Water-, Soil- & Plant-Associated Infections

Why the exposure history matters

For environmental infections, one line of history (seawater, leeches, aquarium, gardening, fish handling, hot tub, soil, rodents) usually identifies the organism and its specific treatment.

Water-associated bacteria

Vibrio vulnificus. This curved, salt-loving gram-negative rod thrives in warm coastal water, especially along the Gulf Coast in summer. Raw oysters cause primary septicemia; seawater exposure of a wound causes wound infection. Cirrhosis and iron overload (hemochromatosis) are the dominant risk factors, since the organism thrives on iron. Within a day or two come fever, hypotension and spreading cellulitis with hemorrhagic bullae, often necrotizing fasciitis; septicemia kills about half of patients. Treat immediately with doxycycline plus ceftriaxone or ceftazidime and early debridement or fasciotomy. Patients with liver disease should avoid raw oysters.

Aeromonas hydrophila. This gram-negative rod lives in freshwater and in the gut of medicinal leeches. It causes rapidly progressive cellulitis or myonecrosis after freshwater injuries and after leech therapy for congested flaps or replanted digits. It is ampicillin-resistant; ciprofloxacin or TMP-SMX is used for treatment and as prophylaxis during leech therapy.

Hot-tub folliculitis. Pseudomonas aeruginosa in a poorly chlorinated hot tub or whirlpool causes pruritic follicular papules and pustules concentrated under the swimsuit 8–48 hours after exposure. It resolves on its own within 1–2 weeks; ciprofloxacin is reserved for severe disease or immunocompromised hosts, and the real fix is proper water treatment.

Lymphocutaneous and occupational infections

Mycobacterium marinum. Weeks after a scrape while cleaning an aquarium or handling fish, a papule or nodule appears on a finger or hand and may spread as nodules along the lymphatics ("fish tank granuloma"). Biopsy shows granulomas. The lab must incubate cultures at about 30 °C, because the organism grows poorly at 37 °C. Treatment is clarithromycin, often with ethambutol, for several months. M. marinum is intrinsically resistant to isoniazid and pyrazinamide.

Sporothrix schenckii. This dimorphic fungus contaminates rose thorns, sphagnum moss, hay and soil. A painless papule at the inoculation site is followed by nodules and ulcers ascending along lymphatics, with few systemic symptoms. Tissue culture confirms the diagnosis; histology may show cigar-shaped budding yeast. Itraconazole for 3–6 months is first-line; saturated potassium iodide is an older alternative, and amphotericin B is used for disseminated disease.

Nodular lymphangitis differential. Sporotrichoid spread is also produced by Nocardia brasiliensis after soil-contaminated trauma and by cutaneous leishmaniasis after sandfly bites in endemic areas. The exposure history plus biopsy with fungal, mycobacterial and bacterial cultures separates them.

Erysipelothrix rhusiopathiae. This gram-positive rod is carried by fish, shellfish, swine and poultry and enters small hand wounds of fishermen, butchers and veterinarians. It causes erysipeloid: a painful, sharply demarcated, violaceous swelling of a finger that spreads outward with central clearing and no suppuration. Penicillin or amoxicillin is curative.

Erysipelothrix is intrinsically vancomycin-resistant, which becomes critical if it causes bacteremia or endocarditis.

Soil, freshwater amebae and rodents

Nocardia. Inhaled from soil and dust, Nocardia is an aerobic, gram-positive, beaded, branching rod that is weakly (modified) acid-fast. It infects patients with impaired cell-mediated immunity (transplant recipients, glucocorticoid users, advanced HIV), causing lung nodules and cavities, then spreads to the brain (abscess) and skin. Every patient with nocardiosis needs brain imaging. Treatment is high-dose TMP-SMX, with imipenem ± amikacin added for severe or CNS disease, for 6–12 months or longer. Contrast Actinomyces: anaerobic, not acid-fast, part of oral flora, forming abscesses with sulfur granules and draining sinus tracts, and treated with penicillin.

Naegleria fowleri. This free-living ameba inhabits warm freshwater lakes, hot springs and poorly maintained pools. Water forced into the nose during diving or swimming, or nasal irrigation with tap water, lets it travel along the olfactory nerve through the cribriform plate. Primary amebic meningoencephalitis follows within days: headache, fever, altered smell or taste, then coma and death in more than 95%. CSF shows neutrophils and low glucose, and a wet mount reveals motile trophozoites; combination therapy including miltefosine and amphotericin B rarely succeeds. Use distilled, sterile or boiled water for nasal rinses; swallowing contaminated water does not cause infection.

Hantavirus pulmonary syndrome. Sin Nombre virus follows the course above; falling platelets with a rising hematocrit after a febrile prodrome are the key clues, and no antiviral helps. Soak droppings with disinfectant or dilute bleach before cleaning; never sweep or vacuum them dry.