Come monsoons and vector borne diseases start making headlines every year in many parts of India, especially mosquito borne diseases like dengue, malaria, chikungunya and Japanese encephalitis.

What are vector borne diseases

Vector borne diseases are caused by infectious agents such as parasites, viruses and bacteria transmitted through the bite of vectors or carriers of these parasites like mosquitoes, ticks, fleas and lice that depend on the blood of humans and animals for their survival. Vectors such as mosquitoes, ticks, fleas or lice ingest disease producing microorganisms while sucking blood from an infected host (human or animal) and later transmit it into a new host, after the disease producing pathogen has replicated [1].

Water and vector borne diseases, especially those caused by mosquitoes, are inexplicably linked as water logging caused due to rains, stagnant polluted water, water collected in pots and pans, old tyres, constructions sites, puddles helps in breeding of mosquitoes - and mosquito eggs, larvae and pupae need water for their survival. Adult mosquitoes - one of the important culprits that spread some of the deadliest vector borne diseases in India, emerge out of the water and live by feeding on human blood.

The incidence of vector borne disease is high in India

The common vector borne diseases in India include malaria, dengue, chikungunya, encephalitis, kala-azar and filariasis that are transmitted through the bite of an infected mosquito. While malaria has shown some decline in 2017-18, it has still led to as high as 85 percent deaths in a year in the country. Dengue outbreaks have continued since the 1950s, but severity of disease has increased in the last two decades. Reported cases of chikungunya in the country have shown a slight decrease in 2018 as compared to 2017. However, the total number of cases and deaths due to encephalitis continue to be high with Assam reporting maximum numbers of cases and deaths [2].

Climate change is predicted to further worsen the situation

Evidence also shows that climate change will further increase the incidence of these diseases in the coming years. Climate change has led to an increase in the average surface temperatures and accelerated the rate of warming along with an increase in extreme rainfall events in many parts of the globe and in India.

Higher temperatures are known to increase the transmission of vector borne pathogens mainly through increasing vector numbers, pathogen replication and vector biting rate, and reducing the incubation period or the time between a vector feeding on an infected host and being able to transmit the pathogen [3].

Mosquito species such as the Anopheles gambiae complex, A. funestus, A. darlingi, Culex quinquefasciatus are very sensitive to temperature changes in the larval as well as adult stage. Thus if water temperature rises, the larvae take a shorter time to mature and develop a greater capacity to reproduce during the transmission period. In warmer climates, adult female mosquitoes digest blood faster and feed more frequently, thus increasing transmission intensity. Malaria parasites and viruses complete incubation within the female mosquito in a shorter time as temperature rises, thereby increasing the proportion of infective vectors [3].

Preventive measures to control vector borne diseases

A number of preventive measures are being suggested to reduce the incidence of these diseases, one being controlling the spread of mosquitoes by destroying their larvae and pupae found in stagnant waters that includes water from ponds, marshes, swamps, ditches, open gutters as well as water collected in pots and pans, buckets, tyres etc.

Rivers in India too, are becoming stagnant and breeding grounds for mosquitoes because of reduction in their flows due to damming and overload of large quantities of urban sewage and harmful industrial effluents [4]. Sewage can lead to eutrophication, which encourages growth of invasive aquatic plant species like water hyacinth, which slows down the water flow due to its long roots and encourages breeding of insects such as mosquitoes.

Use of guppy fish to eliminate mosquitoes

Biological control by using fish such as guppies that feed on mosquito larvae has been proposed and widely used to eliminate mosquitoes and reduce the incidence of these diseases in India and world over [5] and its usefulness debated.

Guppy fish (Poecilia reticulata Peters, 1859) is a species of colourful fish found in parts of Central and South America [6]. The fish are extremely resilient and are found in a wide variety of habitats ranging from large lakes to ditches and can sustain themselves in a wide range of environmental conditions [6, 7].

The fish are now commonly found in India following their introduction as a biological vector to control mosquito larvae and/or in aquarium trade as pets [8]. Being resilient, they can rapidly multiply due to their capacity to adapt to the situations and reproduce at a very fast rate. For example, single females of the fish are able to store sperms of different males for a long duration and give birth to fully developed offspring, under suitable conditions [9]. The fish are so adaptable that even a single introduction of one female into the water can quickly give rise to a viable population [9].

Guppy fish are voracious feeders and prey on a wide range of organisms in the water. Guppy fish are known to control the mosquito population by feeding on mosquito larva, and studies such as those in Cambodia show that introduction of guppies in artificial water containers like water storage tanks reduced the density of Aedes sp. larvae [10]

The effectiveness of guppies has been questioned and widely debated

However, many studies have challenged the effectiveness of using guppies to control mosquitoes. While laboratory studies show their effectiveness in controlling mosquito larvae, these are done under controlled conditions and may not reflect the reality about the efficiency of guppies in controlling mosquito larvae in actual conditions. Laboratory studies are undertaken under certain preconditions that include starving the animals before any feeding experiment which can lead to an artificial increase in their mosquito control efficiency [5].

In a multi prey system, guppies do not show preference for mosquito larvae, sometimes preferring other prey items over the larvae [5, 9]. Hence their introduction would become conditional on where they are released. Indian researchers have also shown that the rate of consuming the larvae can alter based on its body size besides alternative prey items [11].

Besides lack of effectiveness, scientists argue that the introduction of aggressive and highly invasive species such as the guppies can drastically alter the freshwater habitats as they prey on a range of organisms and reproduce rapidly leaving little space for local fish to flourish.

Unpublished observations on some water bodies in Western Maharashtra show that habitats with presence of guppies had much lower richness of certain microcrustacean species than localities that did not have these fish [13]

Are there any alternatives

While scientists are skeptical on the effectiveness of using guppies for mosquito control, many recommend use of indigenous species of fish from their respective regions from genera like Aplocheilus and many small barbs (formerly known as Puntius spp.) as a better option instead of guppies [14].

Besides, there are many other natural invertebrate native predators of mosquito larvae as well such as several species of aquatic bugs and beetles, odonate (dragonflies and damselflies) nymphs and crustaceans like tadpole shrimp (Triops sp.) crustaceans and their efficacy has also been studied [15,16, 17]. They could also be used as alternatives to guppies. Being local species, they are less aggressive and do not have a negative effect on the biodiversity of freshwater bodies.

At the same time, concerted efforts to reduce pollution and eutrophication of water bodies need to be made on an urgent basis.

The authors are extremely thankful to expert biologist Professor Hemant Ghate for his valuable inputs in the article.

References

1.    World Health Organisation (2020) Vector borne diseases-Factsheets. Accessed on 30th July 2020.
2.    Central Bureau of Health Intelligence (2019) National Health Profile - 14th Issue. Ministry of Health and Family Welfare, Government of India.
3.    Karmarkar, M., Pradhan, M. (2019) Climate change and public health: a study of vector-borne diseases in Odisha, India. Natural Hazards, 102: 659-671.
4.    Lokgariwar, C., Purohit, M.(2017) Why rivers don’t flow anymore. Accessed on 30th July 2020.
5.    El-Sabaawi, R. W., Frauendorf, T. C., Marques, P. S., Mackenzie, R. A., Manna, L. R., Mazzoni, R., ... & Zandona, E. (2016). Biodiversity and ecosystem risks arising from using guppies to control mosquitoes. Biology letters, 12(10), 20160590.
6.    Froese, R. and Pauly, D. (Eds) (2019). FishBase. Accessed on 30th July 2020.
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8.    Knight, J. D. M. (2010). Invasive ornamental fish: a potential threat to aquatic biodiversity in peninsular India. Journal of Threatened Taxa, 700-704.
9.    Deacon, A. E., Ramnarine, I. W., & Magurran, A. E. (2011). How reproductive ecology contributes to the spread of a globally invasive fish. PloS one, 6(9), e24416.
10.    Seng, C. M., Setha, T., Nealon, J., Socheat, D., Chantha, N., & Nathan, M. B. (2008). Community-based use of the larvivorous fish Poecilia reticulata to control the dengue vector Aedes aegypti in domestic water storage containers in rural Cambodia. Journal of Vector Ecology, 33(1), 139-144.
11.    Manna, B., Aditya, G., & Banerjee, S. (2008). Vulnerability of the mosquito larvae to the guppies (Poecilia reticulata) in the presence of alternative preys. J Vector Borne Dis, 45(3), 200-6.
12.    Ghate, H. V., & Padhye, A. D. (1988). Predation of Microhyla tadpoles by Gambusia. Journal of the Bombay Natural History Society, 85, 200-201.
13.    Sameer Padhye, Personal observation.
14.    Chandra, G., Bhattacharjee, I., Chatterjee, S. N., & Ghosh, A. (2008). Mosquito control by larvivorous fish. Indian Journal of Medical Research, 127(1), 13.
15.    Su, T.., & Mulla, M. S. (2002). Introduction and establishment of tadpole shrimp Triops newberryi (Notostraca: Triopsidae) in a date garden for biological control of mosquitoes in the Coachella Valley, Southern California. Journal of Vector Ecology: Journal of the Society for Vector Ecology, 27(1), 138-148.
16.    Mogi, M. (2007). Insects and other invertebrate predators. Journal of the American Mosquito Control Association, 23(sp2), 93-109.
17.    Quiroz-Martínez, H., & Rodríguez-Castro, A. (2007). Aquatic insects as predators of mosquito larvae. Journal of the American mosquito control association, 23(sp2), 110-117.