Biological water quality – part 2

Welcome to part 2 of these talks on biological water security. I mentioned that this time I wanted to highlight the spatial and temporal factors which affect the distribution of waterborne pathogens. Spatially, we have covered quite a few of the factors in mentioning some of the environmental factors and anthropogenic factors, such as global change, that affect the distribution of pathogens. Of course, you can’t have a disease without the host. And so host distribution, of course, is going to impact spatial distribution. But it’s not just the presence of that host, but susceptibility of individuals and populations is key in determining whether an infection establishes and whether you have disease outbreaks.

Generally, there are lots of unpredictable effects and interactions of these factors on particular pathogens in particular areas that overall affect spatial distribution. With regard to temporal variation in the presence of waterborne pathogens, this very much depends on the association between the hosts and parasite over time. Taking this classic figure from Curt Lively’s group, in which he describes how the hosts, shown in blue, and the pathogens, shown in red, fluctuate slightly out of sync with one another so that you have an increase in the host population followed shortly afterwards by the peak in parasite outbreak. That pathogen, if it causes disease, then often causes the host population to crash. And so you have less hosts available.

And then, of course, the pathogens go down because they can’t exist without the hosts. And so this cycle continues over time. And that can occur very dynamically on a short-term basis. You can also have dramatic historic variation in parasites due to a whole range of different factors. So disease, such as malaria, that you might not associate with the UK was in fact common in Britain in the 17th century in the time of Cromwell. It was only eradicated through changes in land use, swamp drainage, and development of pesticides and application of those to get rid of the vector. We now tend to think of malaria as its distribution being dependent on climate change, and particularly temperature.

So this map shows predicted areas, highlighted in red, where the pathogen is likely to appear because of changing distribution of vector associated with increase in temperatures, climate change. There are other pathogens also that we might not have associated with particular countries that have changed over time. And that includes cholera, of which there were major outbreaks here in Cardiff in South Wales. And that was largely due to unsanitary conditions. And there was a particularly large outbreak, with almost 400 people dying here in Cardiff due to cholera. And that occurred because during our Victorian era, there was a dramatic increase in population of Cardiff, associated with industrialization.

And the water supplies effectively just couldn’t cope with that level of contamination, and cholera spread. Similar cholera outbreaks were occurring in other cities, and the one in London had particular historic importance. It occurred around a focal point in Broad Street. And it was the physician John Snow who identified contaminated water as the source of cholera. Prior to then, it was thought that cholera was just associated with bad air. So this identification of the cause of an infection was really marked the birth of epidemiology. And John Snow was able to map the number of people with cholera. And so this, on a London map, shows the cases of cholera that he identified.

And he was able to show that it was just a single water pump that was responsible for this outbreak in Broad Street. And with closure of that pump, the problem was solved in that area. So lots of cities around the world, including Cardiff, passed various acts of Parliament to construct freshwater supplies for their cities. And this infrastructure, at least for a while, reduced dramatically the incidence of diseases that we don’t necessarily associate with these countries now. So interesting example of temporal variation of disease, both in the short term and the long term.

Other thing I wanted to mention today is really the importance of parasites in an ecosystem. So parasites themselves, they’re not just vectors of disease. They are also ecologically important. And they have to really be considered when you think about food chains. And so two notable parasitologists, Kevin Lafferty and Armand Kuris, really set about proving that parasites have key roles in the ecosystem. And the food web ecosystem that they chose to show this was a salt marsh in California, which had a fairly simple ecosystem, with algae being grazed on by snails and other invertebrates. Also fish in the system, and they were prey for waterbirds.

And they systematically searched all the water and the biodiversity in the system for parasites and recorded and monitored and identified everything. And to cut a very long story short, one of the key groups of pathogens they found were a particular group of snails– sorry, a particular group of helminths that occurred in snail hosts. And remarkable fact, they found that the larval stages produced from these parasites in the snail– and they were larvae produced, and they were shed by the snails and released into the water. Taking those free-living larval stages of the parasite, the biomass of those parasites was greater than all of that of the avian birds in that system.

So that shows you that parasites are an important component of an ecosystem. So not only are they a cause of disease, but they can also provide food. They can act as prey for hosts that are not susceptible to them. So I think this is a really important factor to consider, that parasites are not just a cause of disease. They are an important component of the ecosystem, and they will always be present.

They will also always be changing. And we constantly have a situation where we also see new and re-emerging infectious diseases in the environment. And as Dan Brooks said, referred to these infectious agents as “evolutionary accidents just waiting to happen.” So my final message to you, really, is to think about how we can avoid the costs of parasites for ourselves and animals. And the old adage, “prevention is better than cure,” is really very important. Hygiene and sanitation, avoiding contact with contaminated water are excellent ways to actually avoid the costs of infectious disease. Good nutrition and health care, essential. And also, resilient infrastructure. We saw how changes in Victorian England had an impact on reducing the impact of disease.

And we should learn from lessons of history to reduce the cost of infection. So on that note, I’d like to say thank you for listening. And if you’d like to find out more about our research on infectious disease at Cardiff, do you have a look at our website and find more. In the meantime, protect yourself against disease. Thank you for listening.