An article in the New England Journal of Medicine looks at the situation in Ghana, where, as in many African countries (and others) many physicians train, then go abroad. Currently there are 532 Ghanaian doctors practising in the USA, which is in absolute terms a small number, but there are only 2600 physicians in Ghana. 25 percent of doctors in the USA trained outside the USA, and the same figure is true for the UK.
The article talks about some of the measures that the Ghanaian government is taking to try and keep Ghanaian trained health professionals in the country.
The article is available free in full text, here.