Admissions to New Zealand hospitals from infectious diseases have jumped dramatically over the last two decades according to just published research from the University of Otago, Wellington.

The new study in the prestigious international medical journal The Lancet shows that hospitalisations from infectious diseases increased by 51% over the 20 year period 1989 to 2008, accounting for 27% of all acute hospitalisations in New Zealand between 2004 and 2008. By contrast hospitalisations from non-infectious diseases have increased by only 7%.

Lead investigator, Associate Professor Michael Baker, says he was 'taken aback' by the size of the increase.

"What we expected to see was a steady decline in serious infectious diseases and a rise in admissions for chronic diseases, such as cancer and diabetes, which is the expected pattern for a developed country."

"Instead we found infectious diseases had risen far faster than chronic diseases. New Zealand now has the double burden of rising rates of both infectious and chronic diseases".

This first-ever study of serious infectious diseases across an entire country and over an extended period was based on analysis of 5 million overnight admissions to New Zealand hospitals over a 20-year period. It tracked trends across major population groups according to age, ethnicity and socio-economic status.

It found that most categories of infectious disease have risen, with the main contributions coming from increases in respiratory, skin and gastrointestinal infections.

The rise in rates was not even over this period. The largest increase in infectious diseases was in the 1990s, then some improvement in 2001-05, followed by a more recent increase. The rise in the 1990s coincided with a period of rapidly rising income and wealth inequalities associated with major restructuring of the New Zealand economy.

"All New Zealanders pay the price of rising infectious diseases. There are those who are directly affected by these infections But these contagious diseases affect all sectors of society. The increased rates are adding 17,000 hospitalisations a year and tens of millions of dollars in avoidable health care costs," Baker says.

The research also showed that ethnic and income inequalities in infectious diseases are large and increasing. Māori and Pacific peoples are more than twice as likely as the European population to be hospitalised with a serious infectious disease. And those living in the most deprived neighbourhoods have almost three times the risk compared with those living in the most affluent areas.

"Fundamentally what this new research reveals is that the poorest sections of our community are bearing the brunt of an increasing burden of infectious disease, with children and older people in particular ending up in hospital; this is especially so for Māori and Pacific peoples," he says.

For children under five years infectious diseases now account for 64% of acute admissions for Māori children and 68% for Pacific children, compared with 55% for European and other children.

"Because Māori and Pacific populations tend to be over-represented in the poorest suburbs there is a multiplier effect regarding infectious disease risk. This has seen a 77% increase in hospitalisations for Māori and a 112% increase for Pacific peoples from the most deprived areas over the last two decades."

Baker says an example is rheumatic fever, which has almost disappeared as a childhood disease in Western Europe and North America, but is still a serious threat for Māori and Pacific children in New Zealand, causing heart disease and early death in adulthood.

The increased health risk for Māori and Pacific peoples, compared with the European and other group, is much higher for infectious than for non-infectious diseases. As a result, prevention of infectious diseases is likely to be a particularly effective way to reduce ethnic health inequalities in New Zealand.

In an accompanying editorial in The Lancet, Stephen Lim and Ali Mokdad from the University of Washington commented:

"These findings challenge the epidemiological transition theory whereby development is accompanied by a shift of health burden towards chronic diseases and have enormous implications for health and social policy in New Zealand. The health of indigenous people in New Zealand has historically been poorer than the rest of the population and these findings suggest that a rising burden of infectious disease may be leading to a widening of this gap. Urgent action must be taken to reverse this trend."

Co-author Professor Philippa Howden-Chapman says there is a need to develop interventions that address major causes of poor health, including additional strategies to reduce poverty, lower household crowding, and improve access to immunisation and other health services.

"Fortunately we know that it's possible to reduce these distressing diseases - lowering household crowding is the first step The government should be motivated to act, as prevention will save expensive hospital costs".

The study was funded by the Ministry of Health and the Health Research Council.

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Detailed findings from the paper:

• Infectious diseases are the most common reason for overnight hospitalisation in NZ (excluding child birth). They accounted for 27% of acute hospital admissions in the 2004-08 period.

• The incidence of hospitalisation for serious infectious disease increased in absolute terms over the 20-year period 1989-2008:
• 1989-93 - annual rate 1,242 per 100,000 population (equivalent to 1.2% of the population admitted overnight for treatment of an infectious disease each year)
• 2004-08 - annual rate 1,880 per 100,000 population (equivalent to 1.9% of the population admitted overnight for treatment of an infectious disease each year)

• The incidence of hospitalisation for serious infectious disease also increased in relative terms over the 20-year period 1989-2008:
• The incidence of overnight admission for infectious diseases rose by 51% compared with non-infectious diseases which rose by 7%
• Consequently, the proportion of acute hospitalisations caused by infectious diseases increased from 21% in 1989-93 to 27% in 2004-08

• The increasing trend observed from 1989 to 2008 appears to have continued in 2009 and 2010. The infectious disease hospitalisation rate in 2009 of 1,992 per 100,000 population (equivalent to 2.0% of the population admitted overnight for treatment of an infectious disease) was the highest reported over this period, and dropped slightly in 2010.

• Hospitalisations for serious infectious diseases are concentrated in children under 5 years of age followed by older adults (70+ years). Rates are relatively low in those aged 5-69 years.

• Māori are 2.2 more likely and Pacific peoples 2.4 times more likely to be hospitalised for serious infectious diseases than the European/Other population (based on 2004-08 data).

• Those living in the poorest neighbourhoods (deprivation scores of 9 and 10 using the NZDep index) are 2.8 times more likely to be hospitalised than those living in the least deprived.

• Māori and Pacific ethnicity and deprivation (poverty) are independent risk factors for serious infectious diseases. For example, at every level of deprivation, Māori are between 58% and 85% more likely to be hospitalised than the European/Other population.

• These ethnic 'gradients' in risk for infectious diseases are much stepper than those for non-infectious diseases. For example, for Māori the excess risk of infectious diseases is about twice as large as the excess for non-infectious diseases. This finding suggests that lowering infectious disease risk across the population may be a particularly good way of reducing the health gap between Māori and non-Māori.

• Ethnic and socio-economic inequalities are increasing:
• In 1989-93 Māori were 2.0 times more likely to be hospitalised for an infectious disease than European/Other, increasing to 2.2 times more likely in 2004-08.
• In 1989-93 Pacific peoples were 1.9 times more likely to be hospitalised than European/Other, increasing to 2.4 times more likely in 2004-08.
• In 1989-93 the poorest 20% of the population (based on living in the 20% most deprived neighbourhoods) were 2.4 times more likely to be hospitalised with an infectious disease compared with the most affluent 20% of the population (based on living in the 20% least deprived neighbourhoods), increasing to 2.8 times more likely in 2004-08.

• Ethnic inequalities in risk have risen fastest for children less than 5 years of age. This is particularly important as pre-school aged children are the most vulnerable to infectious diseases based on having the highest rates.
• In 1989-93 Māori children were 1.6 times more likely to be hospitalised than European/Other children, increasing to 2.1 times more likely in 2004-08.
• In 1989-93 Pacific children were 1.5 times more likely to be hospitalised than European/Other children, increasing to 2.1 times more likely in 2004-08.

• Poverty and being of Māori or Pacific ethnicity both contributed to an increase in risk over the 20-year study period:
• In 1989-93 Māori in the most deprived 20% of neighbourhoods had an annual rate of hospitalisation for serious infectious diseases of 2,479 per 100,000 population (equivalent to 2.5% of the population admitted overnight for treatment of an infectious disease each year), increasing to 4,403 per 100,000 population (equivalent to 4.4% of the population) in 2003-08. This was a 78% increase in risk.
• In 1989-93 Pacific peoples in the most deprived 20% neighbourhoods had an annual rate 1,953 per100,000 population (equivalent to 2.0% of the population admitted overnight for treatment of an infectious disease each year), increasing to 4,147 per 100,000 population (equivalent to 4.1% of the population) in 2003-08. This was a 112% increase in risk.