“Conversation about the weather is the last refuge of the unimaginative.” – Oscar Wilde

I will state my bias up front.  I am convinced by the preponderance of data in favor of man made global warming.  At the most simplistic level, I can’t see how converting humongous tons of fossil fuel into CO2 and dumping it into the the atmosphere cannot have effects on the climate.  To my mind its like determining vaccine efficacy or evolution.  Plausible mechanism(s), good basic science, multiple studies using different lines of evidence that all come to the same conclusion.  There are lots of fine points and nuances to be worked out, but the basic truth is reasonable and well defined. Infectious diseases lend some validation to the concept that the world is warming, since with global warming comes a variety of infectious diseases.

It is one big IF:THEN statement.  IF global warming, THEN infections.  Of course  if the IF is not true, then the THEN doesn’t follow.

There is the weather, which the Action Channel News never seems to get right, and I will spare you the Mark Twain quote even though I think he is our best writer ever,  and there is the climate, the summation of weather over time.

Interestingly, infections have probably altered climate for short periods of times.  Through history humans burned trees releasing CO2, chopped down forests for agriculture and raised animals, releasing methane.  As humans populations increased, both CO2 from burning and methane from animals increased as well.  Every now and then large numbers of people have died off.  It happen when Columbus et. al. brought infections to the New World and when plague came to the Old.  People died.  Maybe 90% in the Americas (estimates vary widely) and 2/3’s of Europe died.  As a result, burning fuel and agriculture decreased, decreasing emissions and forests grew back, sequestering CO2.  And temperature rise slowed or decreased.

“Abrupt reversals of the slow CO2 rise caused by deforestation correlate with bubonic plague and other pandemics near 200-600, 1300-1400 and 1500-1700 A.D. Historical records show that high mortality rates caused by plague led to massive abandonment of farms. Forest re-growth on the untended farms pulled CO2 out of the atmosphere and caused CO2 levels to fall. In time, the plagues abated, the farms were reoccupied, and the newly re-grown forests were cut, returning the CO2 to the atmosphere…Moreover, if plague caused most of the 10-ppm CO2 drops… it must also have been a major factor in the climatic cooling that led from the relative warmth of 1000 years ago to the cooler temperatures of the Little Ice Age.”

Like all good scientists, he notes the problems with his conclusions

“A more complete assessment of the role of plague- driven CO2 changes in climate change during the last millennium would require a narrowing of uncertainties in both the spatial and temporal occurrence of plague and in the amount of farm abandonment (and reforestation), as well as a resolution of the inconsistencies among the CO2 trends from different Antarctic ice cores.”

An Earth Abides die off of humans is not the correction for global warming I would suggest,  but it is a fascinating association between infectious human deaths and climate change.

As the weather changes, for a week, a season, or over a  longer period of time, the incidence and distributions of  infections change.  Infections could increase or decrease due to something as simple as temperature or humidity.

Or it could be more complex.  Increased rainfall could lead to more food, which could lead to a boom in the rodent population, leading to more interactions of humans and mice, and the next thing you know you have bubonic plague in India or a Hanta virus outbreak in the four corners of the US.

The daily weather makes a difference in infection risk.  My favorite example is Legionella pneumonia, which increases shortly after thundershowers and humid weather.  It may explains why we do not have a lot of Legionella in the NW despite all the rain; it is rarely hot and humid.

In Philadelphia,  Legionella

“Cases occurred with striking summertime seasonality. Occurrence of cases was associated with monthly average temperature (incidence rate ratio [IRR] per degree Celsius, 1.07 [95% confidence interval [CI], 1.05-1.09]) and relative humidity (IRR per 1% increase in relative humidity, 1.09 [95% CI, 1.06-1.12]) by Poisson regression analysis. However, case-crossover analysis identified an acute association with precipitation (odds ratio [OR], 2.48 [95% CI, 1.30-3.12]) and increased humidity (OR per 1% increase in relative humidity, 1.08 [95% CI, 1.05-1.11]) 6-10 days before occurrence of cases.”

There was a recent study that showed increases in Legionella in roadside puddles after a rain.  I ask the housestaff to look for Legionella after thundershowers and I usually get a case or two, although it may just be due to increased diagnostic testing.  The last case of Legionella had spent the day mucking about in his backyard puddles after a thundershower.

Can you catch a cold when the weather is cold? Maybe.  It has been a topic of interest since the dawn of the medical literature.

“The average outdoor temperature decreased during the preceding three days of the onset of any RTIs, URTI, LRTI or common cold. The temperature for the preceding 14 days also showed a linear decrease for any RTI, URTI or common cold.”

More interesting are the infections associated with El Nino oscillations, where the ocean temperatures vary on a 3 to seven to year cycle, leading to alternating wet and dry weather.  As a result

“In North America, El Niño creates warmer-than-average winters in the upper Midwest states and the Northwest, thus reduced snowfall than average during winter. Meanwhile, central and southern California, northwest Mexico and the southwestern U.S. become significantly wetter while the northern Gulf of Mexico states and Southeast states (including Tidewater and northeast Mexico) are wetter and cooler than average during the El Niño phase of the oscillation. Summer is wetter in the intermountain regions of the U.S. The Pacific Northwest states, on the other hand, tend to experience dry, mild but foggy winters and warm, sunny and early springs.”

Changes due to the El Nino lead to changes in the incidence of a huge variety of infections: an example, I think, from WHO. This picture is in my files without reference.

Climate change will affect the distribution of disease vectors such as insects and snails.  Vectors may thrive with increased temperatures or they may die off, but more likely the vectors, like mosquitos, will move.  It has been estimated that half of everyone who has ever died has died from a mosquito borne illness (I admit I heard this numerous times at ID lectures but do not have reference, at least there is a solution).  As it gets warmer, mosquitos can either go up in elevation or North.  It seems that they are doing both.

Dengue has appeared at higher altitudes than previously reported in Costa Rica (at 1,250m),and in Colombia and India (at 2,200m).The previous range was temperature limited to approximately 1,000 metres above sea level.

In Mexico, the dengue vector (Aedes aegypti) has been detected at 1,600 metres; transmission of dengue was unknown above 1,200m before 1986. There have been cases of dengue near or above the altitude or latitude limit of transmission and would be vulnerable to the small increases in temperature that have occurred across these regions.

Dengue has, by serology, infected 40% of the populations of Brownsville Texas, as the disease slowly moves north.

“In the fall of 2004, during a period of endemic dengue transmission, a cross-sectional survey was conducted in these two cities,4 and dengue incidence and prevalence were measured. In Brownsville, the incidence was 2%, which, if extrapolated to the 2005 population of the city (using the 95% confidence interval), projected between 837 and 5,862 recent infections. Similarly, the prevalence was 40%, with a range from 56,948 to 75,372; these values are relatively similar to those obtained from Brownsville in 2005. “

Other examples of climate-related changes in the prevalence or distribution of pathogens and their vectors include the resurgence of Mediterranean spotted fever in Spain and Italy, the recent epizootic of African horse sickness in Iberia,the resurgence of plague in parts of southern Africa,increased incidence and geographic spread of algal blooms, outbreaks of opportunistic infections among seals,and the spread and establishment of pathogens and vectors in Switzerland. More than mosquito born illnesses are changing in prevalence.

Hanta is increasing in Belgium.  There has been an increase in the average temperature which has lead to increased broadleaf trees, with increased seeds, with increased voles, which carry Hanta, which infected humans to cause renal failure .

Oceans are getting warmer and supporting infections.  Vibrio was not found in Alaskan oysters as the water was too cold.  The water temperature was always less than 15 C.  No longer.  The mean temperature has increased each year since 1997  and now supports the growth of V. parahaemolyticus with resultant outbreaks.  Other oceanic infectious diseases are increasing as well.

However, not all is doom and gloom.  Some infections may fade with global warming. For example, RSV may be disappearing as England warmsm.

“The seasons associated with laboratory isolation of respiratory syncytial virus (RSV) (for 1981–2004) and RSV?related emergency department admissions (for 1990–2004) ended 3.1 and 2.5 weeks earlier, respectively, per 1°C increase in annual central England temperature ( and .043, respectively). Climate change may be shortening the RSV season.”

Diseases that may increase in the US or become endemic again include malaria, dengue, and Leishmaniasis.  A 4 degree rise in temperature could allow dengue to exist as far north as Winnipeg and malaria to be in all of Europe. Seems to be a good trade off to me: more dengue and malaria, less RSV.

Good times for an infectious disease doctor.

These studies are representative of the literature (such a better phrase than cherry picking), not a comprehensive review of the topic.  Personally, I find this adjunctive data compelling  support of global warming, at least over recent times (deliberately worded to not commit to the meaning of recent.)  This does not include all the other potential interactions between human behaviors and changes in the weather that result in an increase in infectious diseases.  Even simple local changes can lead to the unexpected increase in the risk of diseases.

“Adjustable rate mortgages and the downturn in the California housing market caused a 300% increase in notices of delinquency in Bakersfield, Kern County. This led to large numbers of neglected swimming pools, which were associated with a 276% increase in the number of human West Nile virus cases during the summer of 2007.”

All the neglected pools became mosquito breeding grounds, and the disease spread was exacerbated in part by a drought that altered bird populations from resistant finches to susceptible sparrows that were not immune to West Nile, allowing the disease to spread.  The result, I suppose, of failed flock immunity.

Imagine how war, human migration, starvation will interact with climate change to increase or alter the spread of malaria, Tb and some infection that no one can predict.  If H1N1  and SARS proved anything, it is whatever new infection will sweep  across the world , it will not be the infection we predict. Who would have thought in 1989 that the next decade would see West Nile virus, never seen the the US, arrive to the continent in a migrating goose and become endemic. When I started medical school in 1979, there was no AIDS.

Maybe its just the weather, the season, and not climate change that is causing the change in the epidemiology of infections.  I do not think so.  I think these infectious disease associations lend credence to climate change. Another line of converging evidence in support of global warming.

Since it is getting warmer, maybe I will finish with a little Twain after all.

“When a person is accustomed to 138 in the shade, his ideas about cold weather are not valuable….In India, “cold weather” is merely a conventional phrase and has come into use through the necessity of having some way to distinguish between weather which will melt a brass door-knob and weather which will only make it mushy.”  - Following the Equator