Defeating the little dragon
The Guinea worm is a water-transmitted parasite that develops over a year in its host before painfully emerging from the skin. It is hoped that it will soon become the first parasite to be eradicated but there are a few challenges to overcome first…
Dracunculiasis, or as it is more commonly known Guinea worm disease (GWD), is caused by a parasite the Guinea worm, Dracunculus medinensis. It is the first disease caused by a eukaryotic pathogen to ever get close to being eradicated. Great strides have been made in reducing the incidence of this disease over the last 30 years. In the mid-1980s over 3.5 million human cases were recorded a year, whereas in 2014 there were just 126. Furthermore, this has all been done without the use of drugs or vaccines, but simply by controlling transmission. In 2010, however, scientists realised that eradicating the disease from humans was only part of the story and genomics is now being called upon to aid the eradication effort.
Guinea worm larvae develop inside little water fleas called copepods.
Guinea worm is now found in remote areas of Africa such as parts of Chad, Ethiopia, Mali and South Sudan. Although Guinea worm disease is rarely fatal, it can cause crippling disability. It can also lead to bacterial infections that are hard to treat, therefore increasing the pain and disability associated with the disease. The life cycle of the parasite starts in fresh water where the Guinea worm larvae develop inside little water fleas called copepods. The parasite generally affects communities that rely on this fresh water for their main drinking source. By drinking the water contaminated with these parasite-infected copepods, they are inadvertently taking the Guinea worm larvae into their bodies. Once inside the stomach, the copepods are digested and the larvae are set free. They then move to the small intestine and penetrate the wall of intestine entering the body cavity. Male and female larvae migrate through the body and mate 60-90 days later. They then develop into adult worms.
Over the next 10-14 months the adult female worms grow up to a metre in length and as wide as a piece of spaghetti. The female releases her larvae in milky-white liquid near the surface of the skin, usually on the leg or foot. This triggers an immune response in the host that results in the creation of a blister on the skin. This blister causes an incredibly painful, burning feeling, hence the name Dracunculus or ‘little dragon’, and bursts after about 24-72 hours. This burning feeling compels individuals to immerse the affected limb in water, which is exactly what the worm wants. Contact with water releases lots and lots of larvae back into the water supply.
At the bottom of the open skin blister the female worm’s head can be seen. As a result the best way to remove it from the host’s body is for someone to slowly pull it out – often a searingly painful and slow process. The worm is incredibly resistant to being removed, so over the course of several weeks the worm is pulled out and slowly wrapped around a stick, until it is released completely. If this process is carried out too quickly the worm can break and remain in the host. This can lead to more serious complications as the host has to destroy the remnant of the worm. The extraction of the worm from the open blister can also introduce bacteria into the body. This can lead to bacterial infections that, in severe cases, have the potential to kill the host.
Even after the worm is removed from the body, bacterial infections can be very common
Even after the worm is removed from the body, bacterial infections can be very common because the wound has been open for several weeks. In remote areas of Africa where Guinea worm is generally found, these infections can be very difficult to treat and are sometimes fatal.
It is possible that the common symbol of medicine (snakes coiled around a staff) could be based on early representations of the process used to remove Guinea worm.
Taming the dragon
The Guinea worm transmission cycle is relatively long at 12 months
One of the main reasons it has been possible to get so close to eradicating Guinea worm disease, but not other parasitic diseases like malaria, is because the Guinea worm transmission cycle is relatively long at 12 months. Simple interventions have also made it easier to block the transmission cycle although changing the behaviour of whole communities has been challenging. Providing communities with access to clean water and educating them about the parasite has stopped it being transmitted. Communities are issued with water filters and pipe filters (personal filters that children can wear around their necks), to filter the water that they drink so that they do not consume the copepods containing the parasite.
Active surveillance is being carried out in all countries that still have Guinea worm. This means that every time there is a case of an infection, an investigation is carried out to try to find out where the worm came from. This has contained the disease and prevented contamination of drinking water with parasite larvae. This can be quite a challenge, not only because the worms take a year to grow to full size and emerge, but also because some communities in Africa migrate hundreds of miles every year, or live in the middle of conflict zones.
The rapid decline in the number of cases of Guinea worm disease led many to believe that the problem of the little dragon had been solved, so very few scientists now work on Guinea worm. However, recent findings in Chad have meant investigations have had to be opened once again...
What does eradication mean?
Chad in Central Africa is one area that was thought to have completely eradicated Guinea worm as no cases were reported for an entire decade at the start of the 21st century.
For a country to be certified as free of Guinea worm disease, it has to have a robust and comprehensive surveillance system in place and be able to provide proof that there have been no cases of the disease for three consecutive years. This is the equivalent of three incubation periods for the Guinea worm. This means showing evidence of active surveillance over those three years to demonstrate that the surveillance system has been hunting for cases of Guinea worm disease over that time. Once they’ve done this, the World Health Organization (WHO) sends a team of experts who independently check the findings. If the necessary standards are met they then recommend to the International Commission for Certification of Dracunculiasis Eradication (ICCDE) that the country is certified free of the parasite. If the ICCDE are satisfied they will then recommend to the WHO that the country be declared free of Guinea worm disease.
Although Chad went 10 years without a single reported case of Guinea worm disease, the WHO felt that the surveillance system during that time hadn’t been rigorous enough. They therefore did not certify that the country was free of the disease. Sure enough, Guinea worm disease was found once again in Chad in 2010. However, this time there was an unusual difference in the pattern of Guinea worm infections not previously seen in Chad or other countries.
In 2012 dogs infected with Guinea worms began to be detected
The re-emergence of Guinea worm disease in Chad led to the establishment of new surveillance in the areas that had reported cases of the disease. In April 2012 dogs infected with Guinea worms began to be detected. Historically, there have only been a few reports of non-human cases of Guinea worm but over 100 infected dogs were reported in 2014 and this number has increased in 2015. In contrast there have been far fewer human cases of Guinea worm disease. It was unknown whether the infection in Chad was reintroduced by dogs or humans, or had just gone undetected for 10 years.
Observations of infected dogs in Chad seem to suggest they cope with the worms better than humans. They exhibit a milder response to infection, the tissue around the worm is much less inflamed compared to infected humans and, as a result, the worms are easier to pull out. Dogs also tend to be infected with more worms than humans at one time. This suggests that they are being exposed to greater numbers of Guinea worm larvae.
The re-emergence of human infections, and appearance of dog infections in Chad has raised a number of questions about the possible association between the infections in people and dogs. Do the dogs serve as a reservoir for human infections that is somehow spilling over into people, or are the dogs infected with a different, distinct species of Dracunculus?
The concern is that the dogs are now driving the spread of Guinea worm. Dog cases seem to be spreading southwards down the Chari River. Now they’re just 20 kilometres away from the border of the Central African Republic that has been free of Guinea worm and currently has no surveillance programme. But how do you stop a dog from drinking larvae-infested waters or releasing larvae into the water? To reduce the potential transmission of Guinea worm between dogs and human new measures have been put into place. These are similar to the methods used to prevent humans transmitting the worm and include:
- applying insecticide to water contaminated by dogs
- offering a reward to people who notify authorities of dogs infected with Guinea worm
- tethering of dogs with Guinea worm infections to keep them out of water sources.
A fishy tale
One aspect of the human cases in Chad is that they don’t seem to have an obvious water source in common. In fact, cases have been found outside the at-risk zone along the Chari River. This raises the question of whether a paratenic host might be channelling the infection to both dogs and people.
Paratenic describes a host that is not essential for the development of a particular parasite but just serves as a place for it to sit until it can find its actual host. In this case, scientists suspect a host might be involved that lives in water such as fish or even tadpoles or frogs. This would help explain why efforts put in place since 2011 have not reduced the number of annual human infections and would also explain the emergence of infection in dogs.
Fish feed on copepods, some of which may contain the Guinea worm larvae. Fish are one of the primary sources of protein for people in Chad and surrounding areas. In fact, every year when the rivers start to dry out communities set down nets and drag out all the fish before having an enormous fishing party. Usually communities cook the fish by just light smoking or drying in the sun. This could enable the Guinea worm larvae to survive and continue their life cycle in the human body. Finally, the raw entrails of the fish are often thrown to the dogs to eat which could explain how the dogs also become infected.
However, this fishy theory presents a couple of practical challenges. Measures to kill the copepods in large water sources aren’t realistic in terms of the livelihood of these communities. Generally if you kill the copepods, you kill the fish and then you leave the people without a protein source and this could put them at risk of starvation.
In addition to this, actually finding the Guinea worm larvae inside the fish to prove that they are a paratenic host is very difficult because they are so tiny. Research is trying to establish a way of detecting the larvae.
How can genetics help us?
The fact that the latest cases of Guinea worm in Chad are different to previous ones in Chad and other countries is worrying for the global eradication campaign. It may indicate that eradicating Guinea worm is not as straightforward a process as was once thought. However, studying the genome of the worm could help provide a more detailed insight into exactly why it has come back so that interventions can be adjusted accordingly.
DNA sequencing: the key to success
In 2011, scientists at the Wellcome Trust Sanger Institute initiated a collaboration with the Carter Center and the US Centers for Disease Control and Prevention (CDC) to sequence the entire genome of the Guinea worm. The aim was to record genetic knowledge of this historically important parasite after its eradication. With the current challenges to eradication in Chad, however, it is hoped that the high-quality genome sequence released in 2015 will provide a valuable reference for further research into the genetic make-up of the Guinea worm.
Is the species of Guinea worm that affects humans the same as the one that has been found in dogs?
Primarily scientists want to find out if the species of Guinea worm that affects humans is the same as the one that has been found in dogs. Like other parasites that affect both humans and animals, the species is often specific to its host.
Funded by the Carter Center – which leads the international campaign to eradicate Guinea worm disease – and collaborating with scientists at the CDC, scientists at the Wellcome Trust Sanger Institute are currently using population genomics to find out more about the Guinea worms in dogs and humans.
To do this they have sequenced the genomes of a subset of Guinea worms taken from dogs and humans in Chad and other areas of Africa to identify any key similarities or differences. So far they have found that Guinea worms from dogs and humans in Chad are more similar to each other than a Guinea worm from a human in Chad and a Guinea worm from a human somewhere else in Africa. This supports the unusual pattern of the recent Chad outbreak and the idea that the increase in human cases is related to the worm now being found in dogs.
The next step is to perform DNA sequencing on most Guinea worms found in Chad. This isn’t many worms because there are less than 20 human cases of Guinea worm disease each year. Much of the lab work has focussed on generating enough high quality Guinea worm DNA sequence to be able to carry out a proper, detailed analysis. It is important to ensure that the Guinea worm samples received are as usable as possible because so few are available each year. Once they have examined the genetic data from every human case of Guinea worm in Chad and compared it with many samples of worms from dogs it will be easier to work out where the infections originate from. The hope is to provide direct evidence that worms emerging in dogs one year are the parents of worms emerging from people in the year after, and vice-versa. This would demonstrate that the same worms are involved in both hosts and measure how important dogs are in leading to new human cases of the disease.
If genome sequencing can prove it is the same Guinea worms that are infecting both humans and dogs in Chad it would confirm that it is the dogs that are responsible for maintaining the level of Guinea worm larvae in human water sources. This will inform the Guinea worm eradication programme in Chad.
If the studies confirm that there is a major ‘zoonotic’ (animal) component to the transmission of Guinea worm it will provide a much better understanding of how the Guinea worm spreads. This may help to establish how eradication could be achieved. One way may be to design further interventions to prevent infection in dogs as well as any potential paratenic hosts, such as fish.
When can we expect eradication?
Currently the Gates Foundation, one of the main funders for the eradication programme, has said that they want the Guinea worm to be fully eradicated by 2020. Eradication efforts cost a lot of money as they require many health workers on the ground in every area where Guinea worm is present. However a better understanding of the biology of the Guinea worm will help to strengthen these efforts. New techniques in genomics and analysis of the Guinea worm genome are valuable tools to help solve this puzzle and find a way to block the way of the dragon.
This page was last updated on 2016-06-13