Category Archives: Urban

Malaria, Dengue, Mosquitoes – evolving in the urban environment

As the world increasingly urbanizes, we need to address the role of urban ecosystems and the evolution of disease vectors and organisms.  Marina Alberti and colleagues explained that …

“Recent studies show that cities might play a major role in contemporary evolution by accelerating phenotypic changes in wildlife, including animals, plants, fungi, and other organisms. Many studies of ecoevolutionary change have focused on anthropogenic drivers, but none of these studies has specifically examined the role that urbanization plays in ecoevolution or explicitly examined its mechanisms.”

In their own study they looked at “five types of urban disturbances including habitat modifications, biotic interactions, habitat heterogeneity, novel disturbances, and social interactions.” The researchers learned that, “clear urban signal; rates of phenotypic change are greater in urbanizing systems compared with natural and nonurban anthropogenic systems.” They concluded that there is need to continually “uncover insights for maintaining key ecosystem functions upon which the sustainability of human well-being depends.”

Of particular concern in the area of tropical health are the unique urban manifestations of diseases like yellow fever, dengue and malaria. Although Zika virus, for example, was first discovered in forests, it has adapted to an urban cycle involving humans and domestic mosquito vectors in tropical areas where dengue is endemic. Musso and Gubler in their review further explain that although there may be sylvatic cycles of Dengue, “Arboviruses such as DENV have adapted completely to humans and can be maintained in large tropical urban centers in a mosquito-human-mosquito transmission cycle that does not depend on nonhuman reservoirs.”

Weaver et al. note that Zika in spreading to Asia, “emerged on multiple occasions into urban transmission cycles involving Aedes (Stegomyia) spp. Mosquitoes.” In addition it can be hypothesized that phenotypic changes in Asian lineage ZIKV strains made rare disease outcomes such as congenital microcephaly and Guillain-Barré more common and visible.

According to Estelle Martin and co-researchers, “Puerto Rico, a major metropolitan center in the Caribbean, has experienced increasingly larger and clinically more severe epidemics following the introduction of all four dengue serotypes.” They found that Dengue serotype 4 replaced earlier strains and that “this epidemic strain progressed rapidly, suggesting that the epidemic strain was more fit, and that natural selection may have acted on these mutations to drive them to fixation.”

In addition to virus evolution, mosquito changes have been documented by Caroline Louise and colleagues in “One of the world’s largest urban agglomerations infested by Ae. aegypti … the Brazilian megalopolis of Sao Paulo.”  They detected microevolution despite a short observational period and stress the implications of the “rapid evolution and high polymorphism of this mosquito vector on the efficacy of control methods.”

“The adaptation of malaria vectors to urban areas is becoming a serious challenge for malaria control,” is a major concern of Antonio-Nkondjio and co-workers. They found, “rapid evolution of pyrethroid resistance in vector populations from the cities of Douala and Yaoundé,” Members of this team also learned that the M form of Anopheles gambiae predominated in the centre of urban agglomerates in Cameroon. Previously it was known that larval habitats polluted with decaying organic matter as found in densely populated urban agglomerates, were unsuitable for Anopheles gambiae. The recent study showed that the “M form showed greater tolerance to ammonia (arising from organic matter) compared to the S form. This trait may be part of the physiological machinery allowing forest populations of the M form to colonize polluted larval habitats.”

The evolutionary response of vectors and disease organisms to urban environments needs continued monitoring. Urban disease control and elimination efforts must adapt to such adaptations in the disease process.

Earth Day, Green Cities, Urban Agriculture and Malaria

According to the UN Food and Agriculture Organization (FAO), about six years ago following a global food crisis, a United Nations high-level task force called for a paradigm shift in urban planning, to one that encourages urban and peri-urban food production. By including urban agriculture in urban land use considerations, planners hoped to achieve benefits ranging from improved food security, economic development to greener environments.

FAO recognizes that there are complex political issues involved in meeting the vision of promoting urban agriculture, including basic access to land, water and other resources, before the benefits can be realized. The issue is further complicated by malaria in some urban environments.

Prathiba De Silva and John Marshall observed that, “Malaria transmission in urban and periurban areas is highly focused around vector breeding sites, which tend to be more numerous in areas of lower socioeconomic status. Control strategies should therefore adopt an element of spatial targeting rather than targeting a wide urban area uniformly. Vector breeding sites are common in areas with slum-like conditions and in areas where urban agriculture is practiced.”

In another study Christophe Antonio-Nkondjio and colleagues concluded that, “The data confirm high selection pressure on mosquitoes originating from urban areas and suggest urban agriculture rather than pollution as the major factor driving resistance to insecticide.” Similarly, Seidahmed et al. observed

— Urban gardens along the banks of the Niger River in Bamako, Mali —

different urban agriculture land use patterns between urban and peri-urban areas as well as differences in insecticide resistance.

These studies suggest that urban planners not only need to consider urban agriculture in land use planning but also need a strong working relationship with public health authorities. Both need to work on pesticide use issues. Green cities can be places that help feed their populations, but they can also be places that a strong focus on local land use can be used to prevent malaria.

Mumbai – is transmission season increasing?

The Times of India reports that, “Malaria is no longer restricted to just monsoon months as in the past. Spurred on by widespread construction activity and the resulting poor sanitation, the disease has becomes a round-the-year feature in Mumbai, killing less people but afflicting more.”

An increase was noted: “In all, 76,755 contracted the ailment in 2010, 74% more than the 2009’s figure of 44,035,” but with fewer deaths (better case management?), but it is not clear whether these cases were parasitologically diagnosed.

A member of the medical association attributes the increase, especially the off-season rise, to human activity – construction projects. The official stated that, “Construction sites have puddles of water in which mosquitoes breed. Since construction work goes on throughout the year, so does the breeding. This obviously increases the incidence of malaria.”

Worry was also expressed about, “resistance developed by the Anopheles albimanus mosquito that the civic body’s insecticide fumigation has no effect on it.” This has led the city to consider using “bacillus thuringiensis variety israelensis” for control.

Ironically, in pointing out that, “Another reason for the spread of malaria, which is caused by a parasite called plasmodium, during non-monsoon months is that plasmodium can stay in the body for a long period,” the article raises the possibility that the upswing may not be fully due to new transmission.

asia-in-wmr-2008.gifAside from these possible limitations on the validity of the data,  the potential for increased transmission is worrisome, especially in a part of the world that has received less (but increasing) attention from the Roll Back Malaria Partnership. The map from the 2008 World Malaria Report shows the extent of the problem in Asia.

India has a double problem with malaria, hosting both P. vivax and P falciparum.  A recently published article reports that while the national control program has introduced artemisinin-based combination therapy for P. falciparum as a first-line treatment, the older drugs, chloroquine (CQ) and Sulphadoxine-Pyrimethamine (SP) are still available. Unfortunately Shrabanee Mullic and colleagues found that, “In Jalpaiguri District the overall failure rate of CQ was 61% and of SP 14%, which was well above the WHO recommended cut-off threshold level (10%) for change of drug policy.”

Other research in India examined vector control with positive effects. “A study was conducted to evaluate the preventive efficacy of insecticide-treated mosquito nets (ITMNs) and mosquito repellent (MR) in a malaria-endemic foothill area of Assam, India, with forest ecosystem.” The researchers found that, “The total vector population in the three intervention sectors decreased significantly compared with that of the non-intervention one.”

Overall, malaria in India is a complex phenomenon with different forms of the parasite, different ecological settings and different levels of government involved. More attention is needed to address this complex situation is malaria is ever to be eliminated.

As Africa Becomes More Urban, What Happens to Malaria?

Africa is one of the fastest urbanizing regions of the world. Estimates are that nearly 40% of Africans live in urban areas today. This number is expected to exceed 50% by 2030. UN Habitat reports in State of the World’s Cities that African urbanization is often focused on the major cities like the capital, with major slum development, not a place where anopheles mosquitoes are comfortable.

Studies have shown that malaria is not generally an urban disease because dense, congested and dirty urban settings do not favor the breeding of anopheles mosquitoes. When malaria occurs in urban areas, it is often found in very focal transmission sites, for example, in places where urban agriculture is practiced.

luanda-sm.jpgLuanda in Angola presents a good example of the urban phenomenon as seen in the chart.  Although much of the surrounding country to the north and east are highly endemic areas, Luanda itself was found to have a prevalence of only 3.5%.  There are variations as expected with the somewhat less dense suburbs having greater, but still not high levels of prevalence.

What is important is that this city contains up to half of the country’s 16+ million inhabitants and is growing. Ironically, while prevalence is low, national strategy documents cite malaria as a cause of nearly one-quarter of child deaths in Luanda.  Clearly there are diagnostic challenges.

Unfortunately the absence of malaria in urban areas does not preclude spending money on malaria treatment, as was documented in Nairobi. We documented a similar challenge in Lagos, Nigeria where prevalence among children aged 1-6 years was only 0.9%, but community members had spent thousands of dollars in preceding weeks on antimalarial medicines to treat fevers that they suspected as being malaria.

Moving forward toward malaria elimination will require countries to account for increasing urbanization.  Increased use of diagnostic tools will be required to ensure appropriate and targeted use of anti-malarial drugs.  Vector control activities will need to be strategic and focus specifically on anopheles’ verified breeding sites.

While increasing urbanization may result in proportionately fewer people at risk from malaria, population growth generally will unfortunately guarantee that large rural populations remain at risk.  National malaria strategies need to take these varying ecologies into account if they are going to eliminate malaria.

Urban Malaria – where can we find it?

In Nairobi’s Korogocho slum Yazoume Ye and colleagues looked for evidence of malaria parasites among 1,069 residents. Among those with data, 16.9% had a recent fever episode. Half were treated, primarily with sulphadoxine-pyrimethamine or amodiaquine, while four received artemether-lumefantrine. Ironically, “Three were positive for Plasmodium falciparum using RDT; however, all were confirmed negative on microscopy. Microscopic examination of all 953 readable slides showed zero prevalence.”

These results were similar to a study we did in low income neighborhoods of Lagos in 1998 screening only children between 6 months and 5 years of age [Afr. J. Med. med Sci (2001) 30, suppl. 7-15].  Blood film investigation of 916 children yielded a parasite prevalence of 0.9%.  Knockdown and night landing collections of mosquitoes in houses in these neighborhoods found no anopheles species. Very low densities of A. gambiae larvae were found in breeding sites.

urban-ag-dscn4842.JPGThese findings should not have surprised us since as far back as 1946, researchers had found that urban Lagos was too dirty to host the more finicky anopheles (Muirhead Thomson RC. Studies on Anopheles gambiae and A. melas in and around Lagos. Bulletin of Entomological Research. 1946; 38: 527-558).  Those cases we did find may have resulted from Lagosians visiting their relatives in the village during a recent wedding or funeral ceremony.

We do know that urban environments are not free from malaria, but one needs to identify anopheles-friendly sites and then look for focal transmission sites around those.  Urban agriculture (see photo from Bamako), flower gardens in higher income areas and some of the less dense urban peripheral settlements might be places to look.  This shows the need to plan urban malaria control with full understanding of the micro-ecologies of an urban setting.

urban hunger –> urban agriculture –> urban malaria

The growing problem of urban hunger and urban food insecurity was featured in the Wall Street Journal today. In Monrovia, Liberia, “The cost of a cup of rice has risen to nearly 50 cents from 20 cents, a huge leap for many families who live on less than $1 per day.” The result: “Escalating hunger in African cities is forcing aid agencies accustomed to tackling food shortages in rural areas to scramble for strategies to address the more complex hunger problems in sprawling slums.”

One of these strategies, according to IDRC is urban agriculture:

Urban agriculture (UA) is wrongly considered an oxymoron. Despite its critical role in producing food for city dwellers around the world, urban food production has largely been ignored by scholars and agricultural planners; government officials and policymakers at best dismiss the activity as peripheral and at worst burn crops and evict farmers, claiming that urban farms are not only unsightly but also promote pollution and illness. Contradicting this image, recent studies document the commercial value of food produced in the urban area while underscoring the importance of urban farming as a survival strategy among the urban poor, especially women heads of households.

Urban farming requires water. The International Water Management Institute reports that, “Manual water fetching with watering cans is most common.” They often get water from “polluted streams or they do farming along storm water drains and gutters.” This sometimes leads to “wastewater irrigation.”

Of course malaria vectors need water. In urban Accra, Ghana, Klinkenberg and collaegues found that Anopheles and Culex “outdoor biting rates were respectively three and four times higher in areas around agricultural sites (UA) than in areas far from agriculture.”

The solution to the problem of urban malaria is not to stop urban agriculture, but to intensify integrated vector management interventions.  We certainly don’t want to protect people from malaria and then have then suffer from food insecurity.