In this blog post, we’ll examine the types of pesticides detected in eggs, how they work, their effects on the human body, and the causes of the pesticide-contaminated egg scandal.
Why did the pesticide-contaminated egg scandal occur?
In 2017, an incident in Europe involving the distribution of eggs and processed foods contaminated with fipronil caused a major global uproar. Subsequently, in South Korea, the detection of unauthorized pesticide components, such as fipronil, at some egg-laying farms led to widespread consumer anxiety. At the time, the government conducted a comprehensive inspection of all egg-laying farms nationwide, and a social controversy ensued as various opinions were raised regarding the safety of the eggs and their potential harm to human health.
Since this incident, South Korea has continuously strengthened its regulatory standards for egg-laying farms and its pesticide residue testing system. Currently, regular safety inspections and a traceability system are in place, and eggs sold on the market are produced under a much stricter regulatory framework than in the past. However, since there is still a possibility of occasional non-compliance cases, ongoing management and monitoring are being carried out.
The fact that pesticide residues were detected in the eggs we consume daily came as a major shock to consumers. So, what exactly are these pesticides, and just how dangerous are they to humans?
What kind of pesticide is fipronil?
The main pesticide components detected in domestically produced eggs at the time included fipronil, bifenthrin, flufenoxuron, etoxazole, and pyridaben. Among these, the component that sparked the most controversy was fipronil.
Fipronil is an insecticide in the phenylpyrazole class that selectively disrupts the nervous system of insects to eliminate pests. It is still used on a limited basis in some agricultural sectors and in veterinary medicines, and is also widely used in topical flea and tick treatments for pets. However, its use is strictly restricted or prohibited in food-producing animals, such as laying hens.
How does fipronil kill insects?
Fipronil acts on the central nervous system of pests to produce a powerful insecticidal effect. To understand this, we first need to examine how nerve cells transmit information.
In an organism’s nervous system, vast amounts of information are transmitted back and forth between nerve cells. Nerve cells use neurotransmitters to send signals to the next nerve cell; these neurotransmitters are stored at the ends of nerve cells and are released when an electrical signal is transmitted.
The released neurotransmitters bind to specific structures on the next nerve cell to transmit information. These structures are called receptors. To put it simply, it’s easier to understand if you think of neurotransmitters as keys and receptors as the locks that fit those keys.
Some receptors also function as ion channels. When a neurotransmitter binds to a receptor, a channel opens, allowing ions to move in and out of the cell, thereby altering the neuron’s electrical state. Depending on these changes, the neuron becomes excited or inhibited, and as a result, the neural signal is transmitted to the next cell.
In other words, neurotransmitters act as crucial switches that regulate the electrical signals of the nervous system.
How does fipronil affect the nervous system?
Gamma-aminobutyric acid (GABA) is a neurotransmitter that inhibits the excitation of neurons in the central nervous system of insects.
However, fipronil binds to the GABA receptors—where GABA would normally bind—and blocks the chloride ion (Cl⁻) channels. Similarly, in the case of glutamate, another neurotransmitter, fipronil acts on glutamate receptors to block the channels through which chloride ions pass.
If chloride ions cannot enter the nerve cell in sufficient quantities, the nerve cell becomes relatively more positively charged than normal. This causes nerve cells and muscle cells to become excessively excited, and ultimately, the insect is unable to maintain normal nervous function and dies. This mode of action is piperonyl’s primary insecticidal mechanism.
GABA receptors are, more precisely, GABA-gated chloride channels; when GABA binds to them, they open a pathway allowing chloride ions to move into the cell.
Glutamate receptors are also glutamate-gated chloride channels; when glutamate binds to them, they open a pathway allowing chloride ions to move.
In the absence of fipronil, GABA binds to the receptor, allowing chloride ions to move normally and inhibiting excessive excitation of the nerve cell. However, when fipronil binds to the GABA receptor, the movement of chloride ions is blocked, causing the nerve cell’s excitation to persist, and ultimately preventing the insect from maintaining normal physiological functions.
Why is it permissible for use on companion animals but not on laying hens?
According to regulations governing veterinary drugs, fipronil cannot be used on laying hens, which are food-producing animals. On the other hand, it can be used on companion animals as a treatment for external parasites under certain guidelines. The reason the same active ingredient is applied differently is that the method of application and the process of absorption into the body differ.
Most fipronil products currently on the market for companion animals are applied directly to the skin. In mammals such as dogs and cats, fipronil primarily remains in the sebaceous layer of the skin, where it effectively eliminates fleas and ticks. Under normal conditions of use, these products are designed to limit absorption into the body.
In contrast, when fipronil is used as a spray in laying hen farms, there is a risk of contamination not only of the hens’ bodies but also of their feed, eggs, and the rearing environment. Furthermore, if fipronil is absorbed into a chicken’s body, it can be partially transferred to eggs through metabolic processes, which may raise food safety concerns.
For these reasons, most countries strictly prohibit or restrict the use of fipronil on laying hens and implement measures to ensure that this substance is not detected in eggs intended for human consumption.
