The last two decades have seen unprecedented changes in the pattern of serious fungal disease. Infections with the major opportunistic pathogens are increasing in incidence, new pathogens are being reported in unprecedented numbers, and drug-resistant pathogens are emerging. The HIV epidemic is a major factor that has contributed to the dramatic increase in the occurrence of IFI, but medical developments, such as the widespread use of immunosuppressive agents in transplantation and cancer treatment, have also resulted in an increase in size of the population at risk.
Enhanced surveillance is essential to improve our understanding of the epidemiology of IFI. Although passive surveillance has been conducted in a number of countries, this often underestimates incidence rates and healthcare costs, and can lead to inaccurate description of risk factors for fungal diseases. Active surveillance is expensive and difficult to conduct, but it has enabled accurate incidence rates to be determined for a number of opportunistic IFI.
Sentinel and population-based surveillance programs have documented significant changes in the rates of Candida bloodstream infections (BSI) among different patient groups, with the emergence of non-albicans Candida species as important pathogens. More cases of Candida BSI now occur among critical care patients than are diagnosed among neutropenic cancer patients or stem cell transplant (SCT) recipients. C. glabrata has become the second most common cause of Candida BSI in the USA, now accounting for 20-25% of cases. However, for reasons that are not well understood, C. glabrata remains an infrequent cause of Candida BSI in Latin America and Asia, where C. parapsilosis is the second most frequent organism. The emergence of C. glabrata as an important cause of Candida BSI is of concern because of reports that about 10% of incident bloodstream isolates of this species are resistant to fluconazole.
An uncommon infection before the HIV epidemic, cryptococcosis has emerged as an important cause of illness and death among persons with AIDS. In the USA, population-based surveillance documented the declining incidence of cryptococcosis during the 1990s, largely as a result of improvements in antiretroviral drug treatment. In contrast, in resource-limited countries with large HIV epidemics, the burden of this disease is increasing.
Surveillance programs have begun to provide important information about the incidence of invasive aspergillosis (IA) and other serious IFI after SCT. In a report from the TransNet program, the aggregate cumulative incidence of IA at 19 transplant centers throughout the USA, in 2001 and 2002, ranged from 0.5% after autologous SCT, to 3.9% after transplantation from an unrelated donor. Wide variations in incidence were seen between different centers. Many factors could account for this, including variations in rates of follow-up and diagnostic practices between sites. Transplant- and host-related factors may also impact the incidence of IA at individual centers.
A number of other IFI have been seen after SCT. Between 2001 and 2003, IA, zygomycosis and fusariosis accounted for 68%, 8%, and 6%, respectively, of the identified mould infections reported to the TransNet program.
IFI exact a tremendous toll in terms of human life and healthcare costs, despite the fact that these diseases are still under-diagnosed. Establishing and maintaining surveillance programs for IFI, both on a national basis and within individual medical centers, will be essential if we are to determine the burden posed by these diseases. Together with risk-factor studies and cost-effectiveness analyses, this should provide the information needed to develop appropriate intervention and prevention strategies.