Bisphenol A, a chemical used for softening plastics, shortens the mosquito life cycle and leads to population explosion
Illustration: Yogendra Anand; Graphic: Sanjit / CSE The adverse effect of bisphenol A, or BPA, on human health is well known. When ingested, the chemical that is widely used to soften plastics, paints and other products, disrupts the endocrine system by interfering with the hormones and affects the brain and prostate gland of foetuses, infants and children.
Several studies establish that the chemical can cause high blood pressure, diabetes and cardiovascular disease in adults. Now, a study conducted in Thiruvananthapuram, Kerala, suggests it can indirectly aid in the spread of vector-borne diseases in humans and animals.
It establishes that the human-made chemical can significantly shorten the breeding time of southern house mosquitoes (Culex quinquefasciatus), a major carrier of the West Nile virus, Rift Valley fever virus and avian pox in tropical and subtropical countries, and thereby aid in its quick multiplication.
This is a matter of concern as most urban wastewater canals, which serve as breeding sites for mosquitoes, are choked with BPA-laden plastic waste. The research, published in the Environmental Science and Pollution Research journal by Springer on May 31, studied the water samples from four breeding sites in Thiruvananthapuram and found high concentrations of BPA (0.86 to 1.14 mg/L) in them. At these levels, the chemical shortens the life cycle span of mosquitoes from 13 to 10 days, suggests the study.
The finding is unique as till date, BPA is known to impair reproduction and development in aquatic organisms. BPA exposure is delays larval development and pupation time in common fruit fly (Drosophila melanogaster).
In Drosophila ananassea, another type of fruit fly, high doses of BPA increases the life cycle duration from seven to nine hours. High doses of BPA also cause several phenotypic abnormalities in adults that include asymmetrical antennae, miniature aristae, absence of aristae and others.
Southern house mosquitoes, in contrast, thrive in the presence of the compound. The larvae of the mosquito can tolerate BPA without causing lethal effects up to a concentration of 5 mg/L, a level much higher than what is normally found in the environment.
Adults developed at 1 mg/L BPA, the average found in the four breeding sites that were studied by the researchers, are normal and do not exhibit any significant differences with respect to sanguivory (dietary habit of blood feeding) and fecundity (potential for reproduction).
The life cycle of mosquitoes has four stages: egg, larva, pupa and adult mosquito. BPA has a significant impact on embryonic and larval development. Female mosquitoes lay eggs on the surface of water or on floating wet objects four days after a blood meal. In 26- 310C, the temperature range in Thiruvananthapuram during the summer showers in April, eggs hatch into larvae after 36 hours.
In the presence of BPA, the hatching time is reduced to 18 hours. The larvae then moult (shedding of cuticles) four times to grow into pupae. The intermoult period is reduced from the normal 60 hours to 48 hours in the presence of BPA in the water. BPA has no influence at the pupa stage, the last 50 hours in the life cycle when the aquatic organism turns into an adult mosquito.
During the peak of summer, when the temperature in Thiruvananthapuram hovers between 30 and 38 degrees Celsius, the mosquito life cycle is completed within 12 days.
This gets shortened to nine days by BPA. Mosquitoes usually complete two life cycles in a month and BPA in breeding sites can facilitate three life cycles every 30 days.
The shortening of the life cycle takes place because BPA has a strong influence on the physiology of the larvae.
The study discovered that larvae developed in BPA wastewater had three additional protein bands and high levels of the phospholipase A2 enzyme (responsible for insect immunity and larval growth), which accelerated larval instar development.
BPA also increases the secretion of 20-hydroxy ecdysone (20HE), a steroid hormone that plays a major role in the moulting of the insect larvae, and this shortens the time between larva and pupa.
The hormone has a direct effect on many genes involved in the development of the mosquito. In the presence of BPA, genes that are specifically influenced by 20HE, such as EcR and E75 exhibit double-fold expression and this allows for rapid moulting and growth.
The study findings are expected to have far-reaching consequences as BPA is manufactured in high quantities globally and its industrial discharge is a known surface freshwater contaminant.
Though the chemical degrades quickly, it is always present in the environment and organisms easily get exposed to it.
Evans Asirvadam is the corresponding author of the study ‘Bisphenol A acts as developmental agonist in Culex quinquefasciatus Say’, along with Ayana Gayathri, associate professor, Department of Zoology, University College, Thiruvananthapuram, Kerala
This was first published in the October 16-31, 2022 print edition of Down To Earth
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