Germ warfare against mosquitoes. What now?

Abstract:
Mosquitoes and the diseases they transmit are still major public health concerns in many countries in the tropics and subtropics resulting in significant morbidity and mortality in man and animals. The use of chemical control agents is still the major forerunner in mosquito control strategies. Despite success achieved in chemical control, the use of chemicals has resulted in two major impacts, namely, environmental disruption and development of insecticide resistance in target mosquitoes. This dilemma has prompted the search for alternative control agents/methods. Among these, the use of bacterial control agents is the most effective and proven method that have been successfully replicated in many parts of the world. These bacterial agents, Bacillus thuringiensis and Bacillus sphaericus have now been used widely in various control programmes. Both bacteria are spore-forming rod-shaped bacilli and are known as crystal- forming bacteria as they produce the mosquito-killing toxins as crystals. Bacillus thuringiensis is known to consist of >70 serotypes but the most effective mosquitocidal serotype is H-14 or Bacillus thuringiensis var. israelensis (Bti). Similarly, Bacillus sphaericus (Bs) has a number of serotypes; the H-5 and H-25 serotypes exhibit the highest mosquitocidal activity. Both bacteria are only larvicidal and kill by the crystal toxin they produce. Essentially, the toxins are activated in the gut of the larvae by the specific proteolytic enzymes and alkaline pH and go on to destroy the gut cells resulting in scepticaemia (blood-poisoning). Death occurs within 30 minutes at high dosages. Both agents are highly and specific selective only against larvae of mosquitoes. Bti is known to be toxic against all mosquito larvae, the most susceptible are Aedes aegypti and Culex quinquefasciatus, while Bs has a narrower spectrum in that it is most effective against Culex larvae. Both agents, however, are not harmful to all other non-target organisms. After >20 years of use, no resistance to Bti is reported, though Culex resistance to Bs is known. Today, these agents are formulated as aqueous suspension, wettable powder, water dispersible granules, tablets etc. to suit the application in the various breeding habitats of mosquitoes. Future research will focus on the development of more effective formulation and method of application, isolation of bacteria that exhibit higher mosquitocidal activity and combined use of bacterial agents and other control agents.