What a strange feeling looking at this map of the 1832 cholera pandemic. It looks like a blotchy bruise on the country. A little surprise at how restricted the pandemic was in America. As it turns out this map is incomplete, ending in October 1832; cholera eventually traveled down the Mississippi to reach New Orleans and stops along the way. One of my first experiences with history of medicine discovered in my own research occurred while doing some family genealogy. Cholera is often depicted among the poor in crowded, old cities like London. For me though, cholera is a disease of the American frontier.
My ancestor John Biggs Moore died of cholera on July 4th, 1833. John Moore was the patriarch of a large family in frontier Illinois on the Mississippi River. He came to Kaskaskia Illinois with his parents in 1781 in the last years of the Revolutionary war. His father James Moore had first seen the Illinois country in the militia of George Rogers Clark when they took the area from English control during the Revolution. John Moore was teenager listed among the men on the first census of Americans in the Illinois country in 1787 used in part to prove that Americans were settled as far west as the Mississippi River to Congress. When American territorial boundaries were established, the Mississippi River was set as the western border, putting them on the furthest edge of the American frontier. (The Louisiana Purchase would extend the frontier in 1804.) The Moore family started the American settlement at Bellefontaine (present city of Waterloo), at the site of a big well-known spring, on the trail between Kaskaskia and Cahokia in 1782.
John Moore’s death on July 4th tells us something about its circumstances. July 4th was the biggest community celebration held on the frontier. John Moore was the son, son-in-law, and nephew of Revolutionary war soldiers and a former War of 1812 soldier. There is little doubt that he would have been part of the independence day celebrations. Cholera came to Illinois the previous summer with Gen. Scott’s arrival with federal soldiers to take charge of the Black Hawk War. They arrived at Fort Dearborn (modern Chicago) on the shore of Lake Michigan and the disease traveled with the troops down the Illinois and Mississippi rivers fading out in the fall. When cholera re-emerged the following summer it is recorded in Belleville in July of 1833 it claimed the life of former Governor Ninian Edwards on July 20, 1833. John Moore died of it over two weeks earlier 20 miles from Belleville. Most of the men in John Moore’s extended family where in the Illinois militia on campaign under his step-brother Gen. Samuel Whiteside during the Black Hawk War. So the community gathering for independence day celebrations the summer after the war, that could have went on for days, is the context of his death.
The arrival of the first steamship in Illinois brought with it the double-edged sword of connection with ‘civilization’. Most of the early Illinois pioneers did not come with grand visions of living in an isolated primitive wilderness. They were very focused on land ‘improvements’ and recreating a Virginia-style plantation landscape. Steam ships would have been heralded as a sign of progress since river trade was vital to their economy until the railroad arrived. The cholera brought to the frontier by the federal troops killed more people than the Black Hawk War.
Zoonoses, pathogens with animal reservoirs, exist as part of a complex system of interactions between animal reservoirs, vectors, ecological factors and human interaction. Landscape epidemiology has existed as a field of study since Russian epidemiologist E.N. Pavlovsky coined the term and laid the groundwork in the 1960s. Landscape epidemiology is in essence the study of environmental foci of zoonotic disease, what Pavlovsky called a nidas. Many of the variables have been identified and studied in individual pathogen systems.
Each system seems so complex and unique that it can be easy to think that they each exist as separate entities with little to do with each other. It is necessary to develop some general principles to both see the bigger picture, and guide research and response to less studied and newly discovered pathogens. Lambin et al. set out to do just that by doing a meta-analysis of eight regional case studies of zoonotic diseases in Europe and East Africa: West Nile Virus in Senegal, Tick-borne Encephalitis in Latvia, Sandfly abundance (leishmaniasis vector) in the French Pyrenees, Rift Valley Fever in Senegal, West Nile Virus hosts in Camargue, Rodent-borne Puumala hantavirus in Belgium, human cases of Lyme borreliosis in Belgium, and risk of malaria re-emergence in Camargue. Obviously, as indicated, not all of these studies look at all factors involved in landscape epidemiology so validation is not solely based on the number of case studies that support each principle.
The ten proposed principles by Lambin et al are shown graphically below where they fit into the system of variables.
Proposed general principles (Lambin et al, 2010):
“Landscape attributes may influence the level of transmission of an infection” This proposal is found in all case species. Features of the landscape influence vector and host distribution across the region of study. Distribution and type of water (fresh, brackish, or salt water) is a common landscape feature that influences density of insect vectors.
“Spatial variations in disease risk depend not only on the presence and area of critical habitats but also on their spatial configuration“. The sheer size of the critical area is not the only or necessarily the most important characteristic to determine risk in an area. Some vectors like ticks thrive along border zones between ecosystems, like edges between woodland and grasslands.
“Disease risk depends on the connectivity of habitats for vectors and hosts” Creating contact zones or contiguous zones that create linked areas are also important. The spatial configuration can create corridors for disease persistence in harsh landscapes. Type and connectivity of vegetation is as important as terrain for vector habitats. Connectivity between suitable habitat for rodents and insects allows the disease to spread from one patch to the next amplifying the pathogen to a level that increases risks of human transmission. Connections between patches of critical habitats allows for recolonization after local extinction.
“The landscape is a proxy for specific associations of reservoir hosts and vectors linked with the emergence of multi-host disease.” Their principle could be better fleshed out; their primary evidence coming from West Nile Virus (WNV). Like other multi-host pathogens, WNV has some hosts that are much more important than others for transmission across wide regions. In WNV migratory birds are a key to understanding its spread and epidemic dynamics. WNV is also an example of a disease with different proxies and amplification hosts in different regions of the world.
“To understand ecological factors influencing spatial variations of disease risk, one needs to take into account the pathways of pathogen transmission between vectors, hosts, and the physical environment.” Vector-borne diseases require direct contact between humans and the vector. For other zoonoses like hantavirus contact between humans and animal hosts can be via aerosols of material with rodent feces or dust containing rodent remains. For example, people have contracted hantavirus by vacuuming up rodent remains in homes. When estimating risk of transmission to humans, abiotic (non-living) environmental conditions that can preserve or transmit to humans have to be considered. Climate and moisture content of the soil are common abiotic factors to be concerned about. Additional support for this principle comes from the role of the rodent burrow system on plague (Yersinia pestis) hosts and vectors.
“The emergence and distribution of infection through time and space is controlled by different factors acting at multiple scales” In their discussion of this principle, they focus on human interaction with the environment and particularly urbanization altering disease risk. They note that climate change and natural environmental change do not account for all emerging and re-emerging disease but the activities of humans including urbanization and ecological change like deforestation. Ben-Ari et al‘s study on plague and climate change also looks at the many factors at all levels from micro to macro scales effect the abundance and likelihood of transmission of the plague.
“Landscape and meteorological factors control not just the emergence but also the spatial concentration and spatial diffusion of infection risk” This principle just adjusts the previous principles to take account of primarily rainfall by looking at temporary ponds or wetlands. This particularly affects mosquito abundance, but as the graphic above demonstrates also effects soil moisture.
“Spatial variation in disease risk depends not only on land cover but also on land use, via the probability of contact between, on one hand, human hosts and, on the other hand, infectious vectors, animal hosts or their infected habitats” Land use has been long known to affect mosquito abundance and disease transmission. Clearing land for settlements or agriculture always increases standing water in ditches, tire ruts, railroad ditches, animal troughs, incomplete building projects, and due to loss of water absorbing vegetation. A century of malaria research and management has focused on land use and the elimination of standing water. Mature water management programs for cultivation or flood control can also alter vector abundance and human contact rates. For example flooding fields to grow rice not only provides habitat for mosquito production but also brings people into the fields to cultivate increasing contact rates. Irrigation canals would have a similar effect.
“The relationship between land use and the probability of contact between vectors and animal hosts and human hosts is influenced by land ownership” In Lambin et al, they looked at the contact rates between public (state) land and private ownership. In these studies state ownership increased access to forestland over private ownership.By the same token, state ownership could also prevent deforestation and urbanization by preserving the wilderness or reserving the land for other uses. Forest age and maturity also varies significantly between state forests and private land.
“Human behaviour is a crucial controlling factor of vector-human contacts, and of infection.” Humans bring themselves into contact with vectors by risky behavior and can control exposure vectors and infections. Obviously, vaccination is one of the controlling factors of infection, although many zoonotic infections have either no or poor vaccines. Occupational and recreational exposure to vectors often explains gender difference in infection rates.
In conclusion these principles begin to mark out the three sides of a zoonotic triangle: biology of pathogen, vector and host; ecological system where they exist; and human behavior and ecological interaction. Human behavior including land use and constructed environments is as important as the other two sides of the triangle. Humans are not passive victims or collateral damage.
Dengue Fever is one of the most concerning emerging infectious diseases of the early 21st century. The virus has been spreading with its ever-expanding host, the mosquito Aedes aegypti. For the last several years there have been naturally acquired cases of dengue fever in the United States and Europe, that are not connected to travel.
Aedes aegypti‘s preference for the urban environment distinguishes it from most mosquitoes. It prefers to lay it eggs in small urban pools of water – flower pots, old tires, car ruts, buckets – rather than natural forest pools. As day-light feeders, bed netting would not be useful against A. aegpyti. It has been known for some time that A. aegypti populations are driven by super-producing sites, pools of water that produce the majority of mosquitoes vs. pools that only produce a very few pupae.
It is known that dengue fever is transmitted by super-spreading events but it is unclear how this is tied to A. aegytpi super-production sites and other factors in the environment. To study this phenomena a group of researchers from Yale School of Public Health and the Institiuto National de Salud in Bogota, Columbia chose a dengue fever endemic neighborhood to study the major parameters in transmission. They identified three primary parameters to monitor.
Distribution of super-producing A. aegypti sites across urban plots with similar characteristics. (Mosquitoes generally remain very close to where they hatch and are believed to be exposed to the virus and transmit it within the plot they hatched, or at most a neighboring plot.)
Density of humans domiciled in the plots who can be infected.
Human to mosquito transmission of the Dengue Fever virus. (Humans are the primary reservoir.)
Padmanabha et al devised a new index they call the epidemic potential, secondary infection rate (Ro) per capita. They hypothesize that human density alters the epidemic potential by altering the dengue viral introduction rate and the secondary infection rate. Padmanabha et al. note that viral transfer from human to mosquito depends on the number of mosquito bites per person, while viral transmission to humans from mosquitoes depends on the number of different people an infected mosquito bites.
They selected 16 similar urban plots in an endemic neighborhood in Columbia with a range of 41 to 142 homes (1-3 city blocks) with a human density of 3.2 to 4.5 residents per house. They surveyed A. aegypti pupae in water containers to estimate mosquito production and trapped mosquitoes to look for infected adults. Humans immune response to dengue virus was also surveyed over the season. The mosquito surveys were conducted seven times and human immune surveys three times over the season. They excluded schools, churches and other civil locations were the community gathers from the plots.
Mosquito density results demonstrated super-production sites in each of the seven surveys within each patch. Only 5% of the house surveys accounted for 92% of the total mosquito pupae found. Pupal abundance accounted for nearly 80% of the variation in vector production. Their model predicted an Ro of 0.88 to 3.87 and correlated with the number of infected humans introductions that produced 20 or more secondary infections; this is only 10% of model repetitions. In most cases introduced viruses to the patch did not produce secondary infections. Analysis of human-to-mosquito transmission (viral introduction to the patch) and mosquito-to-human transmissions (secondary infections) suggest that both human density and vector abundance alter the dengue Ro and epidemic potential. Models using data generated by this study showed that the intersection of human density and vectors per household produced the best estimates of epidemic potential (Ro per capita). Padmandabha et al noted that “when viral introduction is accounted for, human density amplified the effect of A. aegypti super-production on dengue risk”. As they monitored the community over the summer with seven surveys they were able to see the decline in super-production decrease the epidemic potential in areas of highest human density.
These super-productive habitats (at the level of individual homes) are seen here to be critical in producing super spreading events of dengue fever. All of the parameters for what makes a super productive habitat including human behavior have not yet been fully explored. This study looked at residential areas with the same socio-economic status. This team is planning further studies that look at a range of socio-economic communities and incorporate community centers like schools and markets. Studies like this one will be useful for designing strategies to target insecticide programs and other efforts to reduce mosquito abundance and dengue risk.
References Padmanabha H, Durham D, Correa F, Diuk-Wasser M, & Galvani A (2012). The Interactive Roles of Aedes aegypti Super-Production and Human Density in Dengue Transmission. PLoS neglected tropical diseases, 6 (8) PMID: 22953017