Tropical infections include a wide variety of bacterial, fungal and viral agents, some with unique geographical distribution, epidemiological and clinical characteristics. Furthermore, over 80% of the 30 million people infected by HIV are estimated to live in sub-Saharan Africa and developing countries of Asia. Tropical regions, which include some of the poorest nations in the world, bear a disproportionate burden of infectious diseases, especially tuberculosis, pyogenic meningitis, Japanese encephalitis, entero and herpes viruses. Other infectious diseases that can rarely affect the brain and CNS include dengue fever, melloidosis, and severe acute respiratory syndrome (SARS).

In addition to bacterial, viral and fungal diseases, parasitic disease of the CNS are an important cause of morbidity. Worldwide, the commonest cause of epileptic seizures is CNS infestation by cysticercosis, and cerebral malaria is one of the most serious and feared complications of this widespread and intractable disease. In addition, amebiasis, toxoplasmosis, toxocariasis, schistosomiasis are also important parasitic diseases with cerebral manifestations.

Singapore is an example of a small tropical island at the crossroads of large people movement, and therefore, potential spread of disease. A large percentage of local cases of tuberculosis are imported, and radiologists may encounter many tropical diseases that are not endemic to our small nation, but are endemic to the region. In addition, in today’s globalized and hyperconnected world, the spread of diseases such as influenza and the Zika virus represents a potential public health hazard.

This presentation will focus on a few typical examples of cerebral infections including schistosomiasis and neurocysticercosis, and outbreaks of Nipah virus, group B streptococcus agalactiae infection, but not tuberculosis or viral diseases of children. Clinical examples, mimics and pitfalls will be highlighted.

Identification of the causative organism from the CSF and biopsy are still the gold standard of diagnosis. However, neuroimaging and polymerase chain reaction (PCR) are recent technologies that have revolutionized CNS infectious disease diagnosis. Improved availability and access to MRI may be helpful for initial diagnosis, delineating the anatomical extent of CNS involvement, narrowing the differential diagnosis, anticipating complications, and improving follow up comparison. MRI patterns may be divided into those predominantly affecting the subarachnoid/ventricular, leptomeningeal and/or pachymeningeal, encephalopathic and mass-like ring-enhancing lesions. Each of these patterns may have features that can be helpful for detection and differential diagnosis, for example the presence of cysts, calcification, hemorrhage and aggregating associated non-imaging investigations.

Diffusion-weighted imaging (DWI), especially, is useful in improving lesion conspicuity, and delineating active disease in cerebral abscess, empyema, ventriculitis and detecting the presence of pus in in the subarachnoid space. Other recent advances in new MRI techniques that may be helpful in assessing infection include perfusion MR and MR spectroscopy (MRS), especially in differentiating neoplastic from infectious diseases. Nevertheless, radiologists should be aware that MRI findings are often non-specific, and that imaging adds value to multi-disciplinary consultations with infectious disease specialists, neurologist and neurosurgeon. Our simplified pattern recognition approach is not exclusive to causative organisms, and there is high complexity and overlap, making clinical diagnosis challenging. It is still hoped that with better access to MRI, early stage diagnosis may be helpful.

Radiologists should be familiar with typical MRI findings of common tropical diseases affecting the CNS; the limitations of neuroimaging in differential diagnosis can be overcome by multi-disciplinary conferences to add value to patient management. Newer techniques, especially DWI, may also be helpful for differential diagnosis.