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There are so many parasites living in tropical regions of Africa, South America, or Asia, but very few in Europe or North America.

Is this due to climate, or are there other reasons?

Many of the tropical diseases and parasites are transmitted by insects, such as flies and mosquitoes. Well there are flies and mosquitoes in Europe as well.

There might be malaria-transmitting mosquitoes in the very south of Europe, and there is encephalitis transmitted by ticks. But that's it. Why don't the hundreds of different parasitic species from Africa spread to Europe ?

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    $\begingroup$ Parasites aren't the only group to show this pattern. Many clades and ecological groups are more diverse in the tropics. This pattern is known as the Latitiudinal Diversity Gradient $\endgroup$ – C_Z_ Feb 21 '16 at 1:06
  • $\begingroup$ Related Q/A: biology.stackexchange.com/q/14472/3624 $\endgroup$ – fileunderwater Feb 22 '16 at 8:45
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The diversity of parasites shows a gradient with increasing diversity from the poles to the equator. Several reasons have been brought forth to explain the latitude-dependency of parasite diversity:

  • An increased diversity overall around the equator; species diversity in general is greater in the rain forests and hence more hosts are available and thus more parasitic species can develop that target specific hosts;
  • Larger amounts of precipitation and higher temperatures around the equator may favor the development and transmission of parasites (Nunn et al., 2005);
  • Increased available energy overall around the equator (Guernier et al., 2004).

References
- Guernier et al., PLOSone (2004): 0020141
- Nunn et al., Diversity and Distrib (2005); 11: 249–56

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    $\begingroup$ Nice answer +1 (and good post +1)! I have two questions: 1) Does "increased available energy" affect parasite diversity other than by increasing overall diversity (in which case the third bullet point would be the same as the first)? 2) I have a good intuition for what higher precipitation would favour parasite transmission but I fail to have a good intuition for high temperature. Can you explain? Thanks $\endgroup$ – Remi.b Feb 21 '16 at 0:08
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    $\begingroup$ @Remi.b - Thanks! 1) I think total energy has more to do with the gross number of animals present. Higher numbers of vectors and hosts will promote development of parasites overall. It's more of a general factor. 2) Most processes, including growth in general, goes faster at higher temperatures. Again, it's more of a general factor. The first bullet point has the strongest association, the 2nd and 3rd less so afaik. $\endgroup$ – AliceD♦ Feb 21 '16 at 6:48

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