A fascinating paper in CDC’s journal, Emerging Infectious Diseases has more details on a problem we first mentioned, on the basis of news reports, back in June. It’s about a possible relationship between West Nile Virus infection and the mortgage crisis, but the paper also gives a dramatic example of how the physical, biological and social environment can affect disease patterns and risks in populations.
Infection with West Nile Virus is primarily a disease of birds. It is transmitted from bird to bird by mosquito bites and the disease is maintained by the cycling between birds and mosquitoes. Other animals, like humans and horses, become infected incidentally when a mosquito bites an infected bird and then us or a horse. We aren’t a reservoir for the disease, just collateral damage.
But the damage can be considerable and in 2007 the Bakersfield area of California became the epicenter of the largest mosquito-borne outbreak of encephalitis of any kind since a 1952 Western Equine Encephalitis episode. Confirmed human cases of West Nile Virus (WNV) infection climbed 200 – 300% over the previous year. This was a surprise because all the signs suggested the season would be mild. The winter and spring were unusually dry and hot and as a result the rural mosquito vector (Culex tarsalis) populations were below average. The river that runs through the region was largely dry. The bird species most likely to be infected were decreased in abundance because of the drought, and the surviving birds had high herd immunity to WNV because of previous infections.
Things didn’t work out that way, though. Infection rates in the main urban mosquito vector, Culex pipiens quinquefasciatus, increased dramatically in June, a month earlier than usual. The decrease in house finches and less predation by jays of sparrow nestlings caused a large increase in house sparrow populations, dominated by young hatchlings with no prior immunity. Soon sparrows were the largest group of WNV infected dead birds reported by the public. As in other years, bird infections were followed shortly thereafter by human cases. What was going on in Bakersfield?
By examining service requests and using aerial surveys researchers from University of California – Davis and in the Kern County Vector Control District (KCVCD) discovered an extensive number of green or neglected pools that were producing mosquitoes:
The likely reasons for neglected pools are the adjustable rate mortgage and associated housing crises in Kern County and throughout California, which have led to increased house sales, notices of delinquency of payment, declarations of bankruptcy and home abandonment. Kern County was especially affected, with a 300% increase in notice of delinquency than in the spring quarters of 2006-2007. Associated with home abandonment was the expanding number of neglected swimming pools, Jacuzzis (hot tubs), and ornamental ponds. As chemicals deteriorated, invasive algal blooms created green swimming pools that were exploited rapidly by urban mosquitoes, thereby establishing a myriad of larval habitats within suburban neighborhoods that were difficult to locate from the ground. These pools frequently were located within new housing tracts and not confined to old neighborhoods. An aerial photograph of a representative Bakersfield neighborhood shows the extent of the problem, with 17% of the visible 42 pools and Jacuzzis appearing green and likely producing mosquitoes. The extent of this problem also was indicated by the marked increase in the number of pools that required treatment by the KMVCD. The increase in August 2007 followed an aerial survey of Bakersfield that enabled identification of previously unknown problem pools. (Reisen WK, Takahashi RM, Carroll BD, Quiring R. Delinquent mortgages, neglected swimming pools, and West Nile virus, California. Emerg Infect Dis. 2008 Nov[DOI: 10.3201/eid1411.080719]; [Epub ahead of print])
Swimming pools can be dangerous and there have been many tragic accidents involving toddlers or children wandering into a neighbor’s pool and drowning. As a result ordinances have been enacted requiring controlled access. This produced an unexpected obstacles to surveillance and treatment as mosquito control personnel encountered the obligatory six foot fence and locked gate.
One further result alarmed public health officials. This last winter, the previous C. pipiens urban mosquitoes were replaced by the more competent rural vector, C. tarsalis, in many of these unmaintained pools. The reason for this isn’t clear. So far California has reported 272 cases this year, the largest number of any state — but none of them have been in Kern County where Bakersfield is located.
The authors conclude:
Anthropogenic landscape change historically has facilitated outbreaks of pathogens amplified by peridomestic vectors such as Cx. pipiens complex mosquitoes and associated commensals such as house sparrows. The recent widespread downturn in the housing market and increase in adjustable rate mortgages have combined to force a dramatic increase in home foreclosures and abandoned homes and produced urban landscapes dotted with an expanded number of new mosquito habitats. These new larval habitats may have contributed to the unexpected early season increase in WNV cases in Bakersfield during 2007 and subsequently have enabled invasion of urban areas by the highly competent rural vector Cx. tarsalis. These factors can increase the spectrum of competent avian hosts, the efficiency of enzootic amplification, and the risk for urban epidemics.
Birds, mosquitoes, viruses, cultural artifacts (swimming pools), climate (drought), urban landscape, adjustable rate mortgages, government regulation. What a tangled web we weave.