Understanding the bloodsucking feeding mechanisms of ticks could result in vaccines against hard-to-treat diseases like Lyme disease.
A sneeze, a cough, a kiss – even a handshake: diseases such as measles, whooping cough and the flu are often passed directly from one person to another in tiny droplets of fluid which contain viruses or bacteria.
However, not all infectious diseases are spread so directly. Some hijack bloodthirsty insects and arachnids, taking the opportunity to infect humans during feeding time.
One such disease risk is the humble tick – an eight-legged parasite often found in Europe’s forests.
If you are bitten by a tick while hill walking this summer, you will probably feel nothing. The trouble is that ticks play host to pathogens which cause Lyme disease, tick-borne encephalitis (TBE), human babesiosis, and many more besides.
Tick-borne diseases can be very serious, particularly if timely treatment is not available or if the victim has a weak immune system. Lyme disease, for example, can cause paralysis if left untreated, while TBE can cause inflammation of the brain and death. Lyme disease can be treated with antibiotics, however TBE is incurable once the symptoms appear.
Ticks perfer shady and humid woodland or forest clearings, and threaten hillwalkers right across Europe, from Great Britain in the west to Russia in the east.
Infection can be all too simple. Ticks latch onto your skin as you brush past a tree or shrub. From there they will find a soft thin area of skin, sink in their proboscis (sucking mouthpart) and begin to feed for several days. The tick then drools saliva into the wound as it feeds, passing infection on to its human host.
Why, you might wonder, do ticks not fall off? Why do humans not brush them away when they feel the bite? Why does the human body’s immune system not block this five-day feeding frenzy?
The answer, said Dr Joppe Hovius, lies in the sophisticated biochemistry of the tick’s saliva.
‘Tick saliva is full of goodies. It has proteins that are like cement and keep it stuck to human skin. It can inhibit the itch reaction, evade the immune system, and even make blood thinner,’ he said.
‘Tick saliva is full of goodies. It has proteins that are like cement and keep it stuck to human skin. It can inhibit the itch reaction, evade the immune system, and even make blood thinner.’
Dr Joppe Hovius, Infectious Diseases Consultant, Academic Medical Center, Amsterdam
Dr Hovius, an infectious diseases consultant at the Academic Medical Center (AMC) in Amsterdam, is leading the EU-funded ANTIDotE project which aims to study the mechanisms of tick feeding, and use this knowledge to develop new vaccines.
The project is in its early stages, so a wide range of tick proteins could be in the firing line. One possibility for vaccine development would be to develop antibodies against a tick protein that suppresses the human immune system during feeding.
Another option would be to neutralise the tick’s natural anticoagulant – which prevents the host’s blood from thickening and forming protective scabs.
‘We are focusing on three tick-borne diseases – Lyme disease, TBE and human babesiosis – but our work could be extrapolated to others,’ Dr Hovius said.
The target diseases are caused by a bacterium, a virus, and a parasite, respectively, but all three piggyback on the tick’s feeding mechanism.
‘We aim for a single vaccine that is capable of preventing transmission to humans either by interfering with transmission or by interfering with tick feeding,’ Dr Hovius explained.
Dr Joppe Hovius, the coordinator of the ANTIDotE project, with a tube containing ticks. © Ivar PelThere are already vaccines against TBE, but the number of people who opt to be vaccinated is low in most European countries. The researchers think that if a single vaccine could protect against a range of diseases at once, uptake rates might increase.
Ticks need to feed on blood in order to reproduce. A vaccine which disrupted tick feeding or the reproductive cycle could bring down the total population of ticks, whereas a vaccine that allowed feeding but prevented diseases from passing to humans would only reduce the number of infections.
But a human vaccine is not the only option ANTIDotE is exploring. It may also be feasible to use a vaccine in wildlife, delivering it to other animals on which the tick feeds, thereby reducing numbers. Fewer ticks mean fewer humans will become victims of tick-borne illnesses.
A combination of human and animal vaccines is another possibility – with perhaps a transmission-blocking vaccine in humans and a tick-feeding vaccine for wildlife – but it will take several years before the solution emerges. The researchers plan to bring together a range of experts to discuss the future exploitation and implementation of anti-tick vaccines in health systems.
Researchers are also examining other disease carriers such as sand flies which spread leishmaniasis disease and the chandipura virus, a deadly cousin of rabies, or tsetse flies which spread human African trypanosomiasis, also known as sleeping sickness.
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