What causes a fishy body odor and how is it treated?

Fishy Body Odor: Causes and Treatment

Primary Cause: Bacterial Vaginosis (Most Common)

In women, a fishy body odor is most commonly caused by bacterial vaginosis (BV), which should be diagnosed using the Amsel criteria and treated with metronidazole 500 mg orally twice daily for 7 days. 1

Diagnostic Approach for BV

BV is diagnosed when three of the following four Amsel criteria are present: 2, 1, 3

• Homogeneous, white, non-inflammatory discharge that smoothly coats the vaginal walls 1
• Presence of clue cells (vaginal epithelial cells with bacteria adhered to their surface, creating a stippled appearance with obscured borders) on microscopic examination 1
• Vaginal fluid pH greater than 4.5 2, 1
• Fishy odor of vaginal discharge before or after addition of 10% KOH (the "whiff test") 2, 1

Key Clinical Features of BV

• BV results from replacement of normal H₂O₂-producing Lactobacillus species with anaerobic bacteria, Gardnerella vaginalis, and Mycoplasma hominis 3, 4
• Up to 50% of women with BV are asymptomatic, so the absence of symptoms does not exclude the diagnosis 3, 4
• BV is the most prevalent cause of vaginal discharge and malodor 2, 3

Treatment Protocol for BV

First-line treatment: 1

• Metronidazole 500 mg orally twice daily for 7 days (95% cure rate) 1

Alternative regimen: 1

• Metronidazole 2g orally as a single dose (84% cure rate, lower than 7-day regimen) 1

Important Caveats for BV

• Do NOT treat male sexual partners - this has not been shown to prevent BV recurrence 2, 3, 4
• BV has a high recurrence rate of 50-80% within one year 1, 3
• Probiotics containing Lactobacillus species as complementary therapy with antibiotics can improve cure rates (RR 1.53,95% CI 1.19-1.97) 3

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Secondary Cause: Trimethylaminuria (Fish Odor Syndrome)

If BV is excluded or the patient is male, consider trimethylaminuria, a rare genetic metabolic disorder causing systemic fishy body odor that requires dietary modification and occasionally antibiotics.

Pathophysiology

Trimethylaminuria is caused by impaired hepatic oxidation of trimethylamine (TMA) to odorless trimethylamine N-oxide (TMAO) due to deficiency of flavin monooxygenase 3 (FMO3) enzyme 5, 6, 7

• TMA is derived from intestinal bacterial degradation of dietary choline, carnitine, and TMAO from marine fish 5, 6, 8
• TMA accumulates in urine, sweat, breath, saliva, and vaginal secretions, producing a rotting fish odor 6, 7, 8

Diagnostic Features

• Disorder is present from birth but becomes apparent when foods high in choline or TMAO (marine fish) are introduced into the diet 5
• Diagnosis can be delayed for years (even decades) due to lack of clinical suspicion 9
• Confirmed by marine fish meal loading test or choline loading test with measurement of urinary TMA/TMAO ratios 5

Treatment Approach for Trimethylaminuria

Primary management is dietary modification: 6

• Avoid foods rich in choline (eggs, liver, kidney, peas, beans, peanuts, soy products) 6
• Avoid foods rich in carnitine (red meat) 6
• Avoid marine (saltwater) fish high in TMAO 5, 6

Adjunctive antibiotic therapy (short courses): 5, 6

• Metronidazole, neomycin, or lactulose to suppress gut bacterial production of TMA 5, 6
• Neomycin is most effective at reducing TMA production 5
• These are used intermittently, not as long-term therapy 6

Critical Pitfall

Do not dismiss persistent fishy body odor complaints, especially in children after fish introduction into diet - this can lead to years of delayed diagnosis and significant psychosocial sequelae 9, 5, 7