This is a cautionary tale. I was recently called out on a Sunday morning to visit a farm where amongst a mob of late calving cows four had died overnight, and three were looking slow and sick.

At this time of the year of course nitrate toxicity is at the top of our list where sudden death is concerned, but on arriving at the farm and taking a history from the farmer it was clear that it didn't fit the picture here. The cows had been grazing mature pasture onto which they'd been moved 36 hours ago. First idea thus ruled out.

Taking the rest of the history made many other common causes of sudden death unlikely.

Mineral overdose? No minerals had been given recently.

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Toxicity? There was a rhododendron hedge, but it was behind hot wire, and no fertiliser had been applied recently.

Clostridial disease? Possibly, but the only injections given recently had been under the skin, which holds very little risk.

Fog fever? This respiratory disease is caused by a sudden change from poor quality pasture to lush, which hadn't occurred here.

The three sick animals were all scouring and dehydrated, with very large impacted rumens that were not contracting normally. Body temperatures were low, and heart and lungs were normal. No abnormalities were felt on a rectal exam.

The cattle were presenting similar to an acidosis or salmonella case, but the acute nature of the outbreak and the lack of supplementary feed meant neither of these possibilities really fit.

Performing a post mortem exam is always important in cases of sudden death. In this case, results were non-specific.

There was inflammation of the rumen, omasum and abomasum (three of the four stomachs) and the intestines were unusually empty, but with mild inflammation only — not typical of the changes seen with salmonella.

Rumen contents were normal with no toxic leaves seen. The liver and kidneys were normal (changes are often seen here with toxicity cases). Unfortunately I didn't have any tools strong enough to chop through the ribs to access the heart, but I could reach a sample of lung, which looked normal.

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After packaging up samples to send to the lab, I decided to take a quick walk through the paddock just to see if the rhododendron hedge was intact. It was — the hot wire was doing its job. The water supply was clean and non-odorous. There was a bonfire pile in the paddock — with, bingo! — oleander branches thrown on it.

All the signs from the history, clinical exam and post mortem exam fit with oleander toxicity.

These leaves are highly toxic. An animal only needs to consume 0.005 per cent of body weight for fatal effects — people have even been poisoned by having used oleander as hot dog sticks.

The toxin interferes with the sodium-potassium pump of the heart, leading to fatally high potassium levels in the blood, and damage to the heart. Gastrointestinal damage also occurs, leading to scours and dehydration.

Effects usually occur within 0-36 hours of ingestion.

This case was a good reminder of the dangers of throwing garden plants into paddocks. Many toxic plants aren't recognised before it's too late.

If in doubt, get rid of them before your animals do.