Wednesday, 23 May 2012

How Some People May Become Infected with Aspergillus

There has been a fairly long standing observation that under some circumstances people with a completely normal immune system who are able to fight off infection (unlike those who are heavily immunocompromised such as those undergoing treatment for some cancers) can become infected with tragic consequences. In the cases I am referring to the unfortunate person has accidentally inhaled a large number of spores of the fungus Aspergillus fumigatus (or similarly temperature tolerant species) and rapidly died.

We all breathe in large numbers of fungal spores every day so how could this happen?

In many cases the fungi we breathe in are unable to grow at our body temperature, so those are unlikely to cause infection. In other cases the fungal material is highly allergenic, so it is possible that they could cause an allergic or asthmatic attack (which can also have serious consequences) but still could not cause infection.

However  if the fungal strain is able to grow at our body temperature it still has to overwhelm one of our deadliest allies - our neutrophils that line our lungs consuming any stray fungal spores or hyphae

This video shows mouse neutrophils (green cells) moving 
through lung cells (red) engulfing fungal spores (blue dots)

A recent paper using cells isolated from birds shows that macrophages (part of the same type of defence system as neutrophils) are only able to provide protection up to a point. When too many spores are present (many 1000's) they become overwhelmed and they are unable to kill the spores that they 'eat'. The spores start to germinate instead inside the macrophages and quickly kill the cells - they are effectively past one of the first lines of defence against infection.

It seems fairly clear then that all of us have a highly efficient immune system capable of killing millions of fungal spores, but once a certain number of thermotolerant spores is inhaled perhaps the fungi can win the race for supremacy. 

It must be said that death from inhaling even quite large numbers of spores is extremely rare and that we have other mechanisms to eliminate spores, but this might be a clue as to why occasionally they aren't enough. Always be careful when opening bags of compost or other plant materials e.g. bark chippings.

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