Leptospirosis in Puerto Rico After Hurricane Fiona, 2022

Leptospirosis is a bacterial disease that people catch from animals. Leptospira bacteria live in soil and water, are carried by animal hosts, and spread through infected animals' urine. Most illness is mild — fever, chills, muscle aches, nausea, vomiting, diarrhea, headache, conjunctivitis — but in about 10%–15% of people who become ill it turns severe and potentially fatal, affecting several organs with kidney failure, liver failure, bleeding in the lungs or meningitis. Puerto Rico reports more cases each year than any other U.S. jurisdiction, and earlier hurricanes there were followed by rises in cases. Worldwide, flooding in endemic areas is linked to outbreaks.

A hard disease to diagnose

Symptoms are nonspecific and easily confused with other diseases, and test accuracy depends on timing. Real-time PCR works best in roughly the first 4–6 days, while the bacteria are in the blood; antibody tests are weak in the first week, since antibodies take 3–10 days to appear. A negative early result does not rule out infection, and patients whose early tests are negative should be retested 7–14 days later — which requires them to come back. Doctors are advised to start antibiotics such as doxycycline while awaiting results, because early treatment reduces illness and death.

The storm and the response

Hurricane Fiona, a Category 1 storm, made landfall on September 18, 2022, bringing heavy rain and severe flooding. Expecting more leptospirosis, the Puerto Rico Department of Health (PRDH):

• asked CDC for a response team on September 20;
• cut the deadline for doctors to report cases from 5 days to 24 hours on September 23;
• consolidated case investigation into a single reporting system;
• issued a clinical management and surveillance guide on September 28 and a virtual training for clinicians on recognizing, reporting and promptly treating leptospirosis — and on telling it apart from dengue, which looks similar and is also endemic;
• stocked its laboratory with more PCR and antibody test reagents and assigned more staff to testing;
• kept up public messaging about avoiding contact with floodwater.

What happened

In the 15 weeks after landfall (September 18–December 31), 823 suspected cases were reported. Of these, 156 were confirmed by PCR (40) or classed as probable by an IgM antibody test (116). Patients' median age was 42, and 74% were male. The commonest symptoms were fever (83%), headache (65%), muscle aches (56%), abdominal pain (47%), nausea (44%) and diarrhea (42%). 112 (72%) were hospitalized, 10 (6%) died, and 95% received antibiotics.

Cases peaked in the first 5 weeks after the storm, at 14.4 a week. Over the whole 15 weeks, the average of 10.4 cases a week was 3.6 times the 2.9 a week in the 37 weeks before landfall.

Weekly rainfall and leptospirosis cases before and after Hurricane Fiona, Puerto Rico, 2021–2023. CDC figure.

Incidence was highest in the inland western municipalities in both periods. Cases stayed elevated afterward, averaging 3.3 a week in 2021, 5.1 in 2022 and 5.2 in 2023.

Leptospirosis incidence by municipality before and after Hurricane Fiona, 2022. CDC figure.

What it means

Contact with contaminated floodwater most likely drove the rise, as the jump in reported water exposure suggests, while greater awareness and testing among doctors probably added to the count. The smaller share of severe outcomes after the storm may reflect detection of milder cases, earlier treatment, or both, and the lasting elevation into 2023 may reflect sustained better detection.

Testing remained a challenge. Some probable cases may have been false positives, since IgM antibodies can persist for a year or more after an earlier infection; 16 people with probable leptospirosis also tested positive for dengue by RT-PCR, consistent with dengue — though co-infection could not be ruled out — being the real cause of their symptoms.

Early identification and treatment are critical. In endemic areas, health departments can strengthen surveillance and raise public and clinician awareness during hurricane season and other high-risk flood periods, and Puerto Rico's surveillance will help it prepare for future storms.

Sources

• Jones FK, Medina AG, Ryff KR, et al. "Leptospirosis Outbreak in Aftermath of Hurricane Fiona — Puerto Rico, 2022." MMWR 2024;73(35).