A field investigation into a suspected outbreak of pyrrolizidine alkaloid toxicosis in horses in western Queensland.

The study investigates a suspected outbreak of pyrrolizidine alkaloid (PA) toxicosis in horses in western Queensland. The disease, which is rare, caused a significant number of horse deaths during 2010, and the research aims to explain and understand various factors contributing to this situation.

Background and Objectives

• The researchers conducted a field investigation in western Queensland to study a rare outbreak of Pyrrolizidine Alkaloid (PA) toxicosis, a disease in horses.
• Approximately 35 out of 132 horses pastured on five properties on the Mitchell grass plains in western Queensland died in the first half of 2010.
• The main objective was to identify the causes of this outbreak and understand the circumstances that led to it.

Methodology and Results

• The researchers found symptoms consistent with PA toxicosis in the affected horses. They also found the presence of a local strain of Crotalaria medicaginea, which is a hepatotoxic plant in the pastures.
• Using the departure and arrival dates of two brood mares, the researchers determined that the horses were most likely exposed to the alkaloid before the rainy season.
• All affected properties experienced very dry springs and early summers, preceded by a significant wet season. The disease outbreak peaked suddenly in March, with a gradual decline in mortality rates towards the end of June.

Findings

• The estimated morbidity rate was quite high at 76%. Mortality rates differed among the horses, with younger horses and brood mares experiencing higher mortality rates of 67% and 44% respectively, compared to an average of 27%.
• Logistic regression analysis revealed that young horses and brood mares and those grazing on bare pastures in December were more likely to die. However, horses that were fed hay or grain-based supplements were less likely to die.
• Emphasizing on the environmental conditions, the study found that the dry weather and overgrazed pastures were significant risk factors for the disease. The researchers also found that supplementary feeding could help reduce the risk of PA toxicosis.

Significance and Conclusion

• This research contributes significantly to understanding the outbreak of PA toxicosis in central western Queensland – its first detailed study. The study’s first-hand insights also set a benchmark for future investigations.
• It was successful in providing evidence that environmental determinants, including weather conditions and pasture quality, contribute to disease mortality.
• Important conclusions drawn include recognizing the crucial exposure period towards the end of the dry season and the protective nature of supplementary feeding.
• The research also highlighted that overgrazing and nutritional deficits in the horses’ diets are risk factors for the disease.