The effect of arsenical dips on Parafilaria bovicola in artificially infected cattle in South Africa.

The possible adverse effect of arsenical tick control dips on Parafilaria bovicola infections was investigated in 48 artificially infected cattle. A treatment group of 24 cattle was dipped in a plunge dip containing 1600 ppm arsenic trioxide. A control group of the same size was dipped in an organophosphate dip containing a mixture of chlorfenvinphos and dioxathion. Regular weekly to 3-weekly dipping had no effect initially on the prevalence of ovipositional blood spots of P. bovicola in either group. However, from 4 months after bleeding commenced there was a significant reduction in blood spots in the arsenic-dipped cattle and, on slaughter at 12-14 months after infection, the arsenic group had significantly fewer live worms and fewer carcass lesions. Arsenic residues in muscle samples of treated cattle were 11.6 times higher than in the controls. It is proposed that arsenic residues in the sub-cutaneous muscle layers increase with repeated dipping until a level toxic to P. bovicola is finally reached. Older cattle would therefore be refractory to infection and their carcasses at slaughter would not be affected.