Vetch is a plant that has often been sown in pastures for its high nutritional value and legume properties or between cropping rotations. There has been limited published data and minimal cases of vetch toxicosis reported in Australia thus far, despite numerous cases detailed overseas. It is likely that vetch toxicosis occurs sporadically; however, due to the vast nature of Australian agriculture these losses are in all likelihood simply not investigated.
This report details a case of vetch toxicosis in Angus heifer weaners and Angus cows grazed on mixed vetch pastures.
Three separate mobs of Angus cattle of various ages were inspected on 17 October 2012. The first mob of 18 heifers contained two affected animals. This mob was situated 5kms from the other two mobs and was grazing on native pastures and vetch. The second was a mob of 70 cows and their calves, of which five cows were affected. These were grazing on a lucerne-based pasture but had previously been on vetch and wheat pastures. The third mob contained 20 cows with calves at foot and had no cases. This mob had no exposure to any pastures containing vetch.
A second visit to this property was required just over 12 months later on the 12 December 2013. At this time three animals from the originally affected mobs were again displaying similar clinical signs. The cows had been grazed on the same pasture as last year, but it was thought that no vetch remained in the pasture.
At the time of the first visit in 2012, two weaner heifers in a mob of 18 were found to have a ventral pruritus, reported to have appeared over the last week or so (Figure 1). They were treated by the owner two days prior to the visit with fenthion (dose not specified; Tiguvon, Bayer), ivermectin back line (dose not specified; Ivomec Pour-on, Merial), diazinon (dose not specified; Coopers Diazinon, Coopers Animal Health), and oxytetracycline (dose not specified; Alamycin, Norbrook). The owner believed the affected animals responded to a degree.
In the second mob of 70 cows with calves at foot (running with the bull), five cows were found to have an intense ventral pruritus. It was reported to be worsening over roughly two weeks. These animals had not received any treatment at the time of the visit (Figure 2).
At the time of the second visit in December 2013, one of the three affected cows showed particularly bad lesions and was in relatively poor body condition at score 2.5. She appeared to be quite unwell, but vitals were normal on clinical exam. All of the affected animals had been treated with penicillin and ketoprofen injections (further details not provided) roughly one month prior to the visit. Whilst they had shown a favourable response to therapy, the lesions had not cleared.
On the first visit, two skin scrapes were taken from one cow and two skin scrapes and a partial thickness skin biopsy were taken from a second cow. Both animals were in the mob of untreated affected animals. The histopathology report showed a granulomatous, eosinophilic dermatitis, with no evidence of mite infection. The skin biopsy showed infiltration of lymphocytes, plasma cells, eosinophils and multinucleated giant cells.
A private vet from Griffith Vet Hospital had also been involved with this case, and took samples for histopathology. Results showed a hypersensitivity reaction with no evidence of mites.
On the second visit in 2013, two full thickness skin biopsies were taken from two separate lesions on the worst affected cow. Histopathology revealed a mixed perivascular dermatitis suggestive of vetch toxicosis.
The private vet also took blood samples for trace elements and biochemistry, all of which came back within normal limits.
Blood samples were taken from the most severely affected animal in 2013. Changes seen were mild and non-diagnostic.
Differential diagnoses included:
Following the second visit and review of the laboratory results it was recommended that the cattle be treated with a dexamethasone injection.
Vetch is a plant that has often been sown in pastures for its high nutritional value and legume properties or between cropping rotations. There are several different species of vetch. The main two that have been found to cause toxicity are woolly pod and hairy vetch. It has been found that three different syndromes result from ingestion of different parts of the vetch plant . Decreasing in severity, the first group of clinical signs appears after ingestion of the seed and results in acute nervous derangement and sudden death. The second syndrome results in subcutaneous swelling of the head, neck and body and respiratory signs. The third syndrome is characterised by dermatitis, diarrhoea and systemic granulomatous disease and most commonly occurs following ingestion of a mixed pasture of vetch . This third syndrome is most similar to the history and clinical signs seen in this case.
Clinical signs of vetch toxicity generally appear 2-6 weeks after exposure, regardless of removal from the original source. It is thought that duration of exposure or repeated exposures impact on the severity of disease more so than total quantity of plant ingested .
Toxicity resulting from the consumption of vetch is caused by prussic acid, however the pathogenesis remains largely unknown. It is thought that following ingestion of one or more substances within the vetch plant that are absorbed as haptens or complete antigens, antigen specific lymphocytes are formed and distributed systemically. Subsequent exposure to the antigens is then thought to evoke a type IV hypersensitivity reaction and granulomatous response as well as multinucleated giant cell formation, indicating that one or more constituents of this process is a persistent reactive substance. An alternative theory is that vetch lectins may directly activate T cells leading directly to granulomatous inflammation through lymphokine production and cytotoxicity [1,2].
Whilst the combination of lesions seen with vetch toxicoses is fairly distinctive, there is no definitive diagnostic test available and a diagnosis cannot be made on skin biopsy alone. A diagnosis by exclusion must be made after review of the herd history and the character and distribution of any lesions present . Confirmation of a diagnosis of vetch toxicosis can be aided by histological examination of other organs at the time of necropsy; however, in this particular case no animals died and therefore necropsy was not possible.
Before the vet was called out to this case, the affected weaner heifers were treated with a wide range of products including antiparasitics and antibiotics. While the owner believed the heifers were responsive to therapy, it is difficult to determine whether their health was improved due to treatment of the underlying cause or simply resolution of any secondary bacterial infection. Furthermore, vetch toxicity in animals less than three years of age has been reported to be much less severe than that seen in older cattle . It also made it impossible to obtain any accurate samples from this mob and hence samples were taken only from the older cows.
No definitive diagnosis was ever confirmed for this case, but given the clinical signs and history other differentials were able to be excluded. Discussion with a pathologist at EMAI revealed the cell types present in the skin biopsy were highly suggestive of vetch toxicosis. A contact allergy or other hypersensitivity would have shown a different cell mixture. Furthermore, all affected animals had access to pastures containing vetch. While no ectoparasites were seen in the skin sections examined, they cannot be ruled out entirely. Angus and Holsteins are reported to be the most susceptible to this type of toxicity, especially those greater than three years old .
Treatment options in this case were relatively limited. In fact, there is minimal data published on cases, especially in Australia. It is possible that given the extensive nature of many Australian farms, sporadic losses due to vetch toxicity have simply not been investigated . Removal from the source of toxin must be immediate, although clinical signs are likely to develop regardless. Avoiding any future exposure to vetch is also important, given the proposed pathogenicity. Furthermore, assuming the pathogenesis of vetch toxicity involves a hypersensitivity reaction, treatment with corticosteroids may be expected to help; however, this treatment option has not been shown to be effective .
The second outbreak seen in 2013 was also thought to be caused by vetch toxicosis. Whilst the owner felt there was no vetch left in the pasture, the animals had previously been exposed and therefore a smaller amount of ingested vetch would have been enough to cause a more significant reaction.
Griffith Vet Hospital and Mel Gabor and the EMAI staff who aided in collection of samples and diagnosis in this case.