How global warming can heat up infectious diseases in Africa

This is according to Barry Schoub, executive director of the National Institute for Communicable Diseases in Johannesburg.

Schoub was presenting a Darwin seminar at the UCT medical school earlier this week on the potential impact of global climate change on human infectious diseases, with special reference to Africa.

He told the seminar that it has been speculated that infectious diseases such as, Malaria, Dengue fever and Rift Valley Fever could spread because of global warming. This has raised fears that diseases that were previously thought of as only tropical or subtropical may start migrating into cooler regions.

“The concern is that global warming could therefore expand the habitat of infectious diseases because replication is enhanced with increased temperature” Schoub told about 150 people attending the seminar.

However he said that the interaction between climate change and infectious disease was a complex subject involving a variety of factors. Those infectious diseases which could be affected by global warming usually involve a number of hosts with several lifecycles. This made it difficult to know for sure how warming would affect the spread of disease. Certainly non-climate related factors would play a more important role.

But while the impact of warming was hard to judge, it was very clear that many diseases are directly affected by the weather.

Influenza, for instance, was restricted almost entirely to winter. Scientists are still not sure why this is the case. “Here we have a disease almost directly related to the weather,” Schoub said.

Measles and rubella occurred largely in the latter part of winter and early spring while rotavirus was primarily a winter disease.

The weather has a more indirect effect when it impacts on the habitats that support the disease.

Schoub explained that the very serious disease Hantavirus Pulmonary Syndrome, which kills almost half of the people who catch it, is carried by the deer mouse, the cotton rat and by other rodents in the Americas. The floods and increased rainfall generated by El Nino led to an increase in nuts, seeds and insects. This increase has led to a rising rodent population, the “reservoir” of the disease.

In spring, fighting rodent males produce dust that has a high virus content. Humans carrying out spring-cleaning inhale the dust and become infected.

In this way, the increase in Hantavirus Pulmonary Syndrome is indirectly but substantially affected by climate.

Another example, according to Schoub, is the Nipah virus which broke out in Malaysia in the late 1990s and which also has a very high fatality rate of 40%. In this disease, two animal ‘reservoirs' are involved: pigs and the flying fox. The flying fox is the natural host and generally lives in dense forest areas. Human deforestation together with the severe haze caused by the ‘slash and burn' clearing of forests in the region - which severely affected flowering and fruiting forest trees - led to a reduced natural habitat for flying foxes. This pushed them to the orchards of the agricultural sector, introduced them to pigs and passed on through both of them the Nipah virus to humans.

The example illustrated how complex the interaction could be between the weather, animal hosts and humans, according to Schoub. “Nearly a million pigs were destroyed as a result, which rapidly brought the epidemic to an end,” he said.

The connection between climate and disease was first established some 150 years ago by a scientist named Jacob Henle, according to Schoub. Since then, the link has become commonly accepted.

But the unrestricted pollution of the atmosphere and the blanket of greenhouse gases now surrounding the globe has “influenced our thinking very significantly,” said Schoub.

“Greenhouse gases form a layer around the earth which restricts heat from going out of the atmosphere and, instead, bounces it back in to cause global warming,” he told seminar-goers.

“The effect of global warming has been recognised for a number of years. The earth now has a higher surface temperature than at any time in the previous 100,000 years. Estimates suggest this could rise another 2 to 5 degrees by the end of the century. That doesn't sound much, but it is the difference between the ice age and where we are now”.

Scientists are still unclear over what the long-term effect of greenhouse gas production will be on infectious disease. “It's complex, there's no definite answer,” said Schoub.

Some scientists believe that global warming will have a devastating impact on malaria with an additional 70-million people living in malarial areas by 2080.

One group of scientists claimed recently that Great Britain would be a malarial region again by 2050, though this view was widely denigrated by the established scientific community.

Other scientists, such as Paul Reiter from the American Center for Disease Control, have argued that such claims are “pseudoscience” and that the spread of malaria is really due to non-climate factors such as failures in malaria control and by poverty.

Cholera is another disease that is affected by climate variability. While cholera has no vector and is carried by water, remote sensing technology has allowed scientists to link seasonal changes in the surface temperature and height of the sea with the outbreaks of the disease.

Where the sea temperature rises, plankton blooms. This has a direct correlation with the amount of cholera in the water. If the sea rises, plankton rich water is carried into new areas leading to further outbreaks.

Even though it is evident that global warming and climate change will have an impact on infectious diseases, it remains hard to judge the extent of the impact.

But, it was equally clear that humans have a critical role in limiting infectious diseases regardless of how rapidly the temperature rises. Providing clean water, improving sanitation and working to reduce poverty were all actions humans could take that would have critical impacts on disease, in Africa and elsewhere.

The Darwin seminars are a partnership between the Africa Genome Education Institute and the Division of Human Genetics at the University of Cape Town.