Systemic antibodies to Clostridium botulinum type C: do they protect horses from grass sickness (dysautonomia)?

The aetiology of equine grass sickness (EGS) is still unknown. There is inc reasing evidence that toxicoinfection with Clostridium botulinum type C is involved. Epidemiological evidence shows that resistance to EGS can occur i n older horses and those that have been on a particular pasture for longer or have been in prior contact with the disease. This resistance may be in t he form of an immune response to the aetiological agent. Levels of systemic antibodies to the surface antigens of C. botulinum type C (using the close ly related and safe C. novyi type A as a phenotypic marker) and to the botu linum type C neurotoxin (BoNT/C) were investigated in horses with and witho ut EGS. Horses with grass sickness were found to have significantly lower levels of systemic IgG to both surface antigens and BoNT/C. Horses with low levels o f systemic immunity to these antigens may be more susceptible to developing EGS. There were no significant differences in antibody levels between the different categories of EGS, suggesting systemic immunity to C. botulinum t ype C does not play a significant role in influencing the severity of the d isease. However, horses that bad been in contact with EGS or that were graz ing land where it had occurred frequently in the past had significantly hig her antibody levels to these antigens. These horses may have been exposed t o subclinical doses of C botulinum type C and BoNT/C, resulting in the prod uction of a protective immune response against the putative aetiological ag ent. This finding is of potential significance for the prospect of preventi on of EGS by vaccination against C. botulinum type C.