Can urine testing aid the early diagnosis of Grass Sickness?

Constanze Fintl, Bruce McGorum (Royal (Dick) School of Veterinary Studies) Elspeth Milne (Scottish Agricultural College, Veterinary Science Division, Dumfries)

Unfortunately, there is currently no definitive ante-mortem diagnostic test for grass sickness. Consequently the diagnosis is based largely on recognising clinical signs that are typical, but not specific, for this disease, and by the elimination of other diseases which cause similar clinical signs. Therefore a test to definitively confirm the diagnosis, ideally early in the disease course, would be of great benefit. Previous studies have investigated the value of blood and peritoneal fluid analysis as diagnostic aids for grass sickness. While it was shown that affected horses have altered levels of some blood and peritoneal fluid constituents, as these changes do not occur consistently and uniquely in grass sickness, they cannot be used to definitively diagnose the condition.

In 1999, Marrs and colleagues, reported that analysis of urine may be of value in the early diagnosis of grass sickness. They found that urine from affected horses had increased specific gravity (an indication of the degree of concentration), and increased glucose, protein and acidity. However this study was subsequently criticised by Kerr (1999) because very few affected horses (only 4) were studied, and the method used to analyse the urine was not optimal. Thus, while Marrs and colleagues raised the exciting possibility that urine analysis could provide an early method to diagnose grass sickness, further work is needed to determine whether their observations hold true when a larger number of horses is studied, and when more accurate analytical methods are used.

The Bossy Boots Memorial Fund has funded a study, commencing in the forthcoming spring, to further evaluate this technique. Urine samples will be collected from a much larger number of horses with grass sickness. In addition urine samples will be collected from those healthy horses that are co-grazing with affected horses. These latter samples are very important since they represent control samples from horses on a similar feeding and management regime to the affected horses. Unfortunately urine sample collection from horses is not without its pitfalls – indeed those readers with previous experience of attempting to collect samples of their horse’s urine will understand that considerable perseverance is usually required! One trick worth trying is to give the horse gentle exercise and then return it to a freshly bedded stable, and shake the bedding around – this often stimulate horses to urinate. Urine should be collected into a clean plastic container that contains no traces of food, sugar or chemicals. As some horses will stop urination if disturbed by the rapid approach of an owner armed with a sampling container, it can help to tape the container to a 1-2 metre long stick so that the sample can be collected without approaching too close.

Similar to the study of Marrs and colleagues (1999), the new study will assess the specific gravity, acidity, protein, glucose, creatinine (a excreted metabolic waste product) and blood content of the urine. However, two methods will be used. Firstly a ‘multireagent urine dipstick’ test will be used – this is a plastic strip with different reagent squares which is dipped into the urine and then immediately compared with a colour chart. Secondly, more accurate, but more time consuming, standard laboratory techniques will be used. The study will determine whether urine from grass sickness horses contains abnormal levels of any of the components tested. Furthermore it will allow us to determine whether the rapid, convenient and cheap urine dipstick technique is accurate when compared with the standard laboratory techniques. If so, theoretically it could be used on a regular basis by owners to monitor urine composition in grazing horses and aid early detection of grass sickness. This may allow appropriate therapeutic and managemental interventions to be instituted earlier than is currently possible, possibly improving the prognosis of this dreadful disease.

This study was funded by the Bossy Boots Memorial Fund.

References Marrs J, John H, Milne E and Irvine R (1999) Urine analysis in equine grass sickness. Veterinary Record 145, 734-735 Kerr M (1999) Urine analysis in equine grass sickness. Veterinary Record 145, 147