Some science behind the scenes

Pyrethroids

A pyrethroid is an organic compound similar to [but not the same as] the natural pyrethrins produced by the flowers of pyrethrums (Chrysanthemum cinerariaefolium and C. coccineum). 

  • The first-generation pyrethroids,  - developed in the 1960s, include bioallethrin, tetramethrin, resmethrin and bioresmethrin. They are more active than the natural pyrethrum but are unstable in sunlight .
  • The second generation pyrethroids  - were released around 1974 and are more persistent compounds notably: permethrin, cypermethrin and deltamethrin. They are substantially more resistant to degradation by light and air which is why they are used in agriculture, but they have significantly higher mammalian and other toxicities.
    Permethrin, [sold under the brand name Nix among others], for example, is a medication and insecticide. As a medication it is used to treat scabies and lice. It is applied to the skin as a cream or lotion. As an insecticide it can be sprayed on clothing or mosquito nets such that the insects that touch them die.  In agriculture, permethrin is mainly used on cotton, wheat, maize, and alfalfa crops.
  • The third generation - Over the subsequent decades these derivatives were followed with other proprietary compounds such as fenvalerate, lambda-cyhalothrin and beta-cyfluthrin.

Pyrethroids are the third most applied group of insecticides worldwide and are extensively used in agricultural and non-agricultural applications. Pyrethroids constitute the majority of commercial household insecticides.  Pyrethroids in theory, exhibit low toxicity to mammals, but have extremely high toxicity to fish and non-target invertebrates.   It has been stated that there is no risk to humans, however, recent papers show this is not the case.

Human toxicity

Pyrethroids are broad-spectrum insecticides. Permethrin intoxication due to topical application has not been documented in humans. We report a 20-month-old infant who had used 5% permethrin lotion topically for scabies treatment. Approximately 60 mL (20 mL/day) was used and after the third application he developed agitation, nausea, vomiting, respiratory distress, tachycardia, and metabolic acidosis. His clinical symptoms and metabolic acidosis normalized within 20 hours. His follow-up was unremarkable. Toxicity of permethrin is rare, and although permethrin is a widely and safely used topical agent in the treatment of scabies and lice, inappropriate use may rarely cause toxicity. Moreover, in cases of unexplained metabolic acidosis, topically applied medications should be carefully investigated.  PMID:  25487692

as far back as 2008 alarm bells were ringing.....

A total of 4,974 cases of acute pyrethrin/pyrethroid-related illness were identified. Incidence rates increased over time, reaching 8 cases/million population in 2008. .... Respiratory effects were the most common symptoms reported (48%). Risk of acute respiratory effects were significantly elevated among persons exposed only to pyrethrins .., only to pyrethroids ..... to a mixture of pyrethroids ....or to a mixture containing both pyrethrins and pyrethroids .... compared to those with illness arising from exposure to other pesticides. The most common factors contributing to pyrethrin/pyrethroid-related illness included exposure from spills/splashes, improper storage, and failure to evacuate during pesticide application.....The magnitude of acute pyrethrin/pyrethroid-related illness/injury is ..... increasing. As such, additional measures to prevent them are needed.  PMID: 23788228

One of the problems with this insecticide is that although it is intended for exterior use in humans, its use in agriculture or other insect eradication programs can result in ingestion via crops or simply via aerosol activity:

Two pyrethroids, permethrin and allethrin, are often combined for large-scale use in public health programs to control vector-borne diseases. In this study, the genotoxic potential of a commercial formulation of permethrin and allethrin was examined using cultured human peripheral blood lymphocytes (PBL). …... Data demonstrated that a mixture of permethrin and allethrin induced concentration- and time-dependent cytotoxic and genotoxic damage to human PBL in vitro. PMID: 25424543

Further ingestion may result from eating animals in the food chain, including marine or water based creatures…

The Benthic zone

Their high hydrophobicity, along with pseudo-persistence due to continuous input, indicates that pyrethroids will accumulate in sediment, for example, posing long-term exposure concerns to benthic invertebrates and ultimately cause significant risk to benthic communities and aquatic ecosystems.

The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone are called benthos, e.g. the benthic invertebrate community, including crustaceans and polychaetes.

The current review synthesizes the reported sediment concentrations of pyrethroids and associated toxicity to benthic invertebrates on a global scale. Geographically, the most studied area was North America, followed by Asia, Europe, Australia and Africa. Pyrethroids were frequently detected in both agricultural and urban sediments, and bifenthrin and cypermethrin were identified as the main contributors to toxicity in benthic invertebrates. Simulated hazard quotients (HQ) for sediment-associated pyrethroids to benthic organisms ranged from 10.5±31.1 (bifenthrin) to 41.7±204 (cypermethrin), suggesting significant risk. The current study has provided evidence that pyrethroids are not only commonly detected in the aquatic environment, but also can cause toxic effects to benthic invertebrates, and calls for better development of accurate sediment quality criteria and effective ecological risk assessment methods for this emerging class of insecticides.  PMID:  27825741

Rivers in general

Pyrethroids are widely used insecticides, but minimal information has been published on their presence in municipal wastewater in the United States. Pyrethroids in wastewater from the Sacramento, California, USA, area consisted of permethrin, bifenthrin, cypermethrin, and cyhalothrin, with a combined concentration of 200 ng/L to 500 ng/L.
Sampling within the wastewater collection system leading to the treatment plant suggested pyrethroids did not originate primarily from urban runoff, but could be from any of several drain disposal practices.
Wastewater from residential areas was similar in pyrethroid composition and concentration to that from the larger metropolitan area as a whole. Secondary treatment removed approximately 90% of pyrethroids, but those remaining exceeded concentrations acutely toxic to sensitive species.
Toxicity to the amphipod, Hyalella azteca, was consistently evident in the final effluent. The large river into which this particular plant discharged provided sufficient dilution such that pyrethroids were undetected in the river, and there was only slight toxicity of unknown cause in 1 river sample, but effects in receiving waters elsewhere will be site-specific.  PMID: 23893650

The sub-lethal effect on bees

We totally rely on bees for our survival.  Very few fruits or vegetables are pollinated by other insects and the honey bee is a key marker bee to indicate the effect of this insecticide.  Managed honey bee, Apis mellifera L., colonies are often used to pollinate fruit and nut orchards improving crop quality and yield.  But colonies placed in orchards are potentially exposed to pyrethroid insecticides used for broad-spectrum pest control.

The manufacturers of Pyrethroids have stated they pose ‘minimal risk’ to bees due to their ‘low application rates’ in the field and ‘putative repellent properties’. This repellency is believed to alter the foraging behaviour of the bees so that they do not go near any crops with the insecticide sprayed on them.  Again, they believe this prevents bees from encountering a lethal dose in the field.  However researchers have started to assess what a sub-lethal dose does to bees

… sub-lethal exposure to pyrethroids may adversely impact bee behaviour potentially resulting in social dysfunction or disruption of foraging. This study quantified behaviours associated with sub-lethal exposure to orchard-applied pyrethroids including, lambda-cyhalothrin, esfenvalerate, and permethrin, using video tracking software, Ethovision XT (Noldus Information Technologies).
Bee locomotion, social interaction, and time spent near a food source were measured over a 24-h period. Bees treated with a pyrethroid traveled 30-71% less than control bees. Social interaction time decreased by 43% for bees treated with a high sub-lethal dose of esfenvalerate. Bees exposed to a high sub-lethal dose of permethrin spent 67% less time in social interaction and spent more than 5 times as long in the food zone compared to control bees. PMID:  25966045

So it may not kill them immediately, but it has the same effect long term because with this sort of behaviour the hive will eventually starve and die off.

Killing cats

Pesticide-grade permethrin is toxic to cats. Many cats die after being given flea treatments intended for dogs, or by simply coming into contact with permethrin. In cats it may induce hyperexcitability, tremours, seizures, hyperaesthesia, hyperthermia, hypersalivation, loss of balance and coordination and death.

Permethrin is a synthetic pyrethroid widely used in flea control products for small animals. Accidental toxicity can occur with off-label usage, and cats are particularly susceptible….  The diagnosis of permethrin toxicity is made on the basis of a history of exposure and characteristic clinical signs, including seizures, muscle fasciculations, and tremors. PMID:  18498556

 Mammalian toxicity in general

....Pyrethroids act on some isoforms of voltage-sensitive calcium and chloride channels, and these effects may contribute to the toxicity of some compounds. Effects on peripheral-type benzodiazepine receptors are unlikely to be a principal cause of pyrethroid intoxication but may contribute to or enhance convulsions caused by actions at other target sites. In contrast, other putative target sites that have been identified in vitro do not appear to play a major role in pyrethroid intoxication.

The diverse toxic actions and pharmacological effects of pyrethroids suggest that simple additivity models based on combined actions at a single target are not appropriate to assess the risks of cumulative exposure to multiple pyrethroids.

PMID:11812616

Observations

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