Summary

For the past couple of months I have been researching the impact of climate change on the risk of liver fluke in livestock. This is a summary of what I have learned:

• Liver fluke infects cattle and sheep predominantly, and causes damage to the liver, and sometimes death of the animal.
• The free-living developmental stages of the fluke, and the host snail need warm temperatures and plenty of rainfall to survive and multiply.
• In the UK, the climate is expected to become warmer and wetter over the next century: this is predicted to increase the risk of liver fluke.
• Researching new drugs and vaccines is important to be able to control the risk of liver fluke, and this receives considerable funding and effort.

Interview with Dr Russel Morphew

I interviewed Dr Russel Morphew of IBERS to find out about the current research that the institute is undertaking regarding Fasciola hepatica.

 

http://www.filedropper.com/drrusselmorphewinterview_1

 

Breeding for Resistance?

Photo taken by Sarah Macmillan.
https://www.flickr.com/photos/essjay/229819241/
Downloaded:11/04/14

 

After discovering the various efforts to control liver fluke, I began to wonder if any resistance is shown by the host animal, and if breeding resistant animals would be a practical method to control the disease. I found a study that suggested that horses show resistance to the establishment of liver fluke infection. When they were infected with liver fluke, infection occurred in only one in ten horses.

In sheep, the Indonesian Thin Tail breed shows innate and adaptive resistance to infection of Fasciola gigantica but not to Fasciola hepatica. Fasciola gigantica doesn’t occur in the UK, or in Europe, however this knowledge increases our understanding of Fasciola resistance in sheep, which may help with combing up with strategies to control infection.

I could find no reports of Fasciola hepatica resistance in native British livestock, so it seems that breeding resistant hosts is not a suitable method for controlling Fasciola infection.

Liver Fluke Research in the UK

I wanted to know what research was being carried out in the UK. I came across GLOWORM (http://www.gloworm.eu/), an EU funded project that aims to develop new, effective strategies for controlling ruminant helminths, including liver fluke. The project is focused on optimising diagnosis, improving disease forecasting, identifying optimal intervention strategies, reducing the economic impact of infections and producing advice for worm control that be understood by the people who need to implement it.

The GLOWORM project started on the 31st of January 2012 and will be completed on 31st December 2014. One key finding of the project is a fecal egg count reduction test for Fasciola hepatica,which will provide a method for testing the efficacy of anthelmintic drugs. A recombinant vaccine for Fasciola hepatica has also reached field trials. So it seems as though this project has had a positive impact on developing the control of liver fluke.

The Institute of Infection and Global Health at Liverpool University has also sequenced the genome of Fasciola hepatica and are attempting to identify the gene or genes that are associated with drug resistance.

The development of more effective methods for liver fluke control could potentially have a large impact on the livestock industry, as the cost of liver fluke is estimated to be around £300 million in the UK alone.

 

How to Cope With Increasing Prevalence of Liver Fluke

In my last post I described how climate change is likely to increase the risk of fasciolosis outbreaks as temperatures and rainfall increase. This raises the question of what methods there are to control and treat liver fluke, and whether we are prepared to deal with the rises in disease expected within the next several decades.

There is no vaccine against liver fluke available, so the focus is on treating infected animals and trying to control. Treating infected animals involves the use of flukicidal drugs, the most popular of which is triclabendazole, which is effective against all stages development stages of the fluke. This drug takes about up to 72 hours to kill all of the flukes in the infected animal. Its use is not without problems however, as there have been reports of drug resistance developing among flukes. There are other drugs available and they work on flukes that are resistant to triclabendazole, however they are only effective against the adult stage of the fluke, not the migratory stage of the fluke. This stage can cause damage to the host, so using a drug that is not effective against it is not ideal.

One method of trying to prevent infection of liver fluke is to remove the Galba truncatula population from the area being grazed. This can be done by draining the pasture, or by using a molluscicide to kill them. Both of these methods are not ideal however as they both cause environmental concerns as they can both negatively impact other species living in the habitat.

It seems that vaccine and drug development  is an important area of research. Good methods for treatment and control are going to be needed as liver flukes and fasciolosis becomes more prevalent throughout the UK.

Climate Change And the Increasing Risk of Liver Fluke

In the previous post, I discussed how environmental conditions can affect the liver fluke and its snail host, and it seemed that temperature and moisture were both important factors in the development and multiplication of the parasite and snail. This is important, because the climate of the UK has changed over recent decades. The MET published a report in 2011 on their observations and projections of climate change, and this report found that over the last 50 years, the UK has become warmer, with average temperatures in the summer and winter rising by 1.4 and 1.15 degrees celsius respectively. These temperature increases are expected to continue on their upwards trend, with a projected increase of 3 degrees celsius in the south and 2.5 degrees celsius in the north by 2100. The temperature is not the only expected change by 2100, with precipitation also expected to increase by up to 10% in the north, although it is possible that there may be decreases in the south.

These changes are important, because they are changes to environmental conditions that can affect the liver fluke. There was a study that tried to predict the impacts of climate change on liver fluke. First, they looked at the impact that climate change had on Fasciola hepatica from 1970-2006, which can be seen in the image below. During the winter, the UK was fasciolosis free for this time period. During the 1970s, there were occasional losses to the disease in the west of the UK, with areas of disease prevalence in the west of Scotland. By 2000, the disease can be found in most of the UK, and had become prevalent in large areas in the west of Scotland and Wales. This shows that climate change has already affected the risk of Fasciola hepatica.

Past changes in Fasciolosis risk
Dark green=Little or no disease.
Light green= occasional losses
Orange= disease prevalent
Red= serious epidemic

Next, they tried to predict the impact that climate change would have on Fasciola hepatica in the future, as seen in the image below. They found that there was a risk of serious epidemics in Scotland by 2020, only 6 years away. This study also predicted that there would be serious epidemics in Wales by 2050. Over the decades, risk from Fasciola hepatica during the winter also begins to appear, with occasional losses along large sections of the west coast predicted by 2070.

Projected change in Fasciolosis risk
Dark green=Little or no disease.
Light green= occasional losses
Orange= disease prevalent
Red= serious epidemic

This study highlights the fact that the changing climate will affect the liver fluke causing increases in the risk of disease in animals.

 

 

Both images from: Fox NJ, White PCL, McClean CJ, Marion G, Evans A and Hutchings MR. (2011) Predicting Impacts of Climate Change on Fasciola hepatica Risk. PLoS ONE 6 (1): e16126.doi:10.1371/journal.pone.0016126

How Does the Climate Affect Liver Fluke?

Now that I knew more about the liver fluke parasite and the disease it can cause, I wanted to find out how the climate can affect the prevalence of the parasite.

The parasite has several stages in its development were it is living free of a host, so it is obviously more susceptible to environmental conditions at these stages of its life cycle. The parasite also relies on the snail host for further development and multiplication, so conditions that affect the snail will have a subsequent impact on the liver fluke.

A temperature of 10 degrees celsius is needed for the snail to breed and for development of the liver fluke within the snail. As the temperature rises above 15 degrees celsius, multiplication of the snails and fluke rises significantly, giving more chance for outbreaks of fasciolosis to occur.

Moisture is also important for the snail host. Galba truncatula is a freshwater snail that can live in permanent (rivers, ponds) or temporary (puddles, wet pasture) habitats. Rainfall can therefore have an impact on where the snail host, and consequently the fluke, can be found.

 

Further Reading:  Taylor et al. (2007) Veterinary Parasitology Third Edition.

 

 

Fasciolosis

Now that I knew more about the liver fluke parasite, I wanted to find out the effect that it can have on the infected animal. The disease that the liver fluke causes is known as fasciolosis, and the impact of the flukes on the disease depends on the number of flukes ingested by the animal. When the flukes are ingested, they must migrate from the digestive tract to the bile ducts. This can cause damage to the tissue of the liver, and can even cause haemorrhaging. The flukes can also cause damage to the host when they are in the bile ducts. They need a source of nutrition, and they obtain this from the host by feeding on the host animals blood. This method of feeding causes damage to the bile duct tissue.

The disease in the animal can either be acute, sub-acute or chronic. Acute fasciolosis is less common than the other types. This occurs 2-6 weeks after the ingestion of a large number of flukes. As these flukes migrate to the bile ducts they cause significant damage to the liver tissue and rupture blood vessels causing haemorrhaging. Acute fasciolosis can cause sudden death of the animal.

Subacute fasciolosis occurs when the host animal ingests flukes over a longer period of time, so the flukes are not all migrating to the bile ducts at the same time, causing less damage to the liver tissue than in acute fasciolosis. The disease occurs 6-10 weeks after ingestion of flukes, which is longer than in the acute disease. Animals with subacute fasciolosis do not die as rapidly as animals with acute fasciolosis, and symptoms of the disease may be present up to 2 weeks before death.

Chronic fasciolosis is the most common form of the disease, and can occur up to 5 months after the ingestion of a small number of flukes. This form of the disease mainly affects the animal because of blood loss, and can cause loss of condition, loss of appetite and development of anaemia. In some animals, clinical signs may not be apparent, but the parasites can still be causing damage to the liver and can production by causing a reduction in appetite.

 

Further Reading:  Taylor et al. (2007) Veterinary Parasitology Third Edition.

What is Liver Fluke?

Fasciola hepatica egg.
Photo taken by Vivien Rolfe.
https://www.flickr.com/photos/biologycourses/7686933314/in/photolist-uGwFy-cs1MUU-gc6sXh-cHgxt1-cHgxfS-cHgxmY/
Downloaded 11/04/14

Before writing about the effect that climate change can have on liver fluke, it was important to find out more about what liver fluke is and the effect that it can have on animals. One website that had a lot of information was http://www.scops.org.uk. SCOPS stands for Sustainable Control of Parasites in Sheep.

From this website, I found out that liver fluke is the name given to the parasitic worm Fasciola hepatica. This parasite can infect all grazing animals, but is mostly found in sheep and cattle. The life-cycle of the liver fluke involves several free-living stages as well as an intermediate host. This host is a snail called Galba truncatula.

Fasciola hepatica worm
Photo taken by Kristi Yim
https://www.flickr.com/photos/121611938@N08/13483712783/
Downloaded: 11/04/14

In the infected animal, the liver fluke lays eggs which are passed out in the faeces. If the conditions are suitable, this egg will develop into a miracidium which needs to find a snail host to survive. The snail is needed for further development and muliplication of the parasite, until conditions are suitable and the infective stage can emerge from the host. The infective stage needs to be in an area where animals graze so  that it can be ingested. Once it has infected the animal, the parasite will migrate to the liver where it can significant damage. After 10-12 weeks of being in the liver, the fluke starts to produce eggs, and the cycle repeats itself.

This snail host is important in determining where liver fluke can be found. The snail prefers wet, muddy, slightly acidic conditions which means that liver fluke is more likely to be present in in wetter areas.

The Effect of Climate Change on Liver Fluke

Climate change undoubtably presents many challenges, and the effect that changes in the weather can have on the organisms present in an environment fascinates me. Liver fluke is an infectious disease of livestock whose epidemiology is changing. The changes seen in the prevalence of the disease are thought to be because of climate change, and I find it interesting to learn how we must increase our knowledge to adapt to these changes, and try to reduce the impact that it will have on the livestock industry in the coming decades. The increase in liver fluke in the UK will present many challenges, and here I will discuss some of them.