Category Archives: climate

El Niño and Possibly New World Primates Contributed to Zika Explosion

by Michelle Ziegler

The explosion of Zika-related birth defects this past year came out of the blue. Zika has been known since the 1940s but was seen as a mild dengue-like illness (Fauci & Morens, 2016). Leaving aside how and why microcephaly has appeared so dramatically, it is undeniable that Zika’s emergence and transmission in the Americas have been unusually rapid and extensive.

Aedes aegypti from Tanzania (Source: Muhammad Mahdi Karim, 2009, GNU Free Documentation License)

Two papers published in December focusing on the Aedes mosquito vectors begin to shed light on how Zika was able to be established so quickly and pervasively. Zika utilizes the same tropical mosquito Aedes aegypti as dengue; it was once known as the yellow fever mosquito. It is also the vector of the chikungunya virus.

As first observed in West Africa many years ago, Zika epidemics followed a chikungunya epidemic by a couple years. Chikungunya was the emerging infectious disease of 2013, the year that Zika is believed to have arrived in South America (Fauci & Morens, 2016). Unrecognized by public health workers at the time, a Chikungunya epidemic was simultaneously chugging along under the radar in at least Salvador, the capital of the Bahai state of Brazil, during the peak of Zika epidemic of 2015 (Cardoso et al, 2017).

El Niño 2015-2016

In the first study by Cyril Caminade and colleagues at the University of Liverpool modeled Zika transmission in the two critical vector species in the Americas, the tropical Aedes aegypti found primarily in South America and the temperate Aedes albopictus found in the southern United States. It is thought that Zika transmits better from A. aegytpi but more research is needed to fully understand the differences. They developed a two vector, one host model where the climate is a variable to compare the effect of climate patterns on Zika transmission. They ran these simulations for each vector individually and together against historic climate data sets.

When they compared the worldwide distribution of the vectors and climate, they were able to show that all of the countries where Zika has been reported were predicted in their model. Ominously, South America was the most friendly region in the world for Zika (Caminade et al, 2016). The model for Zika produced a map that correlates extremely well with the global distribution of dengue. Due to the overlap of A. aegypti and A. albopictus territory, they found a high probability that Zika would transmit well in most of the southern United States.

Risk of Zika transmission based on their models A. winter of 2015-2016 B. Risk over the last 50 years. (Caminade et al, 2016)

The global climate anomaly known as El Niño is known to impact mosquito-transmitted diseases, so they had a particular interest in comparing the 2015-2016 El Niño to historic data sets. The map shows the predicted Ro (reproduction number) for Zika around the world in 2015-2016  and in the bar graph compared to the last 50 years. The conditions for Zika were the best for the last 50 years. Other hot spots that did not experience a Zika epidemic, like India, did have a record year for dengue. They also note that the African hot spot for ideal transmission conditions corresponds and to Angola where there was a Yellow Fever outbreak. In short, it was a very good year for Andes aegypti! And now, as of January 2017, Yellow Fever had added to their misery in a Brazil.

A Sylvatic Reservoir? 

Understanding if Zika will establish a sylvatic reservoir in South America is of vital importance for projections and mitigation of future Zika epidemics in Brazil and elsewhere in South America. Zika was initially detected in a sentinel monkey in Uganda and has since been detected in a wide variety of smaller primates in Africa and Asia. Using a model originally proposed for dengue they were able to show that primates with rapid birth rates and short lifespans are ideal for establishing sylvatic Zika. In primates with short life span, five years or less, and rapid birth rates, the establishment of a sylvatic reservoir is “nearly assured” (Althouse et al, 2016). They predict that a primate population as small as 6,000 members with 10,000 mosquitoes could support a sylvatic reservoir (Althouse et al, 2016). Ironically, since infection rate is dependent upon bites per primate, a small primate population with a large mosquito population is better at maintaining the reservoir than a large primate population. Old World monkeys like the African Green Monkey, a known African host of Zika, are already established in free-living troops in South American forests.  While A. aegypti favors human environments, A. albopictus prefers forested environments and has been spreading in Brazil.  It could be a prime candidate for a bridging vector between a sylvatic and domestic Zika cycle. Studies on Zika vulnerability and incidence in all South American primates has to be a priority. Our ability to manage Zika in the future depends on it.


Caminade, C., Turner, J., Metelmann, S., Hesson, J. C., Blagrove, M. S. C., Solomon, T., et al. (2016). Global risk model for vector-borne transmission of Zika virus reveals the role of El Niño 2015. Proceedings of the National Academy of Sciences of the United States of America, 201614303–28.

Cardoso, C. W., Kikuti, M., Prates, A. P. P. B., Paploski, I. A. D., Tauro, L. B., Silva, M. M. O., et al. (2017). Unrecognized Emergence of Chikungunya Virus during a Zika Virus Outbreak in Salvador, Brazil. PLoS Neglected Tropical Diseases, 11(1), e0005334–8.

Althouse, B. M., Vasilakis, N., Sall, A. A., Diallo, M., Weaver, S. C., & Hanley, K. A. (2016). Potential for Zika Virus to Establish a Sylvatic Transmission Cycle in the Americas. PLoS Neglected Tropical Diseases, 10(12), e0005055–11.

Fauci, A. S., & Morens, D. M. (2016). Zika virus in the Americas—yet another arbovirus threat. New England Journal of Medicine, 374(7), 601–604.

Landscapes of Disease Themed Issue

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For the last couple years, I have been writing about a landscape-based approach to the study of infectious disease in general and historic epidemics in particular. When I first wrote about Lambin et al.’s now classic paper “Pathogenic landscapes” nearly three years ago, I did not know then that it would be so influential in my thinking or that the Medieval Congress sessions would be so successful. In the fall of 2014, Graham Fairclough and I began talking about ways that this first congress session could be represented in the journal he edits, Landscapes. This issue is a departure from their usual approach to landscape studies so I would like to thank Graham Fairclough for entrusting me with a whole issue. It has been a challenge for both of us, and I am proud of our product.

This issue represents the wide variety of studies that can be done all contributing to an understanding of past landscapes of disease. One of the reasons why I like the phrase landscape of disease, rather than simply landscape epidemiology, is that it opens up the array of disciplines that can be involved. In the study of diseases of the past, humanistic approaches can be as valuable as scientific methods. Both are required to build a reasonably coherent reconstruction of the past. Science and the humanities need to act as a check and balance on each other, hopefully in a supportive and collegial way.

The issue was published online a couple days ago. Accessing the journal through your library will register interest in the journal with both your library and the publisher, and would be appreciated. By now the authors should (or will soon) have their codes for their free e-copies if you do not have access otherwise.

Table of Contents

Landscapes of Disease by Michelle Ziegler. An introduction to the concept of ‘landscapes of disease’ and the articles in the issue. (Open access)

The Diseased Landscape: Medieval and Early Modern Plaguescapes by Lori Jones

The Influence of Regional Landscapes on Early Medieval Health (c. 400-1200 A.D.): Evidence from Irish Human Skeletal Remains by Mara Tesorieri

Malarial Landscapes in Late Antique Rome and the Tiber Valley by Michelle Ziegler

Epizootic Landscapes: Sheep Scab and Regional Environment in England in 1279-1280 by Philip Slavin

Plague, Demographic Upheaval and Civilisational Decline: Ibn Khaldūn and Muḥammed al-Shaqūrī on the Black Death in North Africa and Islamic Spain by Russell Hopley

plus seven book reviews. Enjoy!


Lambin, E. F., Tran, A., Vanwambeke, S. O., Linard, C., & Soti, V. (2010). Pathogenic landscapes: Interactions between land, people, disease vectors, and their animal hosts. International Journal of Health Geographics, 9(1), 54.

A winter’s worth of work

Its well into spring now and my blogging has perhaps hit an all time low. I have been working on a project that I will write about more later this year. I’ve been reading a lot about environmental history, not the usual material for this blog. Some of it is listed below. It’s a sample of the kind of thing that I need to be read to understand disease in the past. I think it will be worth it eventually even if pollen diagrams and geology diagrams are not very exciting. 

I do have quite a few ideas for new posts, so I will be back…soon. 

A sampling of some of my recent reading:

Büntgen, U., Myglan, V. S., Ljungqvist, F. C., McCormick, M., Di Cosmo, N., Sigl, M., et al. (2016). Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD. Nature Geoscience, 1–7.

Mitchell, P. D. (2015). Human Parasites in Medieval Europe: Lifestyle, Sanitation and Medical Treatment. Advances in Parasitology (Vol. 90, pp. 389–420). Elsevier Ltd.

Mitchell, P. D. (2016). Human parasites in the Roman World: health consequences of conquering an empire. Parasitology, 1–11.

Brogolio, G.P. 2015. Flooding in Northern Italy during the Early Middle Ages: resilience and adaption, in Post-Classical Archaeologies. 5: 47-68.

Galassi FM, Bianucci, R., Gorini, G., Giacomo M. Paganotti. G.M., Habicht, M.E., and Rühli, F.J. 2016. The sudden death of Alaric I (c. 370–410AD), the vanquisher of Rome: A tale of malaria and lacking immunity, European Journal of Internal Medicine. [Ahead of Print]

Mensing, S. A., Tunno, I., Sagnotti, L., Florindo, F., Noble, P., Archer, C., et al. 2015. 2700 years of Mediterranean environmental change in central Italy: synthesis of sedimentary and cultural records to interpret past impacts of climate on society, in Quaternary Science Reviews, 116(C), 72–94.

Sadori, L., Giraudi, C., Masi, A., Magny, M., Ortu, E., Zanchetta, G., & Izdebski, A. 2015. Climate,  environment and society in southern Italy during the last 2000 years. A review of the environmental, historical and archaeological evidence, in Quaternary Science Reviews, 1–16.

Li, Y.-F., Li, D.-B., Shao, H.-S., Li, H.-J., & Han, Y.-D. (2016). Plague in China 2014—All sporadic case report of pneumonic plague. BMC Infectious Diseases, 1–8.

Statskiewicz, A. (2007). The early medieval cemetery at Aschheim-Bajuwarenring: A Merovinigan population under the influence of pestilence? In Skeletal series and their socio-economic context (pp. 35–56).