| Bacterial infection | Causative organism | Clinical features | Neuropsychiatric symptoms | Diagnosis |
| Brucellosis | Gram-negative coccobacilli: B. abortus, B. mellitensis, B. suis, B. canis. | The incubation period for acute infection is 1 - 3 weeks. Malaise, headache, weakness, generalized myalgia and night sweat. The fever pattern is undulant. Lymphadenopathy, hepatosplenomegaly, spinal tenderness, scaroiliitis. | Behavioral changes, chronic psychosis, stupor, hallucination, delirium, and acute psychosis can be an early presentation of brucellosis. Depression is common in untreated chronic forms of brucellosis. | Blood or bone marrow cultures in acute phase. Serological tests for chronic brucellosis (brucella agglutination test). Polymerase chain reaction. ELISA. |
| Typhoid fever | Salmonella typhi | Abdominal pain, headache, fever | Common: delirium, encephalopathy. Less commonly: Persistent psychiatric symptoms (irritability, psychosis, and personality changes), complete recovery following treatment. | Blood culture (1st 2 weeks), Intestinal secretions and urine culture, Bone marrow culture is rarely required, leucopenia, Widal antigen test can be misinterpreted. |
| Syphilis | Treponema pallidum (T. Pallidum) | Primary stage: hard chancre, regional lymphadenopathy. Secondary stage: fever, malaise, arthralgia, sore throat, generalized lymphadenpathy. maculopapular rash, mucous patches, snail-track ulcers. Tertiary (late) stage: Gummas, aortitis, neurosyphilis. | Neurosyphilis: General Paralysis of Insane, symptoms similar to Alzheimer disease, progressive cognitive decline, seizures, personality change, encephalopathy. | Dark ground microscopy, serological, CSF examination for evidence of neurosyphilis, Chest X-ray. |
| Lyme disease | Borrelia burgdorferi | The first stage (7 - 10 days): Erythema migrans at the site of tick bite, headache, fever, malaise, myalgia, arthralgia, lymphadenopathy. The second stage: neurological symptoms, cardiac symptoms, arthritis. | Decreased memory, poor concentration, difficulties in formulating ideas and difficulty in word findings, irritability, daytime hypersomnolence, depression. | IgM antibodies are detectable in the first month. IgG antibodies invariably present later. |
| Leptospirosis | Leptospira Interrogans | Leptospiraemic phase: severe headache, malaise, fever, anorexia, myalgia, conjunctival suffusion, hepatosplenomegaly, lymphadenopathy, skin rash. Immunological phase:usually mild. Meningism. | Commonly: Confusion and delirium. Mania and psychosis may occur. | Usually clinical only. Blood/CSF culture, leucocytosis, thrombocytopenia, elevated creatine phosphokinase. |
| Mycoplasma pneumonia | Mycoplasma pneumoniae | Headache and malaise preceding the chest symptoms. Extrapulmonary: myocarditis, pericarditis, erythema multiforme, arthralgia, gastrointestinal symptoms, haemolyticanaemia, thrombocytopenia, meningoencephalitis. | Encephalitis, meningitis, myelitis, and polyradiculitis. Other reported presentations include coma, ataxia, psychosis, and stroke. Rarely: Kluver-Bucy syndrome | Chest X-ray, cold agglutinins, rising antibody titre, |
| Whipple’s Disease | Tropheryma whipplei | Arthralgia, diarrhea, weight loss. | Commonly: Depression and personality changes. More common: cognitive dysfunction and dementia. |