This retrospective study reports the occurrence of neurological side effects attributed to the use of polymyxin B in horses treated for endotoxaemia.


Between January 2012 and January 2013 18 horses were treated for endotoxaemia with 5000 iu polymyxin/kg bwt intravenously q. 8 h. For this purpose a sterile polymyxin solution was compounded by a pharmacy: 500 ml 0.9% NaCl contained 2.5 Mio. iu polymyxin B. Depending on the disease which led to endotoxaemia the horses received other treatments as well. Horses were examined at regular intervals and ataxia was graded using the modified Mayhew grading scale.


Sixty-six per cent of patients were mares, 28% were geldings and the rest stallions. Age ranged from one to 23 years, with a mean (± s.d.) of 12 (± 7) years. Ten of 18 horses were treated for colitis, 2 of 18 each for small intestinal strangulating lesion, aspiration pneumonia and large colon torsion. In 6 horses a weak, ataxic gait was noted between 24 and 36 h after start of polymyxin treatment. The only factors significantly associated with ataxia were the number of doses of polymyxin the horses received (P = 0.011) and the concurrent use of neostigmine (P = 0.025). No other treatments were associated with occurrence of ataxia. The level of ataxia observed was correlated with the time necessary for ataxia to resolve. Horses which had shown ataxia after polymyxin treatment had a significantly longer overall hospitalisation time (P = 0.004).

Conclusions and practical significance

Self-limiting weak, ataxic gait was observed in horses treated with polymyxin B for endotoxaemia. Horses receiving multiple doses of polymyxin seem to be at risk. A cumulative effect of polymyxin might be suspected. Furthermore neostigmine could be responsible for potentiating polymyxin side effects. Neurological side effects of polymyxin at dose rates used for anti-endotoxic treatment need to be further elucidated.

Ethical animal research

Not required by this Congress: retrospective study. Sources of funding: None. Competing interests: None.