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Introduction

Histoplasmosis is a fungal infection that primarily affects the lungs. The condition is caused by Histoplasma organisms, which are often found in soil contaminated with bird or bat droppings. On January 17, 2025, a Georgia infectious disease physician notified ÐÇ¿ÕÓéÀÖ¹ÙÍø of suspected histoplasmosis cases among 12 members of an extended family from households in Georgia, Texas, and Washington. The ill family members included six adults aged 42–49 years and six children aged 8–16 years. They had recently returned from Costa Rica, where they toured a cave linked to a previous histoplasmosis outbreak (1).

Investigation and Outcomes

On January 21, ÐÇ¿ÕÓéÀÖ¹ÙÍø launched a multistate investigation in collaboration with the Georgia Department of Public Health, Texas Department of State Health Services, and Washington State Department of Health to identify and characterize cases. This activity was reviewed by ÐÇ¿ÕÓéÀÖ¹ÙÍø, deemed not research, and was conducted consistent with applicable federal law and ÐÇ¿ÕÓéÀÖ¹ÙÍø policy.*

Thirteen family members traveled to Arenal and Manuel Antonio, Costa Rica, during December 21–28, 2024. Investigation revealed that 12 of the 13 family members toured Venado Caves, a popular tourist location, on December 24 (day 0). All reported seeing bats and having direct contact with bat droppings while crawling and squeezing through tight spaces in the cave. All 12 persons became mildly or moderately ill after returning to the United States, 8–19 days after exposure. Signs and symptoms included headache, malaise, fever, night sweats, myalgias, and respiratory and gastrointestinal symptoms (Table). The one family member who did not tour the cave did not become ill. No other activities reported before, during, or after the trip were associated with known risk for exposure to Histoplasma species.

Five adults and one child sought medical care at an urgent or a primary care facility; no travel history was elicited during their examinations. Among these six patients, three received antibiotics, two received corticosteroids, and one received a cough suppressant. One adult was referred to an emergency department and hospitalized because of abnormal chest radiography findings that raised concerns about lung cancer. However, when health care providers requested the patient’s travel history the next day and learned of the visit to a Costa Rican cave with bats, histoplasmosis became the suspected diagnosis. The patient initially was prescribed voriconazole and discharged 2 days after admission; treatment was subsequently changed to itraconazole (2,3). By 28 days after exposure, all 12 affected family members had recovered or were improving, consistent with the clinical course of mild or moderate histoplasmosis.

Fungal antigen testing was performed for four family members. Two received antigen test results positive for Histoplasma capsulatum species complex, including the hospitalized patient; test results for the other two patients were negative. Antigen testing for all four patients occurred within 1 month of symptom onset, the optimal time frame. However, because antigen detection sensitivity for histoplasmosis is lower in patients with mild disease who are immunocompetent, the negative test results might have been false negatives (4). One patient with a positive test result also received positive complement fixation antibody test results on two separate occasions. Also, in addition to the patient with chest radiography findings that initially raised concerns about lung cancer, another patient also had abnormal chest radiography results, including pulmonary nodules and opacities, consistent with histoplasmosis.

The 12 ill family members were classified as having confirmed (one patient), probable (eight patients), or suspected (three patients) histoplasmosis based on the Council of State and Territorial Epidemiologists (CSTE) case definition (5). The CSTE case definition includes both laboratory and clinical criteria; cases can be classified as confirmed or probable. Fungal antigen tests are considered nonconfirmatory laboratory criteria based on the CSTE case definition due to cross-reactivity with other fungal pathogens (5). One patient met the criteria for a confirmed case because the illness was clinically compatible (i.e., had at least two of the following: fever, chest pain, cough, myalgia, shortness of breath, headache, or erythema nodosum/erythema multiforme rash) and laboratory confirmed (fourfold or greater increase rise in Histoplasma capsulatum serum complement fixation antibody titers taken at least 2 weeks apart). Eight patients met the criteria for having a case of probable histoplasmosis because their illness was clinically compatible, and they were epidemiologically linked to the patient with the confirmed case, even though the condition was not laboratory confirmed. In addition, three patients were suspected to have cases because although they each had only one clinically compatible finding (one with shortness of breath and two with headache), they were epidemiologically linked to the patient with the confirmed case.

Preliminary Conclusions and Actions

The same bat-inhabited cave in Costa Rica associated with this 2024 histoplasmosis outbreak was linked to a 1998–1999 outbreak among 51 persons (1), when H. capsulatum was isolated from bat droppings collected from the cave. In response to the outbreak described in this report, ÐÇ¿ÕÓéÀÖ¹ÙÍø alerted health departments through an Epidemic Information Exchange (Epi-X) notification to identify additional cases. ÐÇ¿ÕÓéÀÖ¹ÙÍø has been collaborating with the U.S. Embassy in Costa Rica and the Costa Rica Ministry of Health to incorporate information about histoplasmosis risks at this location into the caving tour waiver forms. In addition, the U.S. Embassy in Costa Rica issued a health alert in March 2025 notifying the public of the risk for contracting histoplasmosis from caving (6). Persons who have already visited Venado Caves might have been exposed to H. capsulatum, and exposure among visitors might be ongoing. In addition, histoplasmosis diagnoses are often delayed (7). Treatment guidelines only recommend antifungal agents for more severe cases of disease (e.g., itraconazole unless the patient is pregnant, in which case liposomal amphotericin B is recommended when treatment is indicated) (2,3). Antibiotic (i.e., antibacterial) medications are ineffective, and corticosteroids might worsen fungal infections; prescription of these medications to certain affected family members suggests that fungal infections were not initially considered as the cause of the illness. Clinicians should consider fungal illness in the differential diagnosis of patients with constitutional or pulmonary signs or symptoms after recent caving or other activities associated with risk for histoplasmosis (8–10) and should report suspected cases of histoplasmosis to local health departments as applicable (5).