HRT-related horse estrogens in wastewater may feminize fish.

Jun 09, 2009

Tyler, CR, AL Filby, LK Bickley, RI Cumming, R Gibson, P Labadie, Y Katsu, KE Liney, JA. Shears, V Silva-Castro, H Urushitani, A Lange, MJ Winter, T Iguchi and EM Hill. 2009. Environmental health impacts of equine estrogens derived from hormone replacement therapy. Environmental Science & Technology doi:10.1021/es803135q.


Squeezyboy, Flickr.


Some horse estrogens used in hormone replacement therapies make their way from people to wastewater and into fish where they may contribute to the feminization of the animals, reports a study from England. Laboratory tests showed the hormones – one of which was more potent than human varieties – can turn on estrogen hormone systems in fish at very low concentrations. This is the first time scientists report that HRT-related horse estrogens in water coming into and leaving sewage treatment plants are estrogenic in fish.




Concerns about hormones in the environment are not new and are well documented (Halford 2008). Municipal wastewater treatment plants discharge hormones used in birth control pills. Reports of feminization of wildlife living in waters receiving such discharges is also well known.

Menopause is the time in a woman's life when menstruation cycles wane in preparation for the reproductive system  to shut down. Typically, women are between the ages of 45 and 55 when the changes occur.  Levels of estrogen and other hormones that guide monthly mensus cycles decrease and level off as menstruation stops.

Women take hormone replacement therapies (HRT) to maintain estrogen and progesterone hormones at levels similar to premenopause levels. This helps control symptoms such as hot flashes and mood swings. While the hormones have health benefits, they are also linked to cancer and other ailments. Longterm HRT studies find protective benefits against bone loss as well as higher risks of cancer, heart disease and stroke (Womens Heatlh Initiative Scientific Resources). 

Horse (equine) hormones are commonly found in HRT taken by women during and after menopause. While HRT seems to be decreasing amidst concerns about breast cancer and heart disease risks, it is still used by approximately 17 percent of women over 50 years old in the US.

Unlike birth control pills, few studies have examined the environmental impacts of estrogen hormones used in HRT. One prior study found the estrogens can accumulate in fish bile (Gibson 2005). Very little to nothing is known about their environmental distribution and effects on wildlife. 

What did they do?

 A British team analyzed levels of six horse estrogens commonly found in HRT in water arriving to and being discharged from four  wastewater treatment plants in England. Two estrogens – equilin (Eq) and equilenin (Eqn) – and four of their metabolites were examined.

The researchers exposed fish to several horse estrogens and then examined blood, gonads and livers for responses. They also measured estrogen levels in the bile of exposed fish to gauge if and how much of the hormones the fish were taking up from the water.

An assay was used to compare the estrogenic potency (the concentration required to activate the estrogen receptor) of the compounds relative to the human estrogen (17B-estradiol). The hormones were tested with both the human estrogen receptor and the estrogen receptor of a fish species called a roach.

What did they find?

Of the six horse estrogens screened, only  equilin (Eqn) and metabolite 17beta-dihydroequilin (17B-Eqn)  were detected at nanogram per liter (ng/L) concentrations in the influent and effluent. Eqn concentrations were higher than 17B-Eqn, but both were found at lower levels after treatment. 

When fish were exposed to the horse hormones, Eqn and 17B-Eqn were found concentrated in the bile. Both hormones also increased the blood levels of the egg protein vitellogenin in the fish. The higher the exposures, the more vitellogenin was measured. 

All of the horse hormones had some estrogenic reactivity with human and fish estrogen receptors.  Additionally, all six compounds activated genes in the fish through the estrogen receptor. Furthermore, exposures to concentrations as low as 0.6 ng/L stimulated estrogenic activity in the fish.

When tested with the the human estrogen receptor, every horse estrogen was less potent than the main human estrogen, 17beta-estradiol. Eqn was only 15 percent as potent as estradiol, while the horse hormone metabolite 17B-Eqn was 77 percent as potent as human estrogen.

When the compounds were tested on the fish estrogen receptor, 17B-Eqn was "35-fold more potent" than estradiol while the rest were weaker. Eqn was 25.6 percent as potent as estradiol.

What does it mean?

Estrogens derived from horse urine and used in hormone replacement therapies are another source of estrogenic compounds in wastewater. This is the most thorough report to date of the environmental levels and estrogenic effects of these hormones.

The two main compounds measured in the inflow and outflow of treatment plants were mostly less potent than the human estrogen estradiol. However, they accumulated in fish, sparked egg protein production (an estrogenic marker) and triggered human and fish estrogen receptors at low levels.

Overall, the results show that estrogens from HRT can make their way through the sewage treatment process, into natural waterways and concentrate in the fish that live there.

The equine estrogens were also capable of producing estrogenic responses in the exposed fish and in lab assays. These results clearly show their potential to affect fish and may be contributing to fish feminization reported in prior studies.

While the 17B-Eqn concentrations tested in the study  is higher than typically found in effluents (about 0.10 ng/L), the understanding that exposures to environmental estrogens act additively makes this yet another potentially harmful compound in the environment.

The environmental health affects reported in this study "should perhaps be considered when weighing up the risks and benefits of HRT for women," the authors' conclude.



Gibson, R, MD Smith, CJ Spary, CR Tyler and EM Hill. 2005. Mixtures of estrogenic contaminants in bile of fish exposed to wastewater treatment works effluents. Environmental Science and Technology 39:2461–2471.

Halford, B. 2008. Side Effects. Chemical and Engineering News 86(February 25):13-17.

Menopause. Mayo Clinic.

 Scientific Resources Website, Women's Health Initiative.

Womens Health Initiative Study, National Institutes of Health.




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