A new study by researchers at Oregon State University found that fish embryos exposed to tiny amounts of pesticides for four days experienced lasting behavioral changes. It also found that the offspring of those fish also experienced changes in behavior and had altered development and gene expression even though they were not exposed to the commonly used pesticides. Joining us to talk about the findings and their implications on human health is Susanne Brander, an associate professor in the fisheries, wildlife and conservation sciences department at Oregon State University and the co-author of the study which was published in the journal Environmental Science and Technology.
Note: The following transcript was created by a computer and edited by a volunteer.
Dave Miller: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Dave Miller. We turn now to a study that provides more data about the way pesticide exposure can affect future generations. Researchers at Oregon State University exposed fish embryos to tiny amounts of pesticides for only four days. Then they moved those embryos to clean water, where the fish grew up and bred. The researchers found that both the exposed fish and their offspring showed effects from the pesticides. Susanne Brander is a senior author of the study. She’s an associate professor and eco toxicologist at OSU’s Hatfield Marine Science Center, and she joins us now. Welcome back to the show.
Susanne Brander: Good afternoon. Thanks so much for having me today.
Miller: Why did you and this team want to study the effects of pesticide exposure on successive generations of fish?
Brander: Good question. This is work that my group and a number of different collaborators have been focusing on for almost the past eight to 10 years at this point. A lot of our focus up until this study was on a chemical called bifenthrin, which is a commonly used pesticide in both households and agriculture. And looking at this pesticide and its potential for endocrine disruption, it turns out that it has estrogenic properties, and therefore can affect fish and other vertebrates just as estrogen would. But it sometimes can also interfere with estrogen-related signaling pathways.
The lead author of this study, Doctor Sara Hutton, who was a PhD student at OSU and has now moved on to work in consulting, really did a great building on previous work by adding two additional pyrethroids pesticides, cyfluthrin and cyhalothrin, in addition to bifenthrin, are all pyrethroids pesticides, which are the third most used class of pesticides globally. And they all have common properties. They’re all similar in terms of molecular structure and the way they cause toxicity.
Miller: Are these pesticides ones that people might use in their home gardens, or ones that would be used on commercial agricultural operations? Where would they be found? And after they’re used, what kinds of bodies of water do they end up in?
Brander: Pyrethroids are used for households as well as agriculture. And that’s been true for quite some time in the US, particularly for bifenthrin. Bifenthrin is marketed under a number of different commercial names. One of those is Talstar. If you go to your local Home Depot or Lowe’s or wherever you pick up pesticides, you can certainly find sprays or other products that contain bifenthrin, and possibly these other two pyrethroids. They’re used for termite control, mosquito control. They’re even sold for roach, carpenter ant, and spider control as well. So they’re pretty broadly applicable in terms of getting rid of whatever pest is bothering you. But there are definitely some potential long-term implications for exposure to nontarget organisms.
And in terms of bodies of water, they run off easily. These chemicals are what you would call hydrophobic, which means they tend to bind to sediment. But they’re mobile enough that when you have a big storm, especially a first flush event - which is what the first big weather event on the west coast is often referred to - when you get that first big storm in the end of October, early November, that’s where you’re flushing all of those chemicals that might be bound to sediments into both freshwater and coastal areas.
Miller: Can you give us a sense for the concentrations of pesticides that you used in your experiments, and how that level might compare to what you’d find in an actual estuary?
Brander: Absolutely. Over the past eight to 10 years, the same period over which we’ve been studying quite a bit on bifenthrin, and then we’ve added these other two related chemicals, we’ve been focusing on nanogram per liter concentrations. And I realized to a lay audience that might not mean a lot. But to put it in perspective, you are thinking relatively about a teaspoon of chemical in roughly an Olympic sized swimming pool.
Miller: And just to remind folks an Olympic sized swimming pool, it’s about twice as big as almost any pool that regular people swim in. 50 meters as opposed to 25. It’s an enormous swimming pool, and one teaspoon of the chemicals that you’re focused on. Which seems like a tiny, tiny amount. Would there be that much in the average estuary?
Brander: Most of my work has focused on levels detected in the San Francisco Bay Estuary, where I began this work years ago as a PhD student. Nanogram per liter levels are roughly what you would see with a grab sample of water from your typical estuary that’s surrounded by a combination of urban and agricultural land use. It can be higher though. For example, after a big storm event, especially if that storm event follows a long period of dry weather where spraying has occurred. You can see higher concentrations, in the, say, 50 to 100 nanogram per liter level, have been documented.
But part of the aim of this study was to get at what the average exposure levels were, and not necessarily look at the extremes. Because the average is what would be more representative of the day-to-day exposure that a fish or another aquatic organism would experience.
Miller: So let’s turn to the results. Can you describe how the directly affected fish reacted?
Brander: Like you said, this was a four day or 96-hour exposure, which is a pretty typical length for acute toxicity testing that’s used for regulatory purposes even across the country, and in Europe as well. But what we saw was after four days, when we evaluated their behavior, their response to stimuli in both light and dark to mimic what they would experience in the external environment, we saw a decrease in behavioral activity. They were less active overall across all of those chemicals. We saw altered gene expression, especially at the lower salinity. We did expose these fish at two salinities, since they’re estuarine. And we did see some slight differences between those salinity.
So altered gene expression looking at things like neurotoxicity related genes, and endocrine related genes. We saw a decrease in adult male gonad size in two of the chemicals. And then we also saw altered fecundity. And what was interesting was that in some of the female fish, we saw an increase in the number of eggs they were producing. But when we looked at the offspring, there wasn’t necessarily an improvement in the health of the offspring. So it almost seemed like the females were maybe overcompensating a bit by producing more eggs because they were being exposed to something that was inducing stress or other responses.
Miller: You found, overall, different responses in the next generation, in the fish that were born from the exposed fish. The males had higher reproductive capability, if I understand correctly. And instead of being less active than average, they were more active. Is it possible to see those results as an example of resilience and adaptation as good news? Or am I misreading the behavior of the second generation?
Brander: Sure, that’s a really good question. And I will say for the second generation, we only looked at the embryos and larvae, we didn’t rear the second generation up to adulthood. So we don’t know what the effects on adults for the second generation would be. It was a short term study, we had about two years of funding for this.
But in terms of the behavior, you’re absolutely right. We saw that the larvae in the second generation had increased activity, in contrast to the decreased activity we saw in their directly exposed parents. That could be viewed as what’s known as a compensatory response. And we think that there is this very short window where a lot of programming of the neuro-endocrine system is occurring in that four day window of exposure. And so it could be that the second generation is compensating for some of that programming that’s happening due to the chemical exposure.
That being said, even though it could be viewed as a sign that the organisms may be able to adapt, it’s also concerning because those behaviors didn’t return to control levels, they’re higher than normal. Both directions are potentially concerning because decreased activity means you’re getting out into the water less, you might be ingesting fewer prey items. There may be issues caused with where you’re settling in your habitat. And then of course with the offspring, with higher activity above the control levels, you’re potentially at higher risk of predation, for example.
Miller: Living fast and dying young, sort of.
Brander: You’re a four millimeter long fish in an estuary. There are plenty of things looking at you as a snack.
Miller: What do you see as the larger repercussions of this study for these fish? But also for the rest of us vertebrates who I understand share very similar chemical pathways with this particular species of tiny fish?
Brander: And I would say not just this particular species, but generally the neuroendocrine pathways are highly conserved between fish and mammals. And so the hormones that fish use, the estrogens and androgens, stress hormones that fuel fish reproduction and behavior and growth, those are all very similar to what we see in humans. And so fish are commonly used as a model for studying the effect of toxic chemicals on humans, especially because we have so many chemicals to test on an annual basis, and it would not be possible to do all of that testing in rodent or mouse models. And so a lot of this is done in fish embryos and larvae, like the zebrafish for example.
So I would say generally share a lot of these pathways. And so anything we’re seeing in terms of perturbations to those pathways, especially over such a narrow time period at these low concentrations, is potentially concerning. It could mean and we can’t say this definitively, of course, but it could mean that some of those same pathways are altered in mammals and potentially humans.
Another thing about endocrine disrupting chemicals, which is what we think some of these pyrethroids are acting as - especially since we saw altered gene expression and altered development and altered growth in the offspring, as well as the behavioral changes - is that they can act at very low concentrations. And what we see with bifenthrin in particular is that it acts more as an endocrine disruptor at low concentrations than it does at higher concentrations. And we think that’s because it’s acting more similar to an estrogen at those lower concentrations than it is at higher concentrations. So that’s another concerning thing that it takes a very small amount of these chemicals to disrupt hormone function, because hormones exist in our bodies at nanomolar or picomolar concentrations.
Miller: Susanne Brander, thanks very much.
Brander: Thanks so much.
Miller: Susanne Brander is an associate professor in the fisheries, wildlife and conservation sciences department at Oregon State University.
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