A woman who was hospitalized with a life-threatening disease in 2019 has her aquarium to blame, say experts with the Centres for Disease Control and Prevention. In a recent report, they detail how her freshwater home aquarium was contaminated by the same bacteria that caused her illness, known as melioidosis. Hers is one of several recent cases in the U.S. that have puzzled doctors, since melioidosis is typically only found in tropical areas.
Melioidosis is caused by the bacteria Burkholderia pseudomallei, which is found abundantly in the soil and water of tropical environments. Symptoms of melioidosis vary significantly, depending on where the infection takes hold, and can resemble many other diseases, like the flu. Adding to its elusiveness, the bacteria can be misidentified by some automated tests. Even with timely antibiotic treatment, melioidosis is often deadly, with a fatality rate ranging from 10% to over 40%. Worldwide, it’s thought to kill at least 89,000 people a year, though this may be an underestimate.
Thankfully, melioidosis usually isn’t contagious from person to person, though the bacteria can become aerosolised under the right conditions. That airborne potential, along with its many other wonderful properties, has led the U.S. to consider the bacteria a Tier 1 select agent, the same designation given to other possible bioterror threats like Ebola and anthrax. Cases of melioidosis remain exceedingly rare in the U.S., however. And up until lately at least, just about all of them could be traced back to travel to somewhere the bacteria is native.
That wasn’t the situation for a 56-year-old woman in Maryland who became hospitalized with fever, cough, and chest pain in mid-September 2019. She was started on antibiotics as soon as she was admitted, and, within a few days, her ailment was identified as melioidosis. Her symptoms gradually improved, and she was discharged from the hospital on day 11. Unfortunately, even after a month of continued antibiotics, the infection wasn’t quite defeated, and she had to be readmitted and given additional antibiotics. Finally, after another week in the hospital and 10 to 12 weeks of antibiotics afterward, the infection was successfully cleared.
The Maryland Department of Health alerted the CDC once melioidosis was suspected. Between October and December 2019, local and federal health officials conducted their investigation, interviewing the women and other household members, as well as examining the woman’s home.
The bacteria in this case appeared to be closely related to strains found in Southeast Asia. But the woman had never travelled outside of the U.S., ruling out the typical route of exposure. The bacteria wasn’t found in environmental samples around her home either, indicating that it hadn’t somehow established a natural reservoir in the subtropical climate of Maryland. But the woman did own two freshwater aquariums, stocked with imported tropical fish. And in the soil and water samples taken from Tank B, the CDC researchers found B. pseudomallei.
Further genetic testing showed that the bacteria from the tank was a dead ringer for the bacteria found in the woman’s body, solving the mystery. Interestingly enough, six fish living in that tank had died between August and November 2019, though it’s unknown whether the bacteria had any role in those deaths. The woman had originally bought the tanks in July, and she recalled sticking her bare hand in to clean the tank as recently as August 2019.
The report, published last week in Emerging Infectious Diseases, is the first to tie an outbreak of melioidosis to a contaminated aquarium. But one French study cited by the authors did find these bacteria contaminating the water used to transport fish to an aquarium, while freshwater fishing practices in countries where the disease is endemic have been singled out as a possible transmission concern.
The investigators ruled out any continued risk of contamination from the local pet store where the woman had bought her tanks. And there are already safeguards to reduce the contamination of aquariums, including the chemical treatment and filtration of the water, according to study author Patrick Dawson, an epidemiologist in the Office of Science at the CDC. It’s even unlikely that much of the water originally used to transport tropical fish to the U.S. will end up in a home aquarium in the first place.
But Dawson does say that further investigation is needed to understand the potential scope of risk that these aquariums may present in importing the bacteria stateside. About 12 million homes in the U.S. own freshwater fish, according to the American Pet Products Association, and the U.S. is the largest importer of exotic fish, which tend to be freshwater and come from Southeast Asia. This recent discovery has now changed the CDC’s perspective on investigating outbreaks of melioidosis moving forward.
“It really broadened our understanding about how the bacteria might be able to travel across borders through imported products. And now that we’ve identified this new route of exposure, that can raise awareness about this risk,” Dawson told Gizmodo by phone.
The CDC, he added, has since included questions about aquarium and tropical fish ownership to their standard investigation of outbreaks with no travel history.
Earlier this year, at least four other cases of melioidosis with no clear origin were reported by local and CDC officials, with two deaths. These cases are still being investigated, though they are likely connected, since the bacteria found in each victim has been closely related to the others. They don’t bear a close resemblance to the strain found in the 2019 case, though. In 2018, another isolated case from Texas was reported, but no origin was ever found.
Some researchers have speculated that a warming climate has or will eventually allow the bacteria to establish a natural reservoir in the hottest parts of the U.S.. Dawson acknowledges that possibility, but he cautions that it’s too early to know whether the threat of melioidosis in the U.S. is truly growing or if we’re simply getting better at detecting the few cases that have occurred sporadically for a while. Globally, the bacteria does seem to be growing its territory.
“Historically, these bacteria were thought to be confined to Northern Australia, or parts of Southeast Asia, like Thailand, where it’s hyper endemic. But as time has gone on, we now suspect that it is endemic in parts of Central and South America, the Caribbean, even Mexico,” Dawson said. “But it’s really hard to differentiate between: Are we increasing our recognition and our detection of what’s already there, or is something changing? I think that’s still unclear.”
At the end of the day, both the risk of catching melioidosis in the U.S. and the risk of catching anything from an aquarium remain rare. But fish owners can still take simple steps to reduce that risk, according to Dawson. He notes that the CDC already recommends owners wash their hands before and after feeding fish or cleaning their aquarium and that they wear gloves if they have any open wounds. Those who are immunocompromised should ideally stay away from handling aquariums at all if possible.
Dawson said he hopes this work will help doctors to keep in mind the potential of melioidosis in patients with relevant symptoms who haven’t travelled recently but do own tropical fish.