Parasites that thrive after floods

No. 1: The ‘jelly-bean’parasites: stomach flukes- (paramphistomes)

After floods, some parasites take advantage of the wet conditions to spread on pastures and infest livestock. A perfect example is stomach flukes, also known as ‘paramphistomes’. These flukes are expanding their distribution across Europe and the British Isles and are getting renewed attention due to their impact on animal health and productivity. In Australia, research has outlined their biology and treatment options, but recent climatic conditions and feral animals may be contributing to their spread.

What are they?

Stomach flukes are trematode parasites that live in the rumen or reticulum (first and second stomachs) of grazing ruminants. They can infect sheep, goats and cattle but have also been found in feral deer. The adult flukes are small (about 5-12mm long), pale pink to orange-coloured and resemble jelly-beans attached to the wall of the stomach.These parasites and their hosts love fresh water and are found across the wetter areas of eastern and tropical Northern Territory and Western Australia.

Fig 1: The ‘jelly-bean parasite’: adult stomach flukes in the rumen. Image courtesy of Univ of Sydney and reproduced from ‘Australasian animal parasites- inside and out’ published by Australian Society for Parasitology.

In Australia, three different species of stomach flukes have been identified (see below).

Table 1: Stomach flukes found in Australia and their intermediate hosts

All species share a similar life cycle (see below), appearance and cause similar impact on infected animals.

Fig 2: Planorbid snails, also known as ‘ram’s head snails’, live in fresh water and are the intermediate host of the stomach flukes.

Figs. 3-5: Distribution of freshwater snail hosts of stomach flukes in Australia (courtesy of ‘Australian Freshwater Molluscs’)

Life cycle (see diagram below)

Adult flukes are hermaphroditic (have both male and female reproductive organs) and lay fertilised eggs, that pass through the gastrointestinal tract of the ruminant host and end up in the dung pat. The eggs hatch in 8 to 10 days. The larva (known as the miracidium) crawls out of the faeces and penetrates the skin around the mantle cavity of the snail. The infective stages (cercariae) of the stomach flukes crawl out of the snails about 6-7 weeks later and form cysts (metacercariae) on grass around ponds and dams, where they are eaten by grazing livestock.

Figure courtesy of NSW Department of Primary Industries.

Impact

Stomach flukes mainly affect young cattle 6-18m of age, as well as young sheep and goats. However, animals of all ages can be badly affected if they have had no previous exposure.

After flooding, high concentrations of snail hosts build up and grasses on the edge of the ponds becomes massively contaminated with infective cysts as the water recedes.

Once eaten by livestock, the immature stomach flukes (1 to 2mm long) travel to the small intestine where they burrow into the intestinal lining and cause intense irritation. This results in bleeding, irritation and signs of diarrhoea, wasting and ill-thrift.

Infected animals may develop bottle jaw and emaciation. Young cattle and previously unexposed animals can die due to complications of this intestinal damage.

Once animals become immune, they tolerate infestation with adult stomach flukes and do not show clinical disease. Recent research however suggests that animals with chronic infestations may suffer weight loss of reduced productivity.

Diagnosis

Eggs of stomach fluke can be seen under the microscope after a ‘sedimentation test’. The stomach flukes are similar in size to liver fluke eggs but can be distinguished by their colourless appearance, compared to the yellow colour of liver fluke eggs. This is a good way of screening mobs of livestock, but is of limited use clinically, as most disease is caused by young flukes before they start to lay eggs.

If you suspect disease is being caused by stomach fluke, your vet will be able to conduct a post mortem examination and search for immature stomach fluke in the small intestines.

Treatments and control

There are no registered treatments for stomach fluke in Australia. However, research in Australia and overseas has shown oxyclozanide (given twice over 3 days) to be effective in controlling stomach fluke. As this is an off-label use of the product, talk to your veterinarian about diagnosis and treatment.

Control is best done by limiting exposure of young stock to environments where stomach flukes are known to occur. The presence of the planorbid snails will give you a guide to where the fluke metacercariae are likely to be. They are mainly found around creeks and other water sources, so are less likely to be seen in hilly or dry paddocks.

However, controlling access to young stock is very challenging following floods, when infective stages of the stomach flukes are dispersed widely across paddocks. One strategy is to only graze highly-contaminated paddocks in spring or early summer and avoid them for young stock in autumn and early winter when contamination of pasture with infective cysts is expected to be highest. Drenching stock with an effective stomach fluke treatment before grazing on high risk paddocks will also limit the number of paramphistomes developing through to the infective stage.

More information

Stomach fluke

NSW DPI Primefact 452

MG Smeal, (1995) ‘Parasites of cattle’ The University of Sydney Postgraduate Foundation in Veterinary Science 

Author: https://paraboss.com.au/news/parasites-that-thrive-after-floods/