1. Experiments which clearly indicate the detrimental influence of solar
radiation on the course of bluetongue are described.
2. Preliminary experiments were carried out during different seasons of
the year in a limited number of sheep. Some of them were photosensitized
against light with methyleneblue. During the course of the disease the sheep
were exposed to solar radiation daily for several hours.
3. The result was, that the exposed sheep showed distinctly severer
reactions than those kept in the stable. The strongest and most fatal reactions
(6 out of 10 sheep died) were observed during a test conducted in February,
when the solar radiation was very intense. Exposure during winter (June)
had practically no influence on the course of the disease.
4. In order to verify these observations, two experiments on a large
scale (60 sheep each) were undertaken on sheep subjected to the immunization
process. Half the sheep were kept in the stable and the other half freely exposed to solar radiation. In each locality half of the sheep were shorn,
the others remained unshorn.
5. The body temperatures and the nature of the clinical symptoms
showed that the reactions were much more pronounced in the camp than in
the stable. The symptoms of bluetongue were most pronounced in the shorn
animals in the sun.
6. Since a distinct influence on the severity of bluetongue reactions was
noticed in sheep photosensitized with methyleneblue, it is believed that sheep
in the field may react very severely to this disease if they become photosensitized naturally.
7. Bronchopneumonia in sheep photosensitized with methyleneblue
occurred on several occasions and developed sometimes very rapidly. This
observation suggests that natural photosensitization in the field may partly
be responsible for the bronchopneumonia which is frequently observed in
sheep m the field reacting to bluetongue.
8. During the course of the above investigations, multiple haemorrhages
and muscular degeneration, a lesion which had not been previously associated
with bluetongue, were observed in the stabled as well as in the exposed sheep.
This observation gives an explanation to the general weakness, torticollis and
stiffness of the sheep reacting to bluetongue.
9. The unthriftiness frequently observed in sheep after immunization or
natural infection can also be attributed to the muscular degeneration.
10. Since the influence of solar radiation on the course of bluetongue has
definitely been established, particular attention should be paid to the time of
the year when the immunization is carried out. On account of the fact that
the severest reactions were observed in shorn sheep, immunization should
preferably be undertaken on unshorn sheep.
11. The provision of shade for sheep which have been vaccinated or
which contracted the disease naturally should be promoted wherever possible
in order to exclude the harmful influence of solar radiation on the course of
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