How Neonicotinoids Promote Deformed Wing Virus And Impair Immunity In Honey Bees
is a summary of an important scientific study by Di Prisco et al examining how low
doses of neonicotinoids impair immunity to diseases and viruses such as DWV, in
"Neonicotinoid clothianidin adversely affects insect immunity and promotes replication of a viral pathogen in honey bees."
Di Prisco G, Cavaliere V, Annoscia D, et al. Proc Natl Acad Sci U S A. 2013;110(46):18466-71. (Ref 1).
- Honey bees are exposed to a range of stress factors,
which acting together, may cause honey bee colony losses that are often associated
with a high level of infectious micro-organisms that cause disease (pathogens).
- Neonicotinoid insecticides have been reported to enhance
the impact of pathogens, although how the immune system is altered to enable
this to happen, is unclear.
- In this study, the scientists identify and describe the molecular
mechanism (the way) through which the neonicotinoid clothianidin impedes the
insect immune response, thus enabling viral pathogens to multiply in honey bees that have a covert infection.
(A covert infection is one that is present, but
not showing symptoms or hampering the host in any way, such that it is not noticeable
and the host can function as normal).
"If the disease is already in a creature's body, surely it's only a matter of time before they begin to show signs of disease, and possibly die?"
Actually, no, this is not correct. All creatures can carry diseases, viruses and bacteria that have the potential to cause real harm and even death, and yet the symptoms of the disease never become apparent.
Here is a comment from a paper published by the esteemed biological research journal, The Royal Society Publishing:
"it is estimated that one-third of the human population is infected with Mycobacterium tuberculosis (Dye et al. 1999); however, only 5–10% of these infections cause overt disease within the first 2 years. Many individuals that have a covert infection will never express the overt, active TB infection that causes disease and has the potential for horizontal transmission (WHO Report 2005)."
In order to understand this paper, a little simplified background information is worth including, so here goes.
The bodies of insects produce a number of different
protein molecules, each performing its own important role. Protein molecules are receptors, meaning they
receive and respond to signals and/or stresses outside the cell itself.
One protein complex called NF-kB is very important for causing immune responses to stresses, such as harmful bacteria and viruses. In other words, this protein is a receptor that responds to harmful bacteria and viruses, so it is very important
for the insect’s ability to fight off diseases.
Nevertheless, if an insect’s response is too sensitive to
stressors, then this is actually harmful to the insect – it could actually
over-react to beneficial gut (stomach) bacteria that the insect needs to keep
it healthy. So, to keep the amount of NF-kB receptors in check, the insect’s
body produces another protein, called LRR (which stands for Lutein Rich Repeat).
However, if the insect has too many LRR receptors, then
it will over-regulate the amount of NF-kB in the insect’s body, such that it cannot
produce enough NF-kB to fight off diseases and viruses that are harmful.
So you can see that insects (including honey bees) need the correct balance
of these 2 proteins in order to have a healthy immune system that fights off
diseases and viruses whilst allowing beneficial gut bacteria to thrive:
- one called
NF-kB, and another called LRR.
So what did the scientists find when honey bees were
exposed to a sub-lethal dose of the neonicotinoid clothianidin?
(Note: A sub-lethal dose is an amount that will not
immediately cause death. However,
sub-lethal doses can have subtle effects that ultimately harm and
eventually cause death).
The scientists found that exposure to the neonicotinoid clothianidin resulted in an imbalance of the 2 important proteins so that:
- The bee has too many LRR receptors, resulting in a negative
impact on NF-kB – the protein responsible for immune responses to pathogens.
- The honey bee’s anti-viral defences are therefore impaired
so that the bee is now vulnerable to multiplication of viruses and disease.
- In the study, this resulted in a multiplication of Deformed Wing
Virus (DWV) infection in the honey bees, where previously only a covert infection of the virus had been present.
- The scientists also performed this experiment with the
organophosphate chlorpyriphos, and found that the chemical had no significant impact on the proteins.
- The scientists fed the honey bees with sub-lethal doses
of neonicotinoid to match concentrations and contamination levels that would
match field conditions, ranging from 0.1 to 10 ppb (parts per billion). Interestingly, they found that neonicotinoids
(clothianidin and imidacloprid) were active in promoting the multiplication and
proliferation of Deformed Wing virus. The scientists make the point that infections were induced
in a similar way (immunosuppression) by the neonicotinoid imidacloprid.
- In causing viruses to multiply, the subtle effects of
very low-dose neonicotinoids, might also be negative in the natural
- Covert infection of DWV is an extremely frequent occurrence
in a large number of apiaries all over the world. Given this fact, the scientists state that using their proposed honey bee
stress models, it is possible to predict that neonicotinoids may promote the proliferation
of DWV in bees bearing covert infections.
"The occurrence at sublethal doses of this
insecticide-induced viral proliferation suggests that the studied
neonicotinoids might have a negative effect at the field level."
In the paper, the scientists refer to a number of other
studies, worthy of mention here, considering their potential impact on proteins that play an important part in immunity:
- Mediterranean Mussel
Ecotoxicological studies have reported that exposure of Mytilusgallo provincialis (meditaerranean mussel) to sublethal
doses of the neonicotinoid insecticide thiacloprid affects proteins important for immune function.
- Common Fruit Fly
In this experiment concerning the larvae of the common fruit fly, Drosophila melanogaster, scientists were interested in what
would happen to the levels of the protein NF-kB (the one responsible for immune
responses) when exposed to standard
dose LD50) of clothianidin.
They found that the percentage of fruit fly
larvae showing an intense response to immune challenge, was
significantly lower in larvae exposed to clothianidin, than in the control
In other words, larvae that had
been exposed to the neonicotinoid, were not producing so much of the protein
NF-kB, which is important for fighting diseases and pathogens.
Again, no such impact on the disease fighting protein was
exhibited when the larvae were exposed to the organophosphate chlorpyriphos.
- This is yet another scientific paper highlighting the dangers of neonicotinoids to invertebrates.
- Neonicotinoids are also implicated in the proliferation of nosema in bees.
- Potentially, neonicotinoids could hamper the ability of bees to deal with Varroa mite.
- The studies on mussels and fruit flies may highlight the dangers of neonicotinoids to other invertebrates including thosse in aquatic environments.
1. Di Prisco G, Cavaliere V, Annoscia D, et al. "Neonicotinoid clothianidin adversely affects insect immunity and promotes replication of a viral pathogen in honey bees." Proc Natl Acad Sci U S A. 2013;110(46):18466-71.
2. The evolution of covert, silent infection as a parasite strategy
Amy B. Pedersen
Rosemary S. Hails
Mike Boots . Published:11 March
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