WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - Introduction
Types of surveillance
Table 1.1 presents the types of surveillance data available for the nine diseases covered in this report. This includes the information collected, years covered, type of surveillance, frequency of reporting, and the strengths and weaknesses of the surveillance system.
Reporting cases and deaths
One of the mainstays of communicable disease surveillance is the reporting and confirmation of cases seen in health facilities. This is known as passive reporting (in contrast to active case-finding methods where cases are actively looked for). For passive reporting to be successful, primary health care providers must be able to recognize the clinical manifestations of reportable diseases. This involves having clear, uniform case definitions available at the peripheral level. In addition, laboratories need adequate resources to make the required laboratory diagnoses.
Passive surveillance has many weaknesses. First, in many parts of the world there is very little access to health care facilities, and many people fall ill or die at home without ever visiting a health facility. Thus many cases are not reported. Second, there are problems of under-recognition of diseases, particularly those that are new to an area or those with non-specific symptoms. Third, in many parts of the world the level of laboratory support is inadequate. Fourth, there are common logistical problems in reporting in many parts of the world, over-worked and underpaid staff, lack of motivation for reporting when no feedback is provided, and a need for further training. Overall, there is considerable variation in the quality of reporting systems from country to country, reflecting economic, social, cultural and epidemiological differences.
There are several typical reporting practices used, depending on the control measures needed, and the specific regulations in the country.
Three diseases are currently subject to the International Health Regulations (2): yellow fever, plague, and cholera. The regulations, which were first adopted by the World Health Assembly in 1951 and then revised slightly in 1969, are a mechanism to provide security against the international spread of epidemic diseases with a minimum interference with world traffic. These are the only binding international legislation for public health and they require that:
Each national health administration should inform WHO within the first 24 hours of being informed of the first suspected case on its territory of a disease subject to the Regulations. This includes both indigenous and imported cases. All subsequent cases and deaths should be reported to WHO.
For these diseases the report from the health professional to the next higher administrative level is done by a rapid method such as phone, e-mail, fax or telex.
Although all cases and deaths from yellow fever, plague and cholera should theoretically be reported to WHO, this does not always happen in practice. In many instances, countries are unwilling to notify WHO because of the fear of economic and political consequences, such as the loss of tourism and trade, and the imposition of travel restrictions. This causes underreporting and reporting delays. Therefore reported data for the diseases covered by the International Health Regulations need to be interpreted with caution.
For diseases not subject to the International Health Regulations, national reporting practices and laws vary across countries. For infectious diseases with potentially high case fatality rates which can spread rapidly (such as meningococcal disease), most countries require rapid reports of the first occurrences of suspect cases. For other diseases, such as pneumonia or AIDS, weekly, monthly, or quarterly case reports are done. Not all infectious diseases are routinely reported, as reporting every infectious disease would place an undue burden on health services.
Some countries have sentinel sites that report more frequently and sometimes on more diseases than the routine reporting system. If these sites are well chosen, they can provide a wealth of information in a timely way – something that would be impossible to expect of all primary health care centres. The disadvantage of relying on sentinel sites alone is that they may not necessarily be representative of the country as a whole.
With the exception of the International Health Regulations which are determined internationally, reporting requirements for infectious diseases are nationally or sub-nationally determined. For example, a disease like leishmaniasis is notifiable in some high risk countries but not in all. Even within countries there may be important differences. For example, reporting of HIV is required in some states in the United States of America but not in others.
As a result there are differences from country to country, and even within countries in how the reporting of each disease is carried out. This makes sense because each country faces a different set of disease related circumstances. However, it does introduce an element of non-comparability into global disease surveillance systems, since information on the same disease is collected in a somewhat different way depending on the country. This must be kept in mind in the analysis of global surveillance data.
Six of the nine diseases in this report depend heavily on reported numbers of cases and/or reported numbers of deaths to track the disease in terms of person, place and time. These include cholera, plague, yellow fever, meningococcal disease, dengue, and leishmaniasis (including leishmaniasis/HIV co-infection).
WHO headquarters maintains disease specific global data bases including the reported numbers of cases and deaths for each country by year. During analysis and interpretation, these data are often supplemented by additional information, and scientific studies. For example, in many instances scientific studies indicate that disease transmission has taken place in a particular country, even though there have been no reported cases. In general, WHO data are adequate to present a broad reflection of disease and mortality trends as is done in this report. More disaggregated data are usually needed for more in-depth analyses.
Surveillance of disease strains
Detection and reporting of disease strains is very important for all infectious diseases, since new strains have the potential to cause new epidemics and pandemics. For some diseases, such as influenza, new strains occur frequently. For influenza a major component of surveillance is to track circulating virus strains, which is key for the development of appropriate influenza vaccines each year. Dengue is another disease where particular importance is given to keeping track of circulating virus strains to assess the potential for outbreaks of dengue haemorrhagic fever.
Surveillance of strains relies on laboratory reports both for the confirmation of clinical diagnoses, and for the assessment of antimicrobial resistance. Good surveillance requires strong laboratory facilities, appropriate resources both human and financial, access to necessary reagents, and strong quality control. Currently, laboratories in many developing countries, particularly in Africa, are not functioning well enough to meet surveillance needs. WHO is making considerable strides in rebuilding infectious disease laboratory capacity in developing countries. In addition, WHO Collaborating Centres and reference laboratories provide international support for such tasks as identifying outbreaks, and identifying problem specimens.
Screening the population for communicable diseases is not often done because it is expensive and potentially invasive of privacy. Sleeping sickness (in particular, gambiense sleeping sickness) is one of the few diseases that uses systematic population screening to find cases. All those who screen positive are referred to treatment centres, where they are re-tested and treated if infected. The certain fatality of untreated sleeping sickness, and the impracticality of other methods of surveillance and control, makes systematic screening of populations living in high-risk areas imperative.
Surveillance of HIV/AIDS
HIV/AIDS surveillance differs from surveillance of other diseases in many ways reflecting transmission patterns, the long latency period, the lack of affordable treatment and cure, high case fatality rates, and the social stigma associated with HIV infection. HIV/AIDS surveillance can be carried out in different and complementary ways. The first surveillance data collected were reported AIDS cases. This was the easiest data to collect, and had the advantage of raising awareness of countries about the disease. In developed countries, AIDS cases were also used for calculating the past prevalence of HIV infection. These data can be detailed enough to provide breakdowns by age, sex and probable mode of transmission. However, because of the long latency period, during which HIV infection is basically asymptomatic, reported AIDS cases reflect infection that occurred many years ago and are not appropriate for tracking current infections. HIV/AIDS surveillance also poses a number of special ethical problems arising mainly from the stigma and discrimination attached to AIDS, and the lack of access of most infected people to treatment. Therefore, unique methods for estimating current prevalence rates have been developed, which involve unlinked anonymous testing. Finally, HIV/AIDS surveillance includes behavioural surveillance, in order to understand trends in behavioural risk factors for HIV.