Data pertinent to this discussion are published in the papers cited in the reference list.
Studies on the pathogenesis of poisoning by carbon tetrachloride have shown that it enters liver cells soon after either oral or parenteral administration. It dissolves into the cell lipids, particularly the phospholipids of the lipoprotein membranes. The entry of carbon tetrachloride into the lipoprotein membranes surrounding the mitochondria, which are the cytoplasmic organelles responsible for cellular respiration and energy conservation, initiates a sequence of events which culminates in increased permeability of mitochondrial membranes, the consequent loss from mitochondria of co-enzymes necessary for respiration, and the death of cells, due to respiratory failure, when the concentration of co-enzymes has fallen too far.
Although it has not been possible to prevent the entry of carbon tetrachloride into the liver cells or its combination with lipoprotein membranes, it has been possible to prevent cellular respiratory failure, and the death of cells, by either preventing the loss of co-enzymes or stimulating their replacement sufficiently rapidly to ensure the maintenance of concentrations, within the mitochondria, adequate for respiration.
One of the factors which govern the state of functional semipermeability of intracellular membranes is the degree of saturation of the constituent fatty acid residues. Peroxidation of the unsaturated fatty acid residues present in the lipoprotein membranes leads to increased permeability. Prevention of lipid peroxidation stabilises biological membranes against the action of factors tending to increase permeability. Thus it has been found that the prior administration of antioxidant substances, including vitamin E, selenium and N.N'diphenyl-p-phenyl-enediamine, largely prevents the loss of liver mitochondrial co-enzymes, respiratory failure, cellular necrosis and the death of animals given lethal doses of carbon tetrachloride.
The administration of the co-enzyme precursors, tryptophane or nicotinic acid, has been shown to stimulate synthesis of the co-enzymes, which are synthesised in the nucleus and transferred to the mitochondria and the cytoplasm. Stimulation of co-enzyme synthesis by administering either of the precursors still occurs at a sufficient rate after the administration of carbon tetrachloride to off-set the leakage from mitochondria and to maintain a concentration adequate for the continuance of respiration, thus preventing the death of liver cells.
Rats were found to be protected against lethal doses of carbon tetrachloride only when the antioxidants or the co-enzyme precursors were administered beforehand; simultaneous or subsequent doses of the prophylactic agents were of no therapeutic value. Nicotinic acid, one of the B vitamins, was easily the best of the prophylactic agents tested, and was chosen for further study with sheep.
Considerable difficulty has been experienced in producing lethal poisoning of sheep with carbon tetrachloride under experimental conditions. However, the greater susceptibility of sheep kept at Armidale and fed a diet different from that given to sheep at the McMaster Laboratory was exploited to evaluate nicotinic acid as a prophylactic agent. Significant protection against death, subsequent to the intra-ruminal administration of a 50ml dose of pure carbon tetrachloride was afforded by 2g doses of nicotinic acid given either intra-ruminally or intra-peritoneally. Greatest protection resulted when the prophylactic treatment was started two days before carbon tetrachloride administration, but significant protection was afforded by a single dose of nicotinic acid given immediately before or with a dose of carbon tetrachloride.
These experiments established the value of nicotinic acid as a prophylactic agent against carbon tetrachloride poisoning of sheep under experimental conditions. However, before a general recommendation for the use of nicotinic acid in the field could be made, it was considered necessary to confirm its protective value under natural field conditions. Consequently field trials involving many thousands of sheep have been conducted on properties on which it was alleged that mortalities commonly followed drenching with carbon tetrachloride. So far these trials, in which have been very grateful for the collaboration of a number of Veterinary Inspectors, have been inconclusive because insufficient control sheep given only carbon tetrachloride have died.
One interesting aspect of the problem of carbon tetrachloride poisoning which has emerged from the field trials, is the importance of careful, correct drenching technique. The careless, hasty technique used by certain farmers prompted investigation of the maladministration of carbon tetrachloride as a contributory factor to mortalities. It has been found that administration of an anthelmintic dose of carbon tetrachloride paraffin mixture into the larynx or trachea, onto the floor of the pharynx in such a way as not to elicit a swallowing reflex, or forcibly against the hard palate, greatly increased mortality and produced pulmonary as well as severe liver and kidney damage. The most likely form of maladministration to occur in the field is that of forced expulsion of the drench against the hard palate. This mode of administration appears to inhibit the swallowing reflex, possibly by virtue of the local anaesthetic properties of carbon tetrachloride, and allows time for some of it to volatilise in the mouth and to be inhaled with inspired air into the lungs, and produce severe damage during absorption through the alveoli. The liver damage produced is as severe, and the kidney damage is more severe than that which results from the same dose of carbon tetrachloride paraffin administered intra-ruminally or carefully by drenching gun so that it is swallowed immediately. The dangers of faulty drenching technique are easily avoided by the exercise of reasonable care with a long-spouted drenching gun. If the tip of the spout is directed down over the back of the tongue and the dose administered reasonably quickly, but without violence, the sheep swallows immediately. Swallowing is elicited readily by inserting the fingers of the free hand into the inter dental space, and this practice should be encouraged. A sheep should never be drenched when struggling, or when the head is not freely accessible to the operator. The use of oesophageal tubes appears to be advantageous provided the tube is inserted with care into the oesophagus and the dose is administered there. Spraying from an oesophageal tube during withdrawal is probably dangerous.
Although it seems likely that careless or faulty drenching technique has been responsible for many deaths following carbon tetrachloride administration, it is unlikely that this has been the only factor involved in field mortalities. Varying susceptibility to carbon tetrachloride poisoning must also have been an important contributory factor. Little is known about the factors involved in susceptibility to carbon tetrachloride poisoning. However, it is reasonable to suggest that sheep, which are already suffering liver disease or which are deficient in such antioxidants as vitamin E or selenium, will be abnormally susceptible to carbon tetrachloride poisoning. This aspect requires further study.