Deterioration in condition of several horses and ponies on different properties in a district.
Above: Case 5 had lost weight and was lethargic, ataxic and jaundiced.
Time and location: February to July 2022, north Queensland.
Case definition: Horses or ponies that are dull and lethargic, typically developing neurological signs within a few days, leading to recumbency and euthanasia.
Disease mapping: Over half a year, the vet attended six mature (mostly older) horses and ponies on different properties across a region became unwell, ataxic and deteriorated rapidly. One horse had visibly jaundiced sclera and oral mucus membranes. There were also other cases with similar symptoms that were not seen by vets. All horses had been in their paddocks for years, some being supplemented with feed. One had been put in a new paddock a week before becoming ill.
Gross findings: Bloods were collected for serology and biochemistry from the cases and some paddock mates that appeared clinically normal. Two post mortems were performed (others were not performed either because the horse was not HeV vaccinated or the owner chose to euthanase it without examination). The gross finding in both horses was diffuse jaundice of the carcasses and abnormal, shrunken livers with ‘nutmeg’ patterning on cut surface.
The differential diagnoses considered initially were: Hendra – especially given a low vaccination rate in the area; various Flaviviruses – Murray Valley Encephalitis, Kunjin, Japanese Encephalitis; and plant poisonings.
Laboratory findings: Histopathology showed hepatopathy (chronic, diffuse, severe with marked fibrosis) and nephropathy, both consistent with pyrrolizidine alkaloid toxicity.
Test results from blood samples on all cases were negative for Hendra Virus (PCR). Tests for other viruses (not all run on all cases) were also negative and included: African Horse Sickness, Alphavirus (Ross River virus), Australian Bat Lyssavirus, Equine Herpes Virus 1, Flavivirus (Japanese Encephalitis, Kunjin/West Nile, Murray Valley Encephalitis), Eastern Equine Encephalitis Virus, Venezuelan Encephalitis, Western Equine Encephalitis. One case was positive for JEV Flavivirus – but not high enough to be decisively diagnostic without paired samples or CSF.
Biochemistry on all cases and two clinically normal paddock mates indicated liver dysfunction, with markedly elevated GGT (5-15 times normal upper range of 40 IU/L). One month later one of the paddock mates progressed to become a clinical case. The diagnosis of pyrrolizidine alkaloid toxicity cannot be definitively made in all of these cases but the majority fit the clinical presentation.
Animal / management / environment risk factors: Unlike many toxic plant presentations these cases seem to primarily be horses that have lived in the same paddocks their whole lives, suggesting chronic exposure played a significant role. No cases were able to be linked to grazed plants know to contain pyrrolizidine alkaloids, possibly because of a relatively long delay between consumption and clinical disease. There did however seem to be a significant increase in incidence over the previous 18 months. Environmental risk factors could have been poor seasons and reduced alternate fodder, or the severe flooding in 2019 leading to weed growth and spread. Crotolaria dissitiflora is a common cause of pyrrolizidine alkaloid toxicity and has been identified in the area, so may have been the plant involved.
Recommendations: The disease investigation ruled out important zoonotic disease differentials. Having a diagnosis has enabled client education on weed awareness and provided a guide for prognosis in other cases.
Below: Liver enzymes were elevated in Case 6 and in clinically normal paddock mates.
Below: Nutmeg patterning of the cut surface of liver seen at two post mortems.