Muhammad Mukhtar, Directorate of Malaria Control. Ministry of Inter-Provincial Coordination, Islamabad, Pakistan & Jaleel Kamran & Mumtaz Ali Khan, Epidemic Investigation Cell. National Institute of Health, Islamabad, Pakistan
In Pakistan, dengue is the most rapidly spreading vector-borne viral disease since 2005 and outbreaks have been attributed by weather and environmental changes, vector species composition, behavior, population dynamics and immunity level in local population. Aedes aegypti and A. albopictus have been considered major vectors of dengue in Southeast Asia including Pakistan. Both species have been specifically associated with human dwellings due to their breeding preference for clean water domestic habitats. In the absence of a vaccine, vector control strategies that include chemical, biological and environmental management is the foundation for dengue prevention and control. However, the use of insecticides has always given the top most priority. Since the existing vector control tools have some limitations in terms of cost, delivery and long-term sustainability, therefore it is compulsory to select “time and target specific” vector control intervention(s) based appropriate entomological and epidemiological evidences and must be carried out in both inter-epidemic and epidemic periods involving local communities.
The use of mosquitofish in a habitat is considered a cost effective and environment friendly biological method of mosquito control. These fishes are small with size range of 0.5-3.0 inches. A large female fish can eat upto 200 mosquito larvae in a day when used properly or correctly. Currently many Vector-Borne Disease (VBD) control programs are using this intervention as one of the main defense-line against mosquito breeding without any harmful effects on the ecology of area or the fauna of indigenous fish and amphibians. Globally, there is always a big debate over the efficacy of mosquitofish and it is strongly recommended not to release mosquitofish in wider habitats like ponds, lakes etc. However, these mosquitofish are effective and successful when use in small clean water habitats like small public ponds, ornamental ponds, animal water troughs, bird bath fountains, big money plants vassals, unused and broken swimming pools which are free from excessive organic and non organic materials. Worldwide Gambusia (Gambusia affinis) and Guppy (Poecilia reticulate) are being used extensively in many mosquito control program after careful and intensive mosquito breeding sits assessment surveys. Gambusia is a large group of fish having 43 species, most of which are found in freshwater habitats, though some species may also be found in brackish or saltwater habitats. Other important species are Goldenfish (G. aurata) and koi however these mosquitofish species are not as effective as G. affinis.
During September 2011, there was a sudden rise in dengue cases in Punjab province particularly in Lahore. Provincial health department took all possible steps on “War-Footing” to contain this outbreak and there was a huge anti-vector campaign with the overall objective to interrupt the disease transmission by reducing the densities of dengue vector mosquitoes. One of the key steps was the release of fishes in open ponds to control mosquito breeding without using chemicals (larvicides) and this was the first time in country that the fishes were introduced on large scale.
In this research article authors give their evidence-based technical comments on efficacy of this intervention and they are very confidant that the release of fish has no significant impact on dengue vector densities and disease transmission in Pakistan. To support their views they produced following extremely important technical points which must be considered while using mosquitofish in country against dengue vectors;
Organic materials and aquatic plants: Large ponds always have excessive organic materials that include leaves, grasses, fruit, dropped branches and flowers etc. Through excessive and rapid decomposition, these materials consume more oxygen making these ponds unfit for mosquitofish and adversely affecting their efficacy to consume larvae, pupae and eggs of mosquitoes. Similarly anaerobic bacteria grow in this kind of water habitats resulting higher level of Biochemical Oxygen Demand (BOD), Electro-conductivity (EC) etc which further compound the situation and subsequently results in a rapid death of mosquitofish. Similarly some plant leaves, like pine, oak, eucalyptus and cheesewoods, also contain chemicals that are harmful to mosquitofish. No doubt the presence of green plant algae in such habitats maintains the oxygen balance in pond and also serves as a good source of food for fish and fry. However the heavy accumulation of these organic materials make mosquitofish too sick to eat mosquito youngones. Generally mosquito larvae always concentrate close to the pond margin where there is shallow water and emergent grasses and other aquatic plants which always provide shelter to mosquito larvae against predators including mosquitofish. Reduced oxygen level and thick emergent grasses are the general characteristics of open large ponds which always results in overall poor performance of mosquitofish.
In Pakistan surface water bodies like open ponds have extremely higher concentration of organic and non-organic pollutants. Among the non-organic characteristics Dissolve Oxygen (DO), EC, BOD, pH, Total Dissolve Salts (TDS), and ammonia concentration are the major characteristics of a habitat which influence the breeding preference of any mosquito species towards a particular breeding habitat. Mostly in Pakistan these ponds, particularly disused ones also have an excessive amount of animal dung which tremendously increase the ammonia level in these habitats. All these physio-chemical characteristics favor the breeding of only those mosquito species which have a wide range of tolerance against above mentioned physio-chemical characteristics of habitats. Culex quinquefasciatus, Cx. pipens fatigans and also Anopheles subpictus are the major species which prefer such polluted breeding sites while these characteristics of habitats also completely prevent the breeding of both dengue vector species A. aegypti and A. albopictus. On the other hand, open and large habitats having clean water like rice field, commercial fish ponds, and irrigation channels provide attractive breeding sites for Cx. tritaeniorhynchus, Cx. bitaeniorhynchus, Cx. pseudovishnui, Cx. vishnui, Anopheles culicifacies, A. stephensi, A. pulcherrimus, A. nigerrimus, A. peditaeniatus, and A. barbirostris in Pakistan.
During 2005-2010, a comprehensive series of field investigations on vector mosquitoes have been conducted across the country and there was no notable record of breeding of A. aegypti in open habitats like such ponds. However, a limited breeding of A. albopictus was reported from some open habitats having fresh and clean water. These entomological finding have been strongly supported by international findings which revealed that both vectors of dengue and DHF A. aegypti particularly and A. albopictus generally have no association with open and organically polluted breeding sites, rather these have statistically significant association with man-made artificial habitats which are placed at shaded places in human dwellings in Pakistan. The most attracted breeding sites inside houses were “Underground water tanks, earthen pitchers, drums, money plant bottles etc.
Natural pond residents: Generally these organically pollutant habitats also provide an excellent attraction for many other aquatic fauna which include; giant water bugs, water beetles and water beetle, diving beetle, damselfly larvae, dragonfly larvae, water scorpions, water boatman, and backswimmers, water strider, tadpole (frog or toad), midge etc. Many of these aquatic animals are very strong predators on mosquitoes.
Globally, there is always great debate over the efficacy of mosquitofish in mosquito control particularly in habitats having these natural mosquito-larvae eating animals. In this regards studies conducted revealed that the natural presence of these native mosquito-larvae eating insects particularly water beetle, diving beetle, backswimmer etc. in organically polluted habitats have shown considerably more effective control on mosquito breeding than introduced Gambusia. Although the use of mosquitofish is a natural way of controlling mosquito breeding, however these fish should never be released into natural waters habitats in which they are not native such as large ponds, lakes, or creeks etc. When mosquitofish will be released in such habitats they will eat other insect youngones instead of mosquito larvae. Therefore mosquitofish will not drastically reduce mosquito larvae populations in large water bodies. Secondly their introduction into these natural habitats has a detrimental affect on other mosquito-eating insect and native fish populations. For example in California, a study indicated that 65% mosquitofish preferred to eat pacific treefrog even when high densities of mosquito larvae were presented in water habitats. Similarly in another study over 2000 mosquitofish were collected from such ponds and were dissected. Only a small percentage of mosquito larvae were found and 75% mosquitofish preferred aquatic insects as their food. The overall results of such studies revealed that mosquitofish are only effective when used in small ponds where other aquatic fauna is limited and mosquitofish have no choice other than mosquito larvae and pupae.
Again it is very important to note that in Pakistan organically polluted water habitats, particularly large and open ponds have huge populations of above mentioned predators of mosquitoes which naturally have tremendous predatory potential on mosquito youngones. Therefore again the release of fish in such habitats in Pakistan will not show any impact on mosquitoes rather this will disturb the ecology of local area and local aquatic fauna. According to Department of Fisheries 11,000 fish seed were released during November 2011 in different ponds, lakes etc of Lahore. Mainly Tilapia fish which is well established in our eco-system was released during this campaign. It is important to note that Tilapia has a wide range of feeding preference on both animal and plants (omnivorous) and habitats in which it was released had heavy accumulation of vegetations and other insects’ larvae which ultimately reduce the predatory potential Tilapia fish on mosquito larvae. Other two kinds of mosquitofish Gambusia and Guppy were also used which are mainly carnivorous (eat animals). However it is well documented in literature that their food preference, particularly of Gambusia depends on the availability of food items rather than choice which also compromised the efficacy of these mosquitofish in big organically polluted water habitats. As reported by Department of Fisheries the total numbers of mosquitofish release in above mentioned large habitats are very less. Normally 5-10-gallon habitat water requires 2 pairs of mosquito fish and a young mosquitofish reach maturity in about 2 months and then females start having babies. Most females give birth to 10-14 minnows as often as 3 or 4 times a year. This means that the number of released mosquitofish is very less to establish to cope the mosquitoes breeding in big water habitats.
CONCLUSIONS: In accordance of global recommendations not to release mosquitofish in wider habitats, the above mentioned facts and figures revealed that there are no chances of breeding of A. aegypti and A. albopictus in large and open areas habitats. Mosquitofish can only be effective when used small ornamental ponds, public ponds free from excessive organic pollutants, and other aquatic fauna. Their use in Pakistan to control dengue vectors breeding in huge open ponds particularly when these ponds have excessive organically polluted materials will not exhibit any impact on reduction of densities of vector mosquitoes and subsequently on dengue caseload in effected areas. Therefore the authors strongly recommend to reconsider the release of mosquitofish in such habitats and also to design effective, evidence-based, community-friendly and sustainable dengue vector control intervention in local scenario to ameliorate this disease from Pakistan.