Future strategies for Lyme disease prevention

Although the prospect for a new human Lyme disease vaccine remains, and research continues in that direction (Thomas and Fikrig, 2002), future efforts to prevent Lyme disease will likely move toward more efficient tick-control strategies, through either enhancing present strategies or developing new ones. These approaches will be based on the understanding that most emerging diseases are zoonotic in origin, with an ecology that should be investigated and considered before informed control practices can be implemented.

Not surprisingly, several approaches targeting white-tailed deer are under investigation. Application of insecticide directly to deer has shown promise in reducing the abundance of I. scapularis in the suburban environment. One such method uses a "four-poster device" - essentially a large central bin holding corn that spills out into two feeding troughs, one on each side of the corn bin. On the two corners of each trough are vertical posts that have been treated with the insecticide. As deer maneuver to feed from the trough, they rub their head and neck against the vertical posts and are passively treated with insecticide, typically a permethrin compound (Pound et al, 2000a, 2000b). This method has successfully reduced the abundance of host-seeking ticks by 69-100 percent over a three-year period on study sites that were between 2.55 and 10.1 km2 in area (Carroll et al, 2002; Solberg et al, 2003). This approach has great potential for control of I. scapularis tick populations, and should be considered by communities that want to reduce Lyme disease risk over a large area. More widespread use of the four-poster device may be achieved in coming years, particularly with the use of newer chemical aca-ricides that provide a longer-lasting residual treatment on deer. Consequently, less labor will be involved for those charged with maintaining the devices.

Much recent research and discussion (see, for example, DeNicola et al., 1996, 1997; Muller et al, 1997; Rudolph et al, 2000) has focused on the possibility of controlling urban/suburban deer populations with immunocontraceptives - essentially vaccines that produce an antibody response to proteins that are involved in reproduction. This approach is appealing because it theoretically offers the potential to control deer in a humane fashion, without the distasteful outcome of "terminal management". It would also have an advantage over methods like surgical sterilization (which requires direct capture and handling of the deer), or the administration of synthetic steroid hormones, where there are some restrictions regarding the status of the doe at the time (pregnant or not) and which are not currently licensed for use with deer (Warren, 2000).

While fertility control may seem to be a logical alternative for reducing deer numbers, and ultimately reducing the risk of exposure from ticks that depend on deer, practical and logistical difficulties of administering the agents have prevented this method from being used routinely (Warren, 2000). Among the problems that researchers have yet to overcome with immunocontraceptives are the following:

1. The lack of a commercial source for the vaccine antigen (PZP: porcine zona pellucida)

2. Difficulty in obtaining FDA approval for a vaccine that uses Freund's Complete Adjuvant (FCA) in its formulation, since this interferes with tuberculosis testing of animals

3. The need to treat deer in spring and summer, before the breeding season but at a time when attracting deer to bait stations so they can be darted is difficult because of abundant natural foods

4. The tendency for treated females to display recurrent estrous cycles, effectively prolonging the breeding season and disrupting normal behavior

5. The fact that incomplete vaccinations can result in a drop in vaccine antibody titers, resulting in late-born fawns that have difficulty surviving

6. The process is time-consuming and costly - Rudolph et al. (2000) estimated the expense at $800-$1100 per treated doe

7. There may not be a population decline unless every female of reproductive age is vaccinated (Warren, 2000).

Work on this front is ongoing, but Telford (2002) cautions that even if an effective immunocontraceptive is developed and it is practical to deliver it to large numbers of animals, herds will almost certainly have to be reduced first, probably to fewer than six to eight deer per square mile (Telford, 1993), by hunting before this measure might be expected to work.

The possibility of a vaccine targeting B. burgdorferi in wildlife, particularly white-footed mice, is intriguing. Tsao et al. (2001) administered a recombinant outer surface protein A (OspA) vaccine to mice that were experimentally infected with B. burgdorferi, and found reduced transmission of the agent to uninfected ticks that were allowed to feed on them. After three vaccinations, infection prevalence in ticks was reduced by 99 percent. Field trials of the vaccine showed a significant reduction in B. burgdorferi-infected host-seeking nymphs the year after vaccination (Tsao et al., 2004). However, the vaccination coverage (percentage of mice vaccinated) was estimated to be only about 55 percent - a difficulty that would be expected in any program requiring the administration of the vaccine by syringe. Results also highlighted the role that other vertebrates may play a part as reservoirs of infection. Tsao et al. (2004) concluded that alternative hosts probably have a greater role in infecting larvae than previously believed. Future work using an oral vaccine (Scheckelhoff et al., 2006) delivered to a variety of host species in the field could resolve these problems.

The use of tick assembly and arrestment pheromones to enhance the efficacy of pesticides (Sonenshine et al., 2003) and the use of biological control agents such as entomopathogenic fungi (see, for example, Benjamin et al., 2002) also show promise in controlling I. scapularis in the field, and these avenues should be investigated further. Both approaches can be used as part of an effective integrated pest management (IPM) program that reduces the risk not only of Lyme disease, but of other tick-borne diseases as well.

On a larger and long-term scale, changes in land-use and development patterns that minimize the creation of ecotones by preserving forest stands intact rather than fragmenting the habitat will go far toward alleviating the future burden of tick-borne diseases. However, our past experience with Lyme disease prevention and control has taught us that the level of active participation and acceptance on the part of community members and government officials (Hayes et al., 1999) must also be considered before a truly effective strategy is developed. In this case, the importance of informing the public about the consequences of current practices and cultivating the will to act in a manner that will improve public health in the future cannot be overstated.

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