Strategic and tactical treatment of liver fluke

Estimated reading time: 7 minutes

• The spread and incidence of liver fluke are farm and climate dependent
• Livestock of any age and at any stage of the production cycle are susceptible to new infections.
• Tactically, immediate action is needed to limit losses, followed by strategic steps to halt the life cycle of the liver fluke.
• Several groups of agents (active components) are available for chemical control, most of which are only active against adult liver flukes.
• Producers should ask their veterinarian to assist in testing for liver fluke infection in summer and autumn.

The spread and incidence of liver fluke are farm and climate dependent. Its incidence and severity on infected farms can increase a hundredfold in wet years. Cattle are more resistant than sheep, but in general animals are not highly resistant to this parasite.

The good rain received in many parts of the country created favourable conditions in which the intermediate host snail could multiply and, with it, the occurrence of severe liver fluke infection this summer and autumn.

Tactically, immediate action is needed to limit losses, followed by strategic steps to halt the life cycle of the liver fluke, while keeping the following objectives in mind:

• Limit production losses in summer and late autumn due to new infections.
• Limit production losses in late winter, as this will also curb reinfection of the intermediate host snails in the coming spring and summer.

Know the parasite

There are two types of liver fluke, namely the common liver fluke (averaging 2,5 x 1,5cm in size) and, to a lesser extent, the giant liver fluke which is sometimes found in the northern regions of South Africa.

The adult flukes live in the animal’s bile ducts. They damage the bile duct wall, which leads to bleeding with the blood serving as a source of nutrients for the flukes. Adult flukes produce eggs which are passed through the bile duct into the intestine and then excreted in the manure.

Rain washes the liver fluke eggs out of the manure, carrying them to areas with stagnant water such as pans and floodplains. The free-living stage of the parasite that hatches from the eggs must enter a freshwater snail (the intermediate host) within hours to survive.

Freshwater snails (Lymnaea species) are mostly present in late spring, summer and early autumn. In winter, when minimum temperatures plummet to below 10°C, the snails burrow into the mud.

The liver fluke reproduces inside the freshwater snail and the offspring are released into the water roughly a month later. This stage of the fluke floats on the water’s surface or encysts on plants in the water, forming a protective coating which allows it to survive for a prolonged period. This is the stage most infective for sheep, cattle, and other animals.

Animals ingest the infective stage while grazing or drinking water in wet areas. Once in the animal’s gastrointestinal track, the young fluke sheds its protective coating and migrates through the intestinal wall into the abdominal cavity and on to the liver. It penetrates the outer membrane of the liver where it proceeds to tunnel its way through for the next one to two months – it feeds on the liver tissue while rapidly maturing. The adult fluke then penetrates the wall of the bile duct to complete its life cycle.

The host (cattle or sheep)

Livestock of any age and at any stage of the production cycle are susceptible to new infections. Sheep are more susceptible than cattle, and animals already infected and with flukes present in their bile ducts are more resistant to new infections.

Immature liver flukes that migrate through the liver while continually feeding on the liver tissue, cause large-scale liver damage and a major inflammatory reaction. Adult flukes cause the bile ducks to thicken and, in some instances, also to calcify. If the infection is long term, the entire liver can harden. The damage means that the liver is unable to function properly.

The liver is responsible for the animal’s energy and protein metabolism as well as the breakdown of toxins. The main signs of disease in severely infected animals are suppressed appetite and anaemia, as the adult liver flukes ingest large amounts of blood as a source of nutrition. Another visible sign in especially sheep, but also in cattle shortly before they die, is bottle jaw brought on by a reduction in protein in the blood.

The outcome of this is weight loss and, in cases of severe infection, a large number of animals dying. Where livestock experience poor nutritional conditions (such as grazing during late winter), the impact is more dramatic and they can lose a lot of weight, severely affecting their reproductive capacity in the next breeding season.

Figure 1: Climatological profile of the Highveld.

Treatment options

For strategic control to be effective, producers must be well acquainted with the conditions in which they are farming, the parasite, and the host. As an example, Figure 1 depicts the general environmental conditions prevalent on the Highveld – it can be adapted for different regions and even different farms.

Firstly, it is important to keep records of the normal rainfall pattern. Rain is needed to dislodge the eggs from the animals’ manure before they can hatch and infect the intermediate host (freshwater snail). Furthermore, rain leads to the formation of wet areas in which the freshwater snails (S) live and multiply.

Secondly, minimum and maximum temperatures must be indicated on a graph. Freshwater snails (S) normally only become active once the minimum temperature rises above 10°C. It takes about a month from the time the freshwater snails (S) become active and are exposed to a new infection, for the infective stage (I) to be released and reinfect the host animal.

Strategic treatment is therefore administered when the intermediate host (freshwater snail) is inactive, and the parasite (adult liver fluke) mainly survives in the host animal.

Tactical treatments can also be administered if grazing is heavily infested and there is a greater chance of animals, especially the young, dying. Treatment should be administered before the liver flukes mature and start producing eggs.

Control agents

Several groups of agents (active components) are available for chemical control, most of which are only active against adult liver flukes, especially in cattle (Table 1). The best option is to make use of the most effective agent that targets both immature and adult liver flukes when new infections have to be treated in summer and autumn.

Table 1: Agents for treating liver fluke.

Important steps

The national disease reporting system that veterinarians participate in enables them to stay informed of:

• Whether the parasite is present in the area in which you farm.
• What the specific risks are, as the occurrence of the parasite can vary considerably in wet and dry years or on farms with several stagnant sources of water that favour infection or others with no standing water.
• The success of treatments using specific agents at different times of the year.

On farms on which liver fluke is a major concern, producers should ask their veterinarian to assist in testing for liver fluke infection in summer and autumn. If animals are infected, a treatment and management plan should be developed in conjunction with the veterinarian.