SEVENTH ANNUAL YEAR BOOK— PART X. 503 



or even six weeks when separated from sheep; if the atmosphere is dry 

 they will generally die in about fifteen days, but death is often only 

 apparent, for the mites may sometimes be revived by warmth and moist- 

 ure even after six or eight weeks; the fecundated females are especially 

 tenacious of life. Various rather contradictory statements may be found 

 regarding their resistance to cold. Krogmann states that they may live at 

 a temperature of 10° C. (14° F.) for twenty-eight days; other authors 

 that they die at 50° C. (122° P.) They are said to have been kept alive in 

 cold water for six days and in warm water for ten days. Several authors 

 admit, however, that the parasites are usually killed by a soaking rain; 

 though it is claimed that in damp, dark stables they "may live for 

 months." 



Experience has shown that in some cases apparently healthy sheep 

 have become infected in places where no sheep have been kept for four, 

 eight, twelve or even twenty-four months. The conditions underlying this 

 infection are not thoroughly understood. Possibly some of the eggs have 

 retained their vitality a long time and then hatched out; possibly the 

 vitality of the fecundated female has also played a role; while it is not 

 at all improbable that an entirely new infection has accidentally been 

 introduced by birds or other animals. Certain authors of high standing 

 soout the idea that birds can introduce an infection of scab, but there 

 is no reason why 'birds should not do this, and there are some reasons 

 for believing that they do. It has been noticed at the experiment sta- 

 tion farm of the bureau, for instance, that crows delight in perching on 

 the backs of scabby sheep and picking at the scab; while so doing it is 

 only natural that small tags of wool would adhere to their feet and 

 thus scatter scab. The fact that snails cling to birds' feet and are car- 

 ried long distances is too well established to need discussion, and it is 

 very probable that many of the cases where sheep are supposed to have 

 become infected with scab on pastures which have not been occupied for 

 one or two years are in reality cases of fresh infection by means of birds. 

 From the data at hand, while "it may be admitted that in some cases, 

 under favorable conditions, the mites may live from spring to fall, it is 

 scarcely within the limits of probability that either the scab mites or 

 their eggs vv^ill live through a winter when separated from the sheep and 

 exposed to the elements. 



All matters connected with the vitality of the scab mite have an im- 

 portant bearing in explaining cases of indirect infection on roads over 

 which scabby sheep have been driven, or in fields and sheds where they 

 have been kept. From the facts now at hand, the following important 

 rules can be presented: 



(1) Scabby sheep should never be driven upon a public road; (2) 

 sheds in which scabby sheep have been kept should be thoroughly 

 cleaned, disinfected and aired, and should be left unused for at least 

 four weeks (better two months) before clean sheep are placed in them; 



(3) fields in which scabby sheep have been kept should stand vacant at 

 least four weeks (better six or eight) before being used for clean sheep; 



(4) a drenching rain will frequently serve to disinfect a pasture, but it is 

 well to whitewash the posts against which scabby sheep have rubbed. 



