Australian health authorities say they're struggling to explain why gastro cases have reached a 20-year high.
The latest outbreak in Sydney in early October prompted a warning from local health officials, after more than 2,700 presentations to NSW emergency departments in just one week.
There have been recorded outbreaks in the last year that have resulted in the closure of pools across Melbourne, and water parks and a child care centre in Queensland.
Medical experts say the outbreaks are often caused by infections such as rotavirus and norovirus, and parasite-borne diseases including cryptosporidiosis and giardia.
However it is the increasing number of patients presenting with cryptosporidiosis that has health authorities concerned.
Up to July this year, 11,747 cases were recorded.
Since then, a further 1,427 have been also been recorded.
The total is close to the number of cases recorded for the entirety of 2010.
[total cases]Cryptosporidiosis is commonly transmitted by bodies of water and through faecal matter, and symptoms include:
- nausea
- sweating
- vomiting
- fever
- headache
The parasite is resistant to certain types of chlorine, meaning just one case can infect an entire swimming pool of people.
Dr Elizabeth Elliott AM, from the University of Sydney and Westmead's Children’s Hospital, said tracing the surge in cases was problematic.
"We don't really understand what's causing the trend," she said.
"It is possible that it's related — directly or indirectly — to climate change.
"In other words, the climate is warmer and people are swimming year round, and more people are outdoors and interacting more — potentially making them more likely to pass an infection on.
"But I think the answer is, we don't know.
"We don't know the reason for the upswing, but we do have some potential explanations for the fact that the infection rate often increases in the summer months or in warm climates."
The Department of Health and Aged Care said it had not planned any studies aimed at understanding the upswing in cryptosporidiosis cases.
"Most cryptosporidiosis infections are mild and self-limiting and most cryptosporidiosis notifications do not receive public health follow up unless detected to be in a cluster or outbreak," a spokesperson said.
The National Notifiable Disease Surveillance System shows the case numbers of cryptosporidiosis varied from state to state, with Queensland and Northern Territory reporting higher infection rates in 2023.
[state by state]This year so far, Queensland has recorded the highest number of cases — 6,020 — which was 2,742 more than NSW, which had the second highest number of infections.
"I think it's really important for governments and policymakers and health departments to be looking at these figures," Dr Elliott said.
"And if there is an outbreak, the best thing to do is to increase community awareness."
The federal health department said numbers were beginning to even out following winter.
"Earlier this year cryptosporidiosis notifications were higher than in recent years, peaking in March 2024, and decreasing through the winter months, with notifications now within normal ranges," a spokesperson said.
"Some community outbreaks linked to various aquatic facilities including swimming pools were identified earlier this year."
The department said 64 per cent of cases in the year to date were recorded in children under the age of five.
Dr Elliott said the data was concerning.
"Gastro is a part of life, kids get bugs," she said.
"But we know that a small proportion of kids that get gastro will will die from it."
Dr Elliott said the only way to control infection was to maintain hygiene practices, and for children to stay home when they have signs of the illness.
"In the case of cryptosporidiosis, also keep your kids out of swimming pools for ... two weeks," she said.
"Even though this is just a part of life, we want to prevent the severe outcomes that may occur in the most vulnerable populations."
