ABORTION IN CATTLE

New Zealand Herald, Volume LXX, Issue 21647, 13 November 1933, Page 5

ABORTION IN CATTLE

LIABILITY TO DISEASE FACTORS OF RESISTANCE I In most outbreaks of contagious j cattle abortion some cows in the j diseased herds are resistant to infec- i tion says a writer in the Livestock Journal. These resistant cows do not abort. Their blood often does not show ' a positive reaction. The blood reaction, if it occurs, is seldom strongly pronounced, and the animals soon cease to ieact. 1 lie question naturally arises, " What factors are responsible for the resistance of these animals which are able to throw oil infection which causes serious disease in the great majority of cows similarly exposed to the infect ion ?" According to Mr. A. W. Hopkins, of Wisconsin University, who has been studying the findings of investigators attached to the United States Department of Agriculture, rabhits were first Used as the experimental animals (as they do not, contract contagions abortion naturally). Earlier studies hv Manresa and his. associates at this station demonstrated that rabbits differed m thojr ability to resist Br. abortus infection, and that resistance to infection was an inherited characteristic. Killing Power of Blood Working on the belief that the power of the resistance animals to throw off infection was in some way associated with the ability of the blood of these resistant animals to kill the invading bacteria, Jrwin and Bell devised a method of measuring in the laboratory the bactericidal power of whole blood. This method involves adding known numbers of Br. abortus organisms to a specific amount of freshly-drawn sterile blood. The mixture of blood and bacteria is incubated at a favourable temperature for a 24-hour period, thus giving the blood an opportunity to exert whatever killing power it has. The number of live bacteria remaining is obtained by the standard bacterial plate count method, using media which are favourable to the growth of Br. abortus. The data, secured show wide differences in the killing power of the whole blood. It was found that 86 per cent of the animals with blood having a low-killing power proved susceptible when actually tested by intraperitoneal injection, while 74 per cent of these having blood with a highkilling power proved subsequently to be resistant to the disease. In other words, high-killing power of the blood seems definitely correlated with resistance to disease, and low-killing power with susceptibility. Similar tests have been made by Irwin and Bell in co-operation with Mr. B. A. Beach (Veterinary Science) of the blood of 35 cows in the station's contagious abortion experimental herd. All the cows have been tested twice, and a part of them three times. As with the rabbit, wide differences have been found in the killing power of their blood. With some animals their blood has virtually no killing power, while for others the bactericidal power is so great that one cubic centimetre of blood (about onethousandth of a quart) kills 8000 Br. abortus organisms. Animals Remain Constant But it is significant that file individual animals have remained constant as to their classification; cows that have shown high-killing power have in one test shown highkilling power in subsequent tests, while cows with medium or low-killing power have also remained relatively constant at these levels. The evidence thus indicates that the killing power of the blood is a specific and definite phenomenon. Most significant is the observation that the few experimental cows which went through the abortion ' epidemic four years ago without becoming seriously affected have blood which shows rather consistent high-killing power. On the other hand, the blood of the cows in the experimental herd which aborted when exposed to infection has invariably shown a little or no killing power. A great many more tests must be made before drawing final conclusions, but these studies give promise of throwing much new light on why some cows are able to resist infection by abortion disease, and also hold out tiie hope that a means may be developed of determining in advance of actual infection whether a particular animal is resistant to abortion. With the evidence strongly indicating that the evidence is a definitely inherited characteristic, it is not at all impossible to believe that dairy farmers in the future may be able to select as herd sires animals which are resistant, and by breeding and selection develop herds which will be resistant.