Benzene hexachloride and DDT are relatively new insecticides and their veterinary value when administered orally to bovines has not yet been fully investigated.

The first series of experiments showed that 0·5 gm./kg. on two successive days of D. 929, was toxic; 0·25 gm./kg. on two successive days followed by 0·125gm./kg. on the 3rd and 4th days was well tolerated, and the benzene hexachloride remained in the blood in sufficient concentration to kill feeding tsetse and ticks up to the 41st day. After a single dose of 0·25 gm./kg. the benzene hexachloride remained in the blood for only 10 to 12 days, and failed to protect the animal from East Coast Fever when it was grazed on heavily infested pasture.

Five calves susceptible to East Coast Fever were dosed with D.929 and exposed to attack by the common tick species under African farming conditions. A dose of 0·125 gm./kg. given weekly protected a calf from East Coast Fever, but there was no protection when the same dose was given once a fortnight. A dose of 0·125 gm./kg. given weekly also protected, whereas 0·1 gm./kg. failed. At all these dosages the concentration of benzene hexachloride in the blood was sufficient to kill feeding G. palpalis during the period the calves were under treatment and for periods of 10 to 25 days after the final dose.

D.929 given to animals suffering from East Coast Fever and Trypanosomiasis had no effect on the course of these diseases.

Doses of 0·3 gm./kg. (39 mg./kg. gamma isomer) are toxic to young animals but older animals showed a greater tolerance. Hypersensitivity, muscular tremors, and general paralysis were the main toxic symptoms, and gross pathologicaal changes were most obvious in the liver and urinary system. The median lethal dose of benzene hexachloride given orally for calves is therefore lower than for small laboratory animals as recorded by Cameron (1945). Only crude benzene hexachloride powder (13 per cent, gamma) was used in the preserit experiments, but Slade (1945) has shown that this is markedly less toxic than the purified gamma powder.

Little experimental work was done with DDT as the results with benzene hexachloride were more promising both in the control of ticks and tsetse.