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Credit: Bacillary white diarrhea / by Thomas Dalling. Source: Wellcome Collection.
![reproductive organs as well as in other tissues and there is stror evidence to show that such male “ carriers ” may play an importai part in the spread of B.W.D. All eggs laid by “ carrier ” hens c not contain Bacillus pullorum and therefore all chicks from infectt hens do not contract the disease. An infected hen may lay mar normal eggs which produce chicks, but sooner or later an infectf egg is laid. The interval between the laying of infected eggs varii greatly : this accounts for the fact that certain hatches bred fro an infected flock may all be healthy, while in others the mortal! is very high. There is a diversity of opinion as to the hatchir capability of infected eggs. It is probably the degree of infectic which decides whether infected eggs will hatch out or whether tl developing chick will die in the egg before fully maturing. Tl chick hatched out from an infected egg is a grave source of dange its general appearance is normal while all the time it is voidir organisms in its excrement and so infecting many of the chic] hatched from normal clean eggs. Experiments show that very fe organisms are required to infect healthy chicks. Infected chic] discharge faeces laden with infection and soil the food, litter etc so that healthy chicks living in the same brooders etc., are bound swallow many organisms and so become infected. Again, all infectc chicks do not die, but continue to harbour the infecting organisn and in their turn lay infected eggs : thus the cycle of disease goes o Contaminated litter may retain infection for a long tim Experiments show that premises inhabited by chicks which ha1 died of B.W.D. will be capable of infecting a healthy lot of chicks p in many weeks after, the premises not having been cleansed meanwhil Chicks usually do not become infected after they are about week old. They may continue to die up to over a month old ar during the time elapsing between their infection and death th< continue to discharge infective excrement which may infect ai young chicks which pick it up. It will be seen from the foregoii discussion that the chief sources of spread are the “ carrier ” he: the chicks produced from infected eggs and the chicks infected late Problem of the “ carrier ” hen. It is the “ carrier ” hen in the flock which keeps the disease goi> from year to year. No outward appearances will lead one to suppo that a hen is a “ carrier.” Because of the ovary being infected would be reasonable to suppose that “ carriers ” would be po layers, but all poultry farmers do not agree on the point, combat B.W.D. effectively it is highly important to know whi of the breeding stock are “ carriers.” Happily, this can be dor and though the test applied may not give complete success on fi: application, it will go a long way to assist the farmer in his effo to “ clean up ” his flock. The most important and satisfactc test at the moment is the agglutination test. It consists roughly mixing various dilutions of serum, obtained from the blood of hej with a suspension of B. pullorum—the organism causing B.W. and noting which dilution of serum, if any, causes a clumping agglutinating of the organisms. The test can only be done in laboratory and with adequate organisation many tests can be carri](https://iiif.wellcomecollection.org/image/b30626663_0002.jp2/full/800%2C/0/default.jpg)
