Myths about the number of casualties due to Ragwort poisoning.

By: Esther Hegt and Dr. Pieter B. Pelser ( University of Canterbury- Biological Sciences, Christchurch, New Zealand)

In order to assess the impact and scale of the Ragwort problem, it is important to find out how many animals per year become victims of Ragwort poisoning. Because only a post mortem can show if an animal has died from Ragwort poisoning, and this is only rarely performed (Gezondheidsdienst voor Dieren, pers. comm.) because it is assumed to be very expensive, there is a lot of speculation in de media (1, 2) and on internet (3, 4) about the number of casualties. Unfortunately, these estimates are often inaccurate and are therefore giving a misleading view of the significance of the Ragwort problem. A nice example of an estimate of the number of casualties under horses resulting from an incorrect, scientifically flawed extrapolation of data can be found on the internet. Several websites report that 6500 horses died of Ragwort poisoning in the UK in 2002 (5). This number turns out to be obtained from a questionnaire sent to all veterinarians that are members of the British Equestrian Veterinary Association. Only 4% of these veterinarians completed the questionnaire. One of the questions that the veterinarians were asked was how many cases of suspected or confirmed Ragwort poisoning were treated. This resulted in 283 reports of SUSPECTED poisoning. This number was subsequently extrapolated to all members of the association: if 4% equals 283 casualties, 100% (if all veterinarians would have completed the questionnaire) would equal 7075. On first sight, this may seem to be a valid conclusion, but unfortunately it is very dangerous to extrapolate this small sample. Especially in this case, because a veterinarian is more inclined to complete such a questionnaire if he/she treated cases of Ragwort poisoning him/herself. This is confirmed by the high percentage of veterinarians (89%) that indicated to have treated suspected (89%) and confirmed (75%) cases. In addition, the possibility cannot be excluded that several veterinarians reported the same case, and therefore casualties were counted more than once.
The only scientifically sound conclusion from this is that in 2002 283 horses presumably died of Ragwort poisoning. It is not known in how many of these cases a post mortem is performed.
The dangers of this incorrect estimate are clearly visible on the internet. A search with Google results in many websites that copy the estimate of 6500 casualties under horses without thinking about it twice and even present this as a fact. Verifiable source information is frequently not even given (6, 7). Also in the Netherlands, because of a lack of verifiable data, numbers of casualties are presented that are not substantiated by any data. The Dutch PETA, for instance, reports hundreds of victims per year (4), although they informed us that they do not have any post mortem reports on file and have not received any reports of confirmed illness or casualties due to Ragwort poisoning.

Suspicion is not enough evidence to prove Ragwort poisoning. Often a combination of observing symptoms of liver failure and finding Ragwort plants in a field or in hay is regarded as conclusive evidence of Ragwort poisoning, but this is, of course, very circumstantial. Ragwort poisoning does not result in symptoms that are very specific and these may have causes other than Ragwort poisoning (such as birth defects, liver infections induced by bacteria, parasites, viruses, or other toxic compounds; 8). Also a liver biopsy cannot prove without doubt that liver failure is caused by Ragwort poisoning, although it may show a pattern of liver damage that excludes many other causes. Another factor that should be taken into account when verifying Ragwort poisoning is that Common ragwort is frequently confused with other yellow flowering plant species (9). Clearly, there are many incorrect estimates of the number of Ragwort victims. The number of casualties may be higher or lower than these. We simply don’t know. And that’s why it is so important that animal owners have a post mortem performed when they suspect Ragwort poisoning.

References:

(1) Leeuwarder Courant 20-7-2005
(2) Leeuwarder Courant maandag 2-8-2004
(3) ANP Perssupport
(4) Landelijke Inspectiedienst Dierenbescherming. 2006. Inspectiedienst waarschuwt voor Jakobskruiskruid.
(5) Equiworld Horsemagazine British Horse Society Ragwort survey reveals disturbing new figures on horse fatalities.
(6) Weetjes Kruiskruid
(7) Bokt Paardenforum
(8) DEFRA/AHT/BEVA. 2004. Department for Environment, Food and Rural Affairs Engeland. Equine Quarterly      Disease Surveillance  Report. Pilot Issue: Focus on equine liver disease 7-9.
(9) Alarm om opmars Jakobskruiskruid Telegraaf 23-07-2007