Prevention and Control of Arboviruses

Surveillance is undertaken to alert us to which viruses are circulating in a given area and is an important first step for the prevention and control of arbovirus infections. It can involve detection of virus activity in humans or other hosts, particularly the vertebrate amplifying hosts, vectors, or all of the above. Information from surveillance provides the basis for ongoing risk assessment and implementation of disease prevention strategies such as distribution of health education materials and vector control operations.

The following examples underscore how surveillance data are used to help prevent arbovirus infections. The work of Moore et al. (1993) provides some excellent examples of how surveillance is conducted for the arboviruses of North America.

In North America, surveillance for West Nile virus has served as the primary gauge of the spatial and temporal risk of human infections through the examination of patterns of infection in dead and live birds (amplifying hosts), mosquitoes (vectors), horses, and humans. (The U.S. Geological Survey maintains an interactive website with maps of West Nile and other arbovirus infections in the United States; see the section ‘Relevant websites.’)

This website shows the progression of West Nile virus across the United States during its introduction and subsequent change to an endemic disease, and has maps for human, bird, mosquito, and veterinary cases. In turn, the surveillance data are used to modulate the intensity of health education (e.g., whether to alert the public and increase personal protection messaging) and to help guide decisions about mosquito abatement (i.e., where, when, how intensely, and what type of mosquito control measures are warranted).

Surveillance for dengue operates in a similar manner to West Nile virus; however, surveillance for disease in people is paramount because humans are the amplifying host for dengue. The serotypes of the virus that people are repeatedly exposed to are important risk factors in predicting whether the outbreaks will result in occurrence of the more severe dengue hemorrhagic fever manifestation.

As a consequence, prevention of dengue hemorrhagic fever outbreaks relies heavily on surveillance for fevers (evidence of dengue activity), recognition of dengue hemorrhagic fever cases and virological surveillance (i.e., knowledge of the serotypes currently and previously circulating in the area), as well as in-depth mosquito surveillance.

Outbreaks of dengue hemorrhagic fever are typically brought under control through the integration of disease surveillance and treatment of people and vector control activities. Thus, in both of these examples, maintaining functional surveillance networks is critical as it provides the ‘science-based’ information required to make sound decisions about the need for, and extent of, intervention activities.

The only methods currently available to reduce the risk of human illness or death as a result of arbovirus infection are vaccination, health education, and vector control strategies. Unfortunately, few vaccines exist for the vast array of arboviruses capable of causing human or animal disease. There are commercially available vaccines for human use only against yellow fever, Japanese encephalitis, and TBE.

Unfortunately, the use of some of these vaccines by travelers can be problematic. For example, two vaccines are available to prevent TBE; however, long-term protection requires three doses (the first two separated by 4-12 weeks and the last one at least 9 months after the second). As a result, relatively few travelers will be in a position to benefit from immunization, though an accelerated schedule can be employed when travelers are likely to be in high-risk situations (i.e., high probability of contact with infected ticks).

Fortunately, yellow fever vaccine is protective after a single dose. A greater number of vaccines are available to protect horses and other livestock from commonly occurring arboviruses. For example, in North America, vaccines are available for use in horses to protect against eastern equine encephalitis, western equine encephalitis, Venezuelan equine encephalitis, and West Nile viruses.

Vaccines for selected arboviruses (e.g., Rift Valley Fever) are available to high-risk individuals under special circumstances (e.g., laboratory workers in an outbreak situation) and research continues to develop and validate human vaccines for several important arboviruses such as West Nile virus and dengue.

Health education is a disease-prevention tool that protects individuals within a target population through the dissemination of information on risk factors for exposure to arboviruses. For example, results of regional and global surveillance programs are often available for selected arboviruses (e.g., reports in local media or through travel medicine clinics or websites) such that citizens can be informed about current and/or seasonal changes in risk of exposure to particular arboviruses and whether vaccines are available for a particular arbovirus. Health education also provides people with information about strategies that can be employed to reduce the frequency of bites from infected vectors.

The suitability and effectiveness of a particular strategy will vary somewhat depending on the type of arthropod vector and include:

- use of appropriate clothing such as long-sleeved shirts, long pants (i.e., to minimize the amount of skin exposed to biting arthropods);
- use of repellents on skin (e.g., DEET) or clothing (e.g., permethrin);
- avoiding habitats infested with vectors and limiting outdoor activity at times of the day when vectors are most active (e.g., dawn and dusk for some mosquito vectors);

- minimizing contact with vectors while indoors (e.g., making sure window screens and doors are in good repair or through the use of bednets);
- physical examination and prompt removal of attached arthropods (e.g., performing a tick check);
- elimination or modification of local microhabitats suitable for vector development (e.g., elimination of standing water to prevent mosquito development or vegetative/landscape management to increase mortality of ticks).

Theoretically, if the majority of people followed these health education messages, human cases of arboviruses would be extremely rare. However, all health education programs suffer from lack of compliance, and although many people hear the message, for a variety of reasons including ‘listener fatigue,’ relatively few modify their behavior accordingly.

In addition, humans cases or infection rates in vectors can reach very high levels in a short period of time, for example, dengue and West Nile virus in adult mosquitoes, such that more proactive responses in the form of vector control operations are warranted to further mitigate risk of arbovirus exposure. That vector control activities are often conducted during a medical health emergency further complicates the operation.

Controlling mosquito populations is not a simple or inexpensive task. For example, mosquito control programs are usually implemented at a municipal level, because local mosquitoes are a nuisance (large numbers of biting females), a threat to public health, or both. The most successful mosquito control programs are those that use an integrated pest management (IPM) approach, involving the integrated use of all available mosquito control methodologies.

The basic components of any IPM plan include: source reduction (i.e., removal of standing water where the larval and pupal stage exist), storm- or wastewater management, use of biological control (e.g., mosquito predators and parasitoids), and the application of larvicides (products to destroy mosquito larvae) and/or adulticides (products to destroy adult mosquitoes).

Mosquito control programs must also monitor spatial and temporal changes in adult and larval populations including distribution of larval development sites as well as environmental parameters such as patterns of rainfall and ambient air temperatures to be effective. Lastly, most IPM programs also incorporate education, extension, and outreach components into their programs to develop a strong connection in the community in which they operate.

Most public health authorities (WHO, Pan American Health Organization, the Centers for Disease Control and Prevention, the Public Health Agency of Canada, etc.) recommend that mosquito control (including larviciding) be used to mitigate the risk of human infections with arboviruses (as well as other mosquito-borne illnesses). Each organization recommends that a ‘graded response’ to virus activity be employed such that mosquito control activities can intensify (from larviciding to the use of adulticides) as surveillance data indicate imminent or ongoing human infections with a particular arbovirus.

The success of larviciding as a means of reducing mosquito populations will depend to varying degrees on knowledge and distribution of, and access to, larval mosquito development sites (i.e., private vs. municipal land) and the total coverage obtained in treated areas (e. g., what proportion of available larval development sites receive treatment).

Coverage obtained will depend in part on the method of application (i.e., ground vs. aerial) and the frequency and intensity of larvicide applications. Because most mosquito species have several generations each year, to be successful, effective coverage needs to be obtained against each larval cohort.

The relative timing of product application can also affect efficacy. In addition, seasonal accumulations of rainfall and temperature extremes have potential to impact the success of mosquito control efforts. Periods of prolonged rainfall, which create large numbers of new larval development sites, followed by warm temperatures, which increase the rate of mosquito development, can create conditions that promote rapid buildup of mosquito populations.

Under these conditions it may be impossible to get to all of the larval development sites to treat them before adult mosquitoes have emerged. Finally, it may not be practical to employ mosquito control measures in all affected localities.

Mosquito control programs are most cost-effective (and have traditionally been operated) in urban rather than rural areas. Many, if not most, small rural communities will have too few people, or too many and/or inaccessible larval development sites to apply larvicides or even adulticides in an effective manner.

Under these conditions, the most practical recommendation for prevention of arbovirus infection might be limited to personal protective measures, or specialized control strategies (e.g., barrier treatments with residual insecticides in or around small communities). Lastly, mosquito control experts continuously need to balance the short- and longterm risks of pesticide use, high costs of program maintenance, sustainability, and resistance of target species to chemical insecticides with the predicted risk of arbovirus disease.

The possible development of resistance by the target vector population to the chemicals used to control larvae or adult mosquito populations has prompted the search for novel control strategies or enhancement or refinements of existing ones. One area that has received considerable attention is genetic manipulation of vector mosquitoes. Because vector competence (i.e., the ability to transmit viruses) is at least partly genetically determined, efforts to create incompetent ‘transgenic’ mosquitoes have been suggested as the key to controlling many vector- borne infections (Beaty, 2000).

The molecular characterization of genes that influence vector competence is becoming routine. With the development of effective transducing systems, potential antipathogen genes now can be introduced into candidate mosquito species and their effect on virus or parasite development can be assessed in vivo.

With the recent successes in the field of mosquito germ-line transformation, it seems likely that the generation of a pathogen-resistant mosquito population from a susceptible population soon will become a reality. However, interventions based on genetic manipulation of vector mosquitoes face serious technical, theoretical, and political hurdles such that traditional methods of vector control will prevail in the foreseeable future.

Efforts to control nonmosquito vector populations to prevent arbovirus infections are rarely attempted or are performed on a much smaller scale. Control of nonmosquito vectors such as ticks may not be justified because the arboviruses they transmit are rare (e.g., Powassan encephalitis in North America), or alternative disease- prevention strategies are highly effective (e.g., vaccination to prevent indigenous cases of TBE or generalized use of personal protective strategies).

Historically, application of chemicals (acaricides) to the environment has been one of the main methods to control tick populations. Undoubtedly, the continuous and heightened burden of human disease caused by mosquito-borne pathogens (including nonarboviruses like malaria) has demanded greater effort be extended to this group of arthropods compared to the others.