency and eventually death. The cattle were moved off the vetch pasture immediately after dermatitis was noticed however the disease continued to progress.
Six of the affected cows died (case fatality rate of 86%). The time between skin lesions being noticed and death varied, the first cow dying in two to three days and the sixth cow dying within six weeks. One cow gradually recovered over a period of months. The calves and a mob of Dorper sheep grazing the same pasture were unaffected.
Two affected cows, one Angus (Figure 2), one Murray Grey, were examined from a distance in the paddock. They were in poor body condition (BCS 1) with moderate to severe generalised patchy alopecia and skin thickening giving a pleated appearance on the neck. Both cows were bright and alert. Loose stools were noted in the cattle yards. These cows both died at a later date and were not necropsied.
Examination of the pasture revealed it to be approximately 90% woolly pod vetch, in flower with green and dry seed pods (Figure 1). The owner of the cattle reported that it had consisted of a mix of grasses and clovers when it was actively growing, but the vetch had overtaken the other species.
One affected cow was presented for necropsy after she became recumbent and was euthanased by the owner. The cow was in poor body condition with severe generalised dermatitis affecting most of her body, worst on the head, ventral abdomen and perineum (Figures 3, 4, 5). The skin exhibited patches of alopecia with crusting and thickening (Figure 6) and a pleated appearance (Figure 2) with abrasions where rubbing had occurred.
Internally there was marked jaundice. The skeletal muscle in multiple sites exhibited multifocal, pale yellow 1-2mm foci disseminated evenly through the muscle body (Figure 7). The heart was of normal size with multifocal pale yellow irregularly round and slightly raised 1-2mm foci distributed across the epicardium. These extended into the myocardium but were not full thickness (Figure 8). The caudal lobe of the left lung was pale yellow and mottled in colour, spongy to touch with areas of emphysema (Figure 9). Both kidneys (Figures 10 and 11) were enlarged and pale. The renal cortex was pale brown with multifocal irregularly circular to elongated raised 1-2mm foci. The medulla was bright purple-red in colour. The renal pyramids were bright yellow in colour. Both adrenal glands were enlarged and dark purple-brown in colour. The liver was enlarged with rounded edges and was mottled tan to dark purple. The intestinal mucosa was thickened.
Biochemistry showed marked elevations in AST and CK, mild elevation in GGT, GLDH and bilirubin and mild decrease in serum protein. Haematology was not conducted as the blood was collected post-mortem. Biochemistry and haematology was undertaken on the one recovered cow four months after the event with no abnormalities detected.
Sections of skin, lung, heart, liver, spleen, skeletal muscle, kidney, adrenal gland, mesenteric lymph node, small and large intestine and abomasum were submitted to the Elizabeth McArthur Agricultural Institute Veterinary Diagnostic Laboratory (EMAI VDL), Menangle for histopathology. Blood and aqueous humour were also submitted along with the brain and fresh spinal cord for TSE exclusion.
All organs except the spleen and small intestine had marked infiltrations of macrophages, eosinophils, lymphocytes and plasma cells. The kidney and liver also contained multinucleated giant cell macrophages. The spleen contained infiltrations of macrophages as the only cell type and the small intestine lacked macrophages in the infiltrations. The skin and liver additionally contained neutrophils. The multi-organ distribution of granulomatous infiltrate is consistent with vetch toxicosis.
Cattle affected by vetch toxicity typically show clinical signs including dermatitis, pruritus, diarrhoea, weight loss and death (Harper et al 1993, Panciera et al 1992), as seen in this case. Most have skin lesions affecting pigmented and unpigmented skin that progress from multiple circumscribed foci to irregular patches of alopecia with thickening and abrasion (Harper et al 1993).
Morbidity in this case was 8.6% with other cases reporting from a single animal up to 30% of the herd affected. The case fatality rate in this case was 86%, with other cases being reported as 50% to 69% (Peet and Gardner 1986, Harper et al 1993). The disease is more severe and prevalent in cattle older than three years of age with Angus, Angus derivative and Holstein breeds over-represented (Panciera et al 1992), as was found in this case with adult Angus and Murray grey cows affected, the calves remaining unaffected.
Cases of vetch toxicity present on necropsy as systemic granulomatous disease affecting multiple organs including skin, skeletal muscle, lymph nodes, adrenal glands, kidneys, heart, liver and spleen. Generalised lymphadenomegaly, marked splenomegaly and hepatomegaly are evident (Panciera et al 1992). Multifocal pale foci have been reported affecting the kidney capsular surface and cortex (Harper et al 1993, Panciera et al 1992) and on epicardial, endocardial and cut surfaces of the myocardium (Panciera et al 1992, Harper et al 1993). Histologically, there is a characteristic granulomatous eosinophilic inflammatory cell infiltrate (monocytes, lymphoplasmacytic cells, occasional multinucleated giant cells, and eosinophils) of multiple organs (Peet and Gardner 1986, Panciera et al 1992). The findings in this case were consistent with these previous reports.
Toxicity associated with consumption of V. villosa has occurred in cattle that have consumed pure vetch seed as well as in animals grazing vetch pasture. A winter growing annual, from late spring to early summer vetch produces abundant small purple flowers, then pods containing two to five seeds, (Matic et al 2008, NSW DPI 2018). Cattle grazing pasture will consume seeds from late spring onwards. Vetch seed is generally considered to be toxic and it is recommended that it should not be fed to any livestock (Matic et al 2008). Claughton and Claughton (1955) reported acute nervous signs (bellowing, repeatedly getting up and down, convulsions) and death in cattle that consumed pure hairy vetch seed. A syndrome of respiratory signs, subcutaneous swelling of head, neck and body and eruptions in the oral mucosae was reported associated with consumption of pasture (Volker and Frohner 1950 as cited in Panciera et al 1992). The most commonly reported syndrome of dermatitis, puritis, diarrhoea, illthrift and death, as seen in this case, follows grazing of vetch pasture (Panciera et al 1992, Peet and Gardner 1986, Harper et al 1993).
The specific mechanism of V. villosa toxicity is unknown (Oruc 2012, Panciera et al 1992). A number of hypotheses have been reported including a type-IV hypersensitivity reaction and granulomatous response (Panciera et al 1992), direct activation of T lymphocytes by vetch lectin initiating a cellular response (Panciera et al 1992) and canavanine toxicity (Enneking 1995).
Epidemiological factors in combination with the histologic lesions and the cell types involved provide evidence for a type-IV hypersensitivity reaction being a major component of the host response (Panciera et al 1992). In addition V. villosa seed contains a lectin that binds specifically to mouse cytotoxic T lymphocytes (Tollefsen and Kornfeld 1983). It has been proposed that this leads to lymphokine production, cytotoxicity and a granulomatous inflammatory response (Panciera et al 1992).
V. villosa and V. benghalensis also contain canavanine, a toxic amino acid with known antifeedant activity (the production of an organic compound which protects the plant from attack by grazing or insect activity). The content of canavanine increases greatly in the plant with formation of pods. Canavanine toxicity gives rise to symptoms similar to those seen in vetch affected cattle such as autoantibody formation, nephritis and alopecia (Prete 1985, Thomas and Rosenthal, 1987a as cited in Enneking 1995).
Given the extensive use of vetch as a pasture source for cattle in Northern NSW (S Boschma, personal communication) the incidence of disease remains low (Harper et al 1993). There is no known treatment and the case fatality rate is high despite removal from the source.
Given the level of toxicity of the seed, grazing management appears to be an important factor in prevention of the disease. The current recommendation (Matic et al 2008) is to only allow cattle to graze Woolly Pod Vetch after it is higher than 50cm and before flowering. It would appear that the pasture is safe for sheep as none of the sheep grazing in this case were affected and there are no reported cases of toxicity in sheep.