'Dirty' city-living animals such as rats may have a reputation for harbouring infectious diseases, but a new study suggests these urban dwellers don't host any more human-infective viruses than their rural species. 

Urban species suffer from 'sampling bias' when assessed for infective viruses, scientists say, so their reputation as disease carriers is likely blown out of proportion.

According to the experts, city wildlife might pose less of a threat for future pandemics than once thought. 

Rats famously carried fleas that caused the Black Death, a devastating bubonic plague pandemic in the 14th century – which may explain their bad reputation that still lingers today. 

The new study was conducted by an international research team led by scientists at Georgetown University, Washington DC and published in Nature Ecology and Evolution. 

The study compared urban adapted mammal species (such as rats) with those species that can't live in urban environments. 

'There are plenty of reasons to expect urban animals to host more diseases, ranging from their food to their immune systems to their close proximity to humans,' said study author Greg Albery at Georgetown University College of Arts and Sciences.

'We found that urban species do indeed host more diseases than non-urban species, but the reasons for this appear to be largely associated with the way we study the ecology of disease. 

'We've looked more at animals in our cities, so we've found more of their parasites – and we've started to hit diminishing returns.'

Scientists have long suspected that cities might be a hotspot for outbreak risk, thanks to species like rats, foxes and pigeons. 

The Covid pandemic has sparked substantial interest in where future outbreaks of viruses are at the highest risk of emerging – not just SARS-CoV-2 (the virus that causes Covid), but other viruses too.

In Washington DC, a growing rat problem caused a respiratory virus called Seoul hantavirus to infect two people in 2018, according to the Centers for Disease Control and Prevention.

For the new study, researchers set out to understand whether animals adapted to living in cities tend to have different viruses. 

After examining pathogens hosted by nearly 3,000 mammal species, they found that urban-adapted animals could host roughly 10 times as many kinds of disease, compared with species that can’t live in urban environments.

However, they found that pattern was partly a problem of sampling bias; the urban-adapted species were nearly 100 times better studied in the scientific literature.