Ill-thrift in horses due to suspect plant toxicity

Time and location: October 2019, Longreach Queensland.

Case definition: Horses of any age (from 2 to 16 years) with rough coats and in poor body condition (2-2.5/5), some drag their hind legs with hooves squaring off, and typically run at the tail of the mob. Clinical signs particularly noted after a rain event the previous month. Unaffected horses were bright, had clean coats and were in good body condition (4/5).

Gross findings: Ill-thrift as per case definition. Vitals within normal limits. The horses were vaccinated against tetanus and strangles, had continuous access to arginine lick blocks (to protect against Birdsville Disease) and constant access to hay. The most likely causes were thought to be toxic plants or arboviruses, and bloods were collected from 10 affected horses (@ red, purple, green and grey top).

Laboratory findings: There was no evidence of the following viruses: Equine Infectious Anaemia, Ross River Virus, Murray Valley Encephalitis, Kunjin Virus. CK (creatine kinase) was elevated in all 10 horses, GGT (gamma-glutamyl transferase) in 8 horses, and AST (aspartate aminotransferase) in 6 horses. Eight horses had a mild leucopaenia. While no definitive diagnosis was obtained, ingestion of a toxic plant was considered most plausible.

Animal / management / environment risk factors: Toxic plants known to occur in the area include Indigofera linnaei ‘Birdsville indigo’, rattlepod varieties ‘trefoil rattlepod’, and caustic bush.

Recommendations: The clients were advised to:

• Not shift horses back to the paddock that they had been in for the last 4 months.
• Continue to feed good quality hay and arginine lick.
• Monitor animals for signs of improvement or deterioration.
• Monitor pastures for toxic plants, especially after rain events.

Above: Monitor pastures for toxic plants, especially after rain.