Potential changes in humidity and temperature could alter the geographic ranges and life cycles of plants, animals, insects, bacteria, and viruses. (For further discussion of forestry and agriculture, see Chapters 5 and 6, respectively.) For example, the range of many plant pests may move northward by several hundred miles. Such changes could occur for insects that spread diseases to both humans and animals. Vector-borne diseases that affect humans are relatively rare in the United States. The incidence of most of those found, however, is increasing. The incidence of some, such as Lyme disease, is increasing dramatically (CDC, 1986).
Both Rocky Mountain spotted fever and Lyme disease are considered to be public health problems in the United States. Although these two diseases are spread by different species of ticks, some overlap exists in their geographic distribution (Figure 12-5). Because tick populations appear to be limited by the size of their intermediate host populations (such as white-tailed deer), the spread of tick-borne diseases may be particularly sensitive to any change that may affect the geographic range of these hosts and, consequently, the range of the vector, or carrier.
In addition to the presence of the host, tick populations also depend upon the seasonality of environmental factors such as temperature, humidity, and vegetation. Optimally, climate must be warm enough to promote progression through the life cycles, humid enough to prevent the drying out of eggs, and cold enough in winter to initiate the resting stage.
As for many tick-borne diseases, the opportunity for a tick to acquire the infective agent from an infected animal is limited to the short period when the level of the agent in the blood of the host is high enough for the tick to receive an infective dose. Higher temperatures may increase the amount of the agent (the organism that is transmitted by the carrier, such as a virus) and the time it remains lodged on the host animal. Both these mechanisms would increase the rate of infection of the carrier. However, although higher temperatures may favor the presence of the agent, there is some indication that they could disrupt the life cycle of some tick species. In these cases, warmer temperatures would reduce both tick survival and the spread of diseases they carry.
Tick populations also vary with the natural vegetation of an area The incidence of Rocky Mountain spotted fever, in particular, has been linked to natural vegetation and changes in climate.
In examining the potential impact of climate change in the United States on Rocky Mountain spotted fever, Haile (Volume G) used a weather-based model, ATSIM, to evaluate the impact of the scenario climate changes on the distribution of the American dog tick, the primary carrier of this disease (Haile, Volume G; Mount and Haile, 1988). The model uses data inputs from the three doubled CO2 scenarios (GISS, GFDL, and OSU) to estimate population dynamics, growth rate, and generation time. Haile assumed that habitats and host density did not change in response to global warming. Sample results for six cities representing the most southern, the most northern, and the two middle latitudes are presented in Figure 12-6. The results indicate that under all scenarios, tick populations would shift from south to north and would be virtually eliminated from the most southern locations (Jacksonville and San Antonio). However, in the middle latitude cities, the results are mixed and depend on the scenario evaluated. The model does not estimate changes in incidence of the disease.
In this analysis, the only model inputs that were changed to simulate climate change were the weather inputs. Other important parameters in the model are the distribution of habitat between forests and meadows and the presence of suitable hosts. Both parameters are likely to be changed relative to current conditions under climate change. As indicated in Chapter 5: Forests, a change from forests to meadows may occur in certain areas of the country; this would depress the tick population. However, the distribution of small mammals also may change. If small mammal populations increased, tick populations would grow. In addition, this study did not consider changes in climate variability, which may have a major effect on the outbreak of diseases.
In a sensitivity analysis of their model Mount and Haile (1988) found that the model predictions could vary sixteenfold, depending on the inputs used for host density, whereas the variability conferred by changes in the weather inputs is about fourfold. Based on the sensitivity analysis, host densities are extremely important to these predictions. Keeping them constant, as was done in this analysis, could have underestimated or overestimated the impact of climate change on the density of the American dog tick.
A second category of vector-borne diseases that can be affected by climate change consists of diseases carried by mosquitoes. Climate changes resulting in more days between 16 and 35deg.C (61 to 95deg.F), with humidity between 25 and 60%, are likely to favor the growth of mosquitoes (White and Hertz-Picciotto, 1985). Mosquito populations are also sensitive to the presence of standing water. It is not clear whether standing water will generally increase or decrease (see Chapter 9: Water Resources).
Worldwide, mosquito-borne diseases are associated with significant illness and mortality. In the United States, however, vector control programs and improved hygiene have virtually eliminated endogenously transmitted cases of these diseases, with the exception of sporadic outbreaks of arbovirus-encephalitis. (Imported cases are seen occasionally.) Numerous mosquito species are present in the United States, however. Recent restrictions on pesticide use, coupled with the influx of visitors and immigrants who can serve as sources of infectious agents, as well as the lack of available vaccines for many of the potential diseases, suggest the potential for reintroduction and establishment of these diseases in the United States--particularly if global warming provides a more suitable climate for their growth and development (Longstreth and Wiseman, Volume G).
At a recent workshop, five of the numerous mosquito-borne diseases were considered to pose a potential risk to U.S. populations if the status quo is disturbed by climate change (Longstreth and Wiseman, Volume G). Malaria, dengue fever, and arbovirus-induced encephalitides were considered to be significant risks, and yellow fever and Rift Valley fever were considered to be possible risks.
Malaria is an infectious disease transmitted by mosquitoes and induced by parasites (Plasmodia). The symptoms are highly variable, depending on the species of the agent. They include chills, sweats, and headache, and in severe cases, may progress to liver damage and even liver and renal failure.
As a result of effective vector control and treatment programs, malaria is no longer indigenous to the United States. However, imported cases occur regularly, and occasionally indigenous transmission has been documented (Longstreth and Wiseman, Volume G). Current U.S. demographic trends, including a large number of legal and illegal immigrants from locations where malaria is endemic, could present a pool of infected individuals that, in conjunction with climate changes, may create sufficient conditions for increased disease incidence.
Haile used the weather-dependent model MALSIM to evaluate the potential impact of climate change on malaria in an infected population living in an area where a competent carrier is present. The model was originally developed to help predict malaria outbreaks in tropical countries such as Kenya This is the first application of the model to the United States. This analysis did not consider changes in climate variability, which may be important for the spread of malaria The MALSIM model showed that several cities in the South (e.g., Miami, Key West, and Orlando), under current climate conditions, are very favorable for malaria transmission. Using the climate change scenarios in MALSIM did little to affect the estimated transmission potential of malaria in the United States (Figure 12-7). In a few cities, e.g., Richmond, Nashville, and Atlanta, the model estimated large increases in one scenario relative to those that would occur normally. However, the results varied with different climate scenarios, did not occur at all locations, and should be considered to be inconclusive.
Dengue fever is an arbovirus-induced illness characterized by fever, rash, and severe pain in the joints. The dengue virus has four different types (DEN 1 through DEN 4). Sequential infection by different types is possible and has been suggested to lead to an increased risk of developing a more severe, hemorrhagic form of the disease that can be fatal in the very young and the elderly. Like malaria, it is not currently endemic in the United States, although potential carriers are present and the disease is imported here regularly by people who have traveled abroad.
The ability of the vector to transmit the agent appears to depend on temperature, and current conditions do not appear to be favorable for this process. Climate changes that raise temperatures, however, may reduce the required incubation period and increase the infectivity of the carrier, increasing the potential transmission of the disease.
Arbovirus-related encephalitides are a group of acute inflammatory diseases that involve parts of the brain, spinal cord, and meninges. In mild cases, these infections result in feverish headaches or aseptic meningitis; in more severe cases, those symptoms can be accompanied by stupor, coma, convulsions (in infants), and occasionally spastic paralysis (APHA, 1985).
At least seven types of viruses causing encephalitis are present in the United States. These include the three forms that also infect horses (the western, eastern, and Venezuelan equine encephalitis viruses) as well as four that are named after the location of their discovery (the La Cross, St. Louis, Powassan, and California encephalitis viruses). Cases range in severity depending on the type of virus, with yearly fatality rates between 0.3 and 60%. These infections are rare. In 1984, 129 cases were reported to the Centers for Disease Control, which maintains an active surveillance program for them (CDC, 1986).
Outbreaks of encephalitis attributable to these viruses are normally limited to specific geographic locations and seasons for several reasons. First, warm temperatures are normally required for the viruses to multiply and to be transmitted to a new host. Higher temperatures may quicken the transmission process and promote epidemic disease. However, the extent of this effect depends largely on the particular virus. Some viruses require cooler weather and higher moisture conditions. Thus, higher temperatures may reduce their prevalence. Second, environmental conditions that favor the presence of carriers and hosts must prevail. For example, relative humidity may affect plant life necessary for the feeding of hosts.
3. The MALSIM estimates of malaria incidence by city under current conditions were based on two assumptions: that there were 100,000 female mosquitoes in the vicinity of each city and that 100 infected people were added to the cities' populations. Under those assumptions, infection of virtually the entire population of Miami was predicted to be possible unless protective measures were taken.
4. An arbovirus is a virus transmitted by an arthropod. Arthropods are a group of animals that includes insects and arachnids. Examples or arthropods that transmit disease include mosquitoes and ticks.