Flower of a hyssop loosestrife plant, Lythrum hyssopifolia.
By Liz Hancock, district veterinary officer, Warrnambool
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Most producers would agree that 2025 was a challenging year for feeding livestock. With limited pasture availability, many animals were grazed in unusual locations and on unconventional forage sources.
It’s often in years like this that unusual disease events occur — as was the case for one unfortunate south-west dairy farmer who experienced suspected lesser loosestrife toxicity.
In autumn 2025, after the afternoon milking, the farmer placed 120 mixed‑age dairy cattle onto the only green feed available — a dried‑up swamp.
This area was usually underwater and had never previously been grazed.
The following morning, when the farmer went to collect the cattle, he found 12 animals dead.
The remaining cattle ranged from slow and trembling to apparently normal.
The herd was immediately moved to a known safe paddock and provided with ad‑lib hay and lick blocks, and local veterinarians were called.
Despite prompt veterinary treatment, the affected animals failed to respond as expected. With no improvement seen, the Victoria district veterinary officer was contacted, and a Significant Disease Investigation was opened.
The local DVO and an animal health officer attended the property promptly. Necropsies were performed, blood samples collected and epidemiological information gathered, including identification of plant species present in the swamp.
A local agronomist confirmed the presence of lesser loosestrife (Lythrum hyssopifolia), an annual spreading herb commonly found in damp or seasonally flooded areas such as swamps.
Map showing the distribution of lesser loosestrife (Lythrum hyssopifolia) in Victoria. Source: VicFlora, Australian Virtual Herbarium. VBA, ALA.
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The plant has small, narrow leaves resembling rosemary, and produces pink, blue, or purple flowers, although flowers were no longer present at the time.
Samples were taken to the AgriBio laboratory in Melbourne for urgent analysis. The following day, results confirmed lesser loosestrife toxicity.
In total, 70 of the 120 cattle died, most within the first 24 hours of exposure.
Lesser loosestrife wasn’t identified in the rumen contents of the deceased animals so it couldn’t be definitively confirmed as the cause.
However, the epidemiology, clinical signs and pathology findings strongly suggest it was responsible.
The 50 cattle in the herd that survived have not since shown any clinical signs.
If you notice sudden deaths, significant illness or unusual clinical signs in livestock, contact your local veterinarian, DVO, or the Emergency Animal Disease Hotline on 1800 675 888.
If you see unfamiliar or unusual plants in your paddocks, seek advice from your local agronomist.
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