“When we hear the word parasite most of us don’t have a positive reaction – we immediately think they are bad and something to get rid of as quickly as possible. Movies, in particular, teach us we don’t want them around,” said Frida Ben-Ami of the School of Zoology, George S. Wise Faculty of Life Sciences at Tel Aviv University. “A world without parasites sounds good but not for long – it would affect every level of our ecosystem. So we don’t want them to disappear but we want to understand them better and to use the knowledge to control them in places we need to.”
Humans have been fighting parasites for a very long time – Ben-Ami pointed to the first actual evidence of their existence – an Egyptian papyrus dating to c1550 BC!
“Parasites and pathogens have a unique relationship with their hosts, albeit the manifestation of this relationship is different, i.e. a parasite is an organism that lives on or in its host, exploiting the host to obtain resources, whereas a pathogen is a microorganism that causes disease after infection,” she continued. “Common to both parasites and pathogens (hereafter referred to as just parasites) is their strong dependency on the host. But this dependency is not the only driver of this relationship, which is also shaped by evolutionary coupled with ecological (biotic and abiotic) processes. To better understand this relationship, it’s necessary to take a step back and employ a holistic view of the wider environment.”
To do this Ben-Ami and her students use various invertebrate model systems, such as Daphnia (water flea), freshwater snails and their parasites (including bacteria, microsporidia, yeast, rotifers and trematoda [flat worms]), to tackle questions in evolutionary ecology. The questions they aim to answer include: What is the impact of co-infections on the host? How can a parasite change the outcome of interspecific competition between two host species? How does host age at exposure at the individual level, and host demography at the population level, affect various host and parasite fitness traits? What happens when a parasite spends a period of its life cycle out of the host? How do other organisms or extreme environmental conditions affect and be affected by host-parasite interactions?
Ben-Ami started by explaining virulence which is the amount of damage the parasite causes its host in which there is, of course, huge variability. The worse damage would be host death or long-term morbidity but parasites can also cause reduced growth and reproduction – influencing the next generation of the host. But this is not a one-way stream – killing the host early means the parasite will have lower fitness (offspring).
“So the parasite and host co-evolve,” explained Ben-Ami. “This means the parasite tries to infect and the host tries to avoid. Over time the host’s immune system develops tools to resist and the parasite is less virulent and causes less damage. By the end there is almost a balance between the damage and how much the host invests in its immune system.”
“Many parameters in nature influence this co-evolution – one is environment. We are asking how ecology affects the evolutionary processes.”
Working together
In nature there is usually not just one parasite – parasites interact and infect together leading to co-infection. To examine this, Ben-Ami’s team exposed Daphnia to multiple parasites finding that in co-infection the less-virulent parasite is pushed out by the more virulent, but the most virulent may kill the host early and also lose part of their own offspring – “so although being more virulent makes you a better competitor to other parasites, virulence may also be costly to a parasite”.
They also found a positive side to co-infection. In Daphnia parasites castrate the host – 80% won’t have offspring but in co-infection only 40% were castrated – so competition between parasites is of benefit to the host.
But most of the evidence on co-infection comes from laboratory experiments and is not conclusive, so Ben-Ami and her team have tried to see how it actually works in nature. The work of tracking parasites and their hosts involves lots of fieldwork. “We are often deep in the water or in small holes in the ground collecting samples,” explained Ben-Ami, “but it’s fun”.
Their sampling is from temporary ponds that have water in winter but are dry in summer. “The Daphnia life cycle works well here, “explained Ben-Ami, “they produce their eggs just before the dry season and they hatch in the next season allowing us to observe disease dynamics over a season.”
They found that co-infection is very complicated in nature – checking 4324 Daphnia individuals and finding up to ten parasites in a pond. The also noted that most co-infections involved only two parasite species with a maximum of five. “Co-infection is a reality and a common and important driver of disease dynamics in the wild,” said Ben-Ami.
Expanding the work to 200 ponds and different Daphnia species, they found that different species had different susceptibilities to parasites and that one – Daphnia similis– was never infected earning it the title ‘Super Daphnia’. They also found that this Super Daphnia had a much lower distribution compared to the more common and parasite-sensitive Daphnia magna which didn’t seem to make sense in terms of its ability to resist parasites. Further work showed that Magna was able to keep reproducing despite infection and that the more-resistant Super Daphnia only had the ability to survive if the parasite was there.
“So parasites matter,” said Ben-Ami. “Parasites can be a crucial factor for biodiversity by giving less competitive hosts a chance.”
The work has also looked at the impact of the age of the host, finding that younger Daphnia are more susceptible to parasites than older. Younger hosts also were more likely to allow co-infection to happen. “So parasites are also competing against the immune system of the host,” said Ben-Ami. “And it’s clear the immune system of invertebrates develops with age.”
This research was also done on Bilharzia schistoma mansoni. Ben-Ami explained that schistosomiasis affects 240 million people worldwide and that more than 700 million people live in endemic areas. “We tested snails in different age groups against the parasite – finding that the older were more resistant.”
Invertebrates are mostly infected for life, but Ben-Ami mentioned very early data that may show Daphnia can cure infection indicating their immune systems may be more complicated than believed.
“Invertebrates are part of our lives,” she added. “If we can understand their immune systems we might be able to use less pesticides in agriculture, for example. They are also important disease vectors to humans.”
The group is also looking at the impact of rising global temperatures on hosts and parasites. The effects of climate change on parasites especially in the off-host stages has received little attention and this is the work that Ben-Ami is focusing on during her fellowship. The aim is to understand the implications of parasite local adaptation to the external environment, and its impact on disease spread.
Michelle Galloway: Part-time media officer at STIAS
Photograph: Ignus Dreyer