BY CHARLES BANKHEAD Infections once considered limited to the domain of tropical medicine have created a worldwide presence that poses a major threat to public health and a challenge to diagnostic imaging. A conglomeration of factors have contributed to the proliferation and spread of infectious diseases that previously have been rare or nonexistent in developing countries, according to Dr. Ann Osborn, speaking at an RSNA refresher course titled "Imaging in the Hotzone," held on Monday.

Scientific posters prompt study from RSNA attendees.

Some of the contributing factors have never gone away: poverty, pestilence, squalor, complacency. Others are more complicated, such as organisms' ability to mutate and develop resistance; emergence of virulent new organisms (such as Ebola and Marburg) and re-emergence of old ones (tuberculosis). Overuse and misuse of antibiotics have played a major role in developed nations.

"The bad bugs are back, and new ones are coming," said Osborn, a radiologist at the University of Utah in Salt Lake City. "They are going to be right outside your window, whether you live in Bosnia or Boston."

In support of her contention, Osborn cites recent cases of dengue fever in Texas, cholera in Boston, malaria in New York, and contamination of the water supply that made water undrinkable in Washington, DC, for several days. She also pointed out that many Americans might have overlooked the fact that infections remain the leading cause of death worldwide.

From the radiologist's perspective, uncommon infections can pose challenges to imaging, especially when the infections involve the brain. As one example, Osborn reviewed CNS tuberculosis.

"TB is a great mimicker," she said. "It can present as a large solitary mass that looks like a brain tumor."

A tip-off to TB is the lesion's low signal intensity on a T2-weighted MRI scan. That finding should make the radiologist think in terms of CNS TB, not brain cancer, she said.

CNS TB presents most often as meningitis, accompanied by meningeal enhancement, vasculitis, and possibly calcifications. A thick exudate may surround and fill the cisternal regions. Granulomas may appear in the basal ganglia and in the gray matter-white matter interface.

CNS fungal infections may cause vasculitis at the base of the brain and at the gray-white interface, Osborn said. The infections are often hemorrhagic because the organisms invade the blood vessels. An example is systemic candidiasis, which can invade bone and mimic a nasopharyngeal tumor.

Among CNS parasitic infections, neurocysticercosis is the most common worldwide. Classically the infection has been portrayed in medical school as arising from dying larvae that incite an intense inflammatory response in the parenchyma. That might not be the case, Osborn said. The infection in fact may arise in the subarachnoid space.

"You will likely see a colloidal vesicular ring-enhancing mass," she said. "Healing nodules may be calcified. Many of them appear in the subarachnoid space. The inflammatory response seals the sulcus around the nodule."

Perhaps more familiar are infections due to tick bites, especially Lyme disease. In the CNS the infection can present as neuroborreliosis, a pervasive inflammatory process that resembles multiple sclerosis, Osborn said. A brain scan may reveal white matter lesions or enhancement in the trigeminal area. The finding might be accompanied by a clinical presentation that includes visual disturbances and disorientation.

CNS complications occur in about 2% of patients who have acute malaria. The infection typically involves the gray matter. Findings include evidence of generalized, watershed, or basal ganglia ischemia, and the infection might cause multifocal petechial hemorrhage.

Among the "really bad bugs" that have begun to emerge, ones to watch include Listeria, E. coli, resistant pneumococci and streptococci, and viral hemorrhagic fevers. Antibiotic resistance is playing a major role in the evolution of difficult infections.

"Resistant bacteria are showing up everywhere because of evolutionary pressure in response to new antibiotics," Osborn said. "In the process of acquiring resistance, organisms may become very virulent and simply overwhelm the host in a matter of hours."

The viral hemorrhagic fevers, which include Ebola and Marburg, have already received considerable attention in the lay and scientific media. The viruses are extremely virulent and have no known cure or vaccine.

"Ebola has a 95% fatality rate," Osborn said. "The infection causes multifocal brain hemorrhaging, and most patients die before they can even be imaged. When a patient does survive long enough to have a CT or MR scan, there is usually evidence of diffuse petechial hemorrhage."

CNS involvement from dengue fever can lead to similar findings on CT or MR, she added.

While the infections she described appear ominous in their own right, they may pale in comparison to what many scientists consider an epidemic waiting to happen. Osborn said knowledgeable infectious disease and medical epidemiology specialists foresee an eventual influenza outbreak that will eclipse the 1917-1918 epidemic that claimed an estimated 50 million lives. The bird flu that Hong Kong public health officials managed to contain not long ago is an example of the type of virulent influenza strain that could cause a worldwide epidemic, she said. Others will inevitably arise.

"Experts predict that it's just a matter of time before a worldwide flu epidemic occurs, and when it does, we will not have 50 millions deaths, but possibly multiples of that," Osborn said.