Infectious diseases are far from defeated. They pose a unique health threat because they are caused by living micro-organisms. This biological fact has two important consequences: firstly it means that these micro-organisms are constantly evolving to exploit new ecological opportunities, and secondly they are transmissible (they spread from person-to-person and from infected animals and contaminated environments).
The constant evolution of micro-organisms is one of the drivers of emerging infectious diseases. There are more than 1400 species of infectious organism known to cause disease in humans . New emerging infectious diseases most commonly arise from infections in animals . These organisms are also evolving in other ways, including the development of antimicrobial resistance . The net effect is a constantly changing set of emerging infectious diseases which often need an urgent public health response that has to be adapted to the characteristics of each new threat.
The transmissible nature of infectious diseases also creates many features that distinguish them from non-communicable diseases (NCDs). An infectious disease case can become the exposure for other potential cases, which is not the situation for NCDs such as cancer and heart disease. This transmission risk creates the need for specialised infection control staff in healthcare institutions. On the positive side, infectious diseases can sometimes be eradicated as a human health hazard if transmission can be stopped, as has occurred with smallpox and will hopefully occur with poliomyelitis and other pathogens in the future .
The two faces of infectious diseases
The unique features of infectious diseases mean that they manifest two distinct patterns of disease threats.
The first face of infectious diseases is their appearance as a seemingly endless series of new threats that surprise and sometimes terrify us. The most frightening manifestations are pandemics which, by definition, are epidemics that spread to affect multiple geographic regions. These may take the form of new epidemics of known pathogens such as pandemic influenza H1N1, Ebola and Zika, or previously unknown pathogens such as SARS and MERS . Far more common are local epidemics of familiar diseases, which result in several hundred reported outbreaks each year in New Zealand .
The second face of infectious diseases is the one that gets less attention, but ultimately accounts for far more human disease and premature death. This is the ongoing impact of established endemic pathogens. Infectious diseases remain the commonest cause of hospitalisation in NZ, accounting for 27% of acute and arranged admissions . The main categories are respiratory, skin and gastrointestinal infections. The highest rates of these infections are in the most vulnerable populations, particularly the young and elderly. They also show a marked social gradient with higher rates among Māori and Pasifika and households living in more deprived neighbourhoods .
Responding to infectious disease threats
An effective response to infectious diseases requires us to manage their two faces: the emerging hazards and the ongoing established threats to the health of vulnerable populations.
Our capacity to manage emerging infectious disease threats is difficult to fully access until it is tested, though simulation exercises can help. New Zealand has multiple recognised outbreaks each year reported via its national outbreak recording system. Most are relatively small, well-contained food and water borne outbreaks . An exception was the recent Havelock North campylobacteriosis outbreak which was remarkable in terms of its size (estimated at 5,200 cases). This event reminded us of our vulnerability to drinking water contamination and the need to consider increasing pressures on our environment, notably the impact of climate change and intensification of animal-based agriculture . The Havelock North outbreak comes in the context of a much larger, prolonged epidemic increase in campylobacteriosis caused by rising production and consumptions of contaminated fresh poultry meat .
A major advance in managing globally important emerging infectious diseases has come from the International Health Regulations 2005. This agreement requires all WHO member states to assess and report emerging infectious diseases that have potential to develop into public health emergencies of international concern (known as PHEIC) . Since the regulations came into force there have been four such emergencies: pandemic influenza H1N1 in 2009, polio in 2014, Ebola in 2014, and Zika in 2016. Of these, only pandemic influenza spread to NZ, though its impact was comparable to normal seasonal influenza .
The Ebola pandemic in West Africa was a grim reminder of the need for a more proactive approach to managing emerging infectious diseases. There is now growing support for the Global Health Security Agenda which is placing more emphasis on prevention and ensuring adequate health services for people living in low and middle-income countries .
Given the dynamic nature of emerging infectious diseases, New Zealand needs to constantly review and refine its response capacity. We have not been good at documenting lessons from major epidemics and pandemics in the past . The Independent Inquiry into the Havelock North campylobacteriosis outbreak is therefore a welcome opportunity to identify and act on the lessons learned from such events .
In terms of the second face of infectious diseases, there is considerable evidence that New Zealand has performed poorly in recent times. We have a history of increasing rates of hospitalisation for serious infectious diseases, particularly during the 1990s. We also have very high levels of inequality in rates of infectious diseases generally, particularly across ethnic and socioeconomic groups . The most extreme example is rheumatic fever where Māori and Pasifika are 14 to 21 times more likely to develop the disease compared to other New Zealanders .
Addressing New Zealand’s high rates of serious infectious diseases of poverty is a major challenge. This situation requires a highly strategic approach with engagement from multiple Government agencies. An effective response includes achieving high coverage with vaccines and other interventions targeting specific infections like pneumococcal disease and rheumatic fever. Ultimately, the biggest gains depend on improving major health determinants, such as housing  and access to healthcare, and reducing child poverty .
Fundamental to all of these measures is having the infrastructure to develop and sustain the policies and programmes needed to manage both the epidemic and established faces of infectious diseases. This infrastructure requires a high level of multi-sectoral coordination, effective multidisciplinary networks including One Health , and good linkages to international prevention and control initiatives. This capacity needs to be supported by highly developed surveillance systems, a skilled workforce (including infectious disease specialists, microbiologists and veterinarians), and a vigorous programme of operational research to keep ahead of these infectious disease threats. Increasing levels of antimicrobial resistance will provide an important test of New Zealand’s ability to mount an effective response to a serious emerging infectious disease problem .
Infectious diseases are a unique threat to human health and wellbeing. We know a lot about how to manage this challenge but are not necessarily acting on this knowledge. Important gaps include the need to address diseases of poverty and the need to keep investing in surveillance and coordinated response capacity that can detect and manage emerging diseases and pandemics.
Michael Baker is a public health medicine specialist and professor of public health at the University of Otago, Wellington. He is Director of the University’s Health Environment Infection Research Unit (HEIRU), Co-director of He Kainga Oranga / Housing and Health Research Programme, and Chief Investigator at the Australian Centre for Research Excellence on Integrated Systems for Epidemic Response (ISER).
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