Residual Damage in Treated Cases of Grass Sickness

By Dr D L Doxey

There has for some time now been an attempt to obtain tissues from cases which were diagnosed as grass sickness at the Royal (Dick) School of Veterinary Studies, successfully treated, sent home and subsequently died. For a variety of reasons we have failed to acquire these tissues in the past, but thanks to the help of Mrs Philippa Gammell and others, we have now had access to four horses and hope to publish the preliminary results of our examination of their tissues, elsewhere.

This short note is to give readers the gist of what we found and because the results are so extreme, we hope that other owners will notify us prior to having their horse put down so that we can confirm our findings in other cases.

In fatal grass sickness, the characteristic lesions seen in the tissues are sometimes loss of, and always damage to, the nerve cells in the autonomic nervous system. There is also severe loss and damage affecting the nerve cells in the ileum and to the lesser extent the jejunum. (The jejunum is the anterior part and the ileum the posterior part of the small intestine).

It has always been assumed that in animals which recover, this damage is less severe than in fatal cases and sufficient nerve cells remain for the patient’s intestinal and other affected functions to eventually return to normal.

Of the four horses we have investigated, three were put down because of severe colic and the fourth as the result of an accident. All animals showed similar pathological lesions. The remaining autonomic nerve cells appeared to be healthy but it was not possible to see if normal numbers were present. The depletion of nerve cells in the ileum, and to a lesser extent in the jejunum, was severe. At best, only 20% of ileal neurons seemed to have survived and at this low level it is difficult to see how the intestine functioned adequately and yet apparently it did – in one case for 13 years.

We need more cases to confirm the findings from this small group of horses, and to investigate in more depth how the intestine works following grass sickness.

Any help would be gratefully received by Dr B McGorum and his colleagues at the Royal (Dick) School of Veterinary Studies.

September 1999