Giardiasis interferes with nutrient absorption in the small intestine, driving chronic diarrhoea and growth delays. Its hardy cysts persist in water and the environment, making outbreaks difficult to control.

There are up to 600,000 cases of giardiasis in Australia each year, and more than 280 million cases worldwide. The disease has a disproportionately high impact on children in poorer communities and in remote Indigenous communities in Australia.

Study lead Professor Aaron Jex said that when a parasite expands from a single host to many, the public health task gets far harder.

“By pinpointing the genetic patterns behind this shift, we can better anticipate where and how future infections may emerge and design smarter surveillance to stop them,” he said.

The new study, published in Nature Communications, has revealed a surprising evolutionary twist: an asexual lineage of Giardia that managed to infect a wider range of hosts than its sexual ancestor, despite being on a genetic path toward extinction.

The study found that the same genetic shortcut may also occur in many other zoonotic parasites.

Giving up sex may have been the key to allowing the parasite to became a generalist – able to infect pets, livestock, wildlife and humans.

This ability to jump between hosts, known as host switching, is a key factor in how new infections emerge in people. But the shortcut comes at a cost: because these parasites don’t exchange genetic material, harmful mutations build up over time, leading to eventual collapse.

“It’s not survival of the fittest – it’s survival of the ‘fit-ish’,” said Prof Jex.

“That brief advantage lets the parasite spread to new hosts before genetic problems catch up.”

This same principle may also help explain how parasites evolve drug resistance.

Mutations that help them survive treatment often make them weaker overall. In parasites that reproduce sexually, these mutants are quickly outcompeted. But in asexual parasites they persist, creating a window for resistant strains to spread.

“Our findings show that when sex stops, selection becomes inefficient,” Prof Jex said.

“That same inefficiency could allow drug-resistant parasites to linger and spread.”

Sex isn’t just about reproduction – it’s about survival.

Sexual recombination allows parasites to stay competitive in the evolutionary “arms race” with their hosts. When that stops, short-term gains like switching to new hosts come at the cost of long-term viability.

The team’s findings raise new questions about how similar mechanisms might influence drug resistance and the emergence of other parasitic diseases.

In the future the team will explore whether the same genetic shortcuts that allow host switching also enable resistant strains to persist, and how this knowledge can guide treatment strategies and surveillance systems to prevent outbreaks before they start.

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Header image: This colourised microscope image shows the Giardia parasite dividing into two, a process that helps it spread and cause diarrhoeal illness.