MICRO-PARASITES 24I 



is probable that wild birds in Britain are also subject to the disease. 



In 1948, Dane recorded a severe epidemic among the manx shear- 

 waters on Skomer Isle. Hundreds of juvenile birds died in outbreaks 

 which occurred in two consecutive breeding seasons. The causative 

 agent was a virus and the visible symptoms included blisters on the 

 webs of the feet, inflammation of the eyes which led to blindness, and 

 extreme exhaustion sometimes accompanied by unnatural extension of 

 the legs. Some similar symptoms had been observed in three juvenile 

 herring-gulls which died on nearby Skokholm Isle, and it seems probable 

 that the disease is not confined to shearwaters. Ducks have been in- 

 fected experimentally. 



A world-wide virus disease of chickens is popularly known as fowl-pox. 

 The organism concerned is related to the virus of small-pox and cow- 

 pox. In the days before vaccination, chickens which contracted the 

 disease were regarded with grave apprehension, as they were considered 

 a possible source of human epidemics, but it is now known that fowl- 

 pox is not transmissible to man. Moreover several different types of pox 

 are known which attack birds — fowl-pox, pigeon-pox and canary-pox. 

 Pigeons are resistant to fowl-pox, but chickens contract a very mild 

 form of the disease if exposed to pigeon-pox, which then renders them 

 immune to the deadly form of their own variety. In the same way an 

 attack of cow-pox immunises man against small-pox. A pigeon-pox 

 vaccine is now used widely to protect chickens against the disease. 

 Various mosquitoes are proved carriers of fowl-pox. They mechanically 

 transmit the virus from one bird to another. The house-gnat remains 

 infective for 58 days after feeding on a diseased bird. It is remarkable 

 that some strain of the virus has not become acclimatised to man, since 

 it must continually be introduced into his body by this insect. The same 

 applies to the avian Plasmodium, Canary-pox is also a disease of wild 

 sparrows in the United States and several outbreaks among them have 

 been studied. Quail, grouse, pheasants, partridges and pigeons are also 

 subject to natural infections of avian pox of one type or another. The 

 disease almost certainly occurs in wild birds in Britain. 



There are of course other viruses recorded from birds, apart from 

 the three selected above; for example, the causative agents of Rous 

 sarcoma, fowl paralysis, fowl leukaemia and fowl pest. In Italy a 

 previously unknown virus has been recorded from wild thrushes and 

 another from owls in the United States. No doubt many others await 

 discovery and investigation in wild birds in Britain. 



