Parasite Watch Autumn Edition: Parasites to Watch Out For in Your Ruminants

This week we’re going to focus on parasites which cause two specific diseases in ruminates. These are parasitic gastroenteritis or PGE, caused by roundworms, and liver fluke, caused (unsurprisingly) by liver fluke flatworms.

What is PGE?

Parasitic gastroenteritis is one of the major contributors of reduced productivity in ruminants. An overburden of parasitic nematodes (roundworms) causes the infection in cattle, sheep and goats. These parasites reside in the abomasum (fourth stomach) and intestines of the animal. They are most common in lambs and first season grazing cattle.

What causes PGE in cattle?

Although several species of nematode may be present in the gut, the principal worm responsible for outbreaks of PGE in cattle is Ostertagia ostertagi. Its common name is the small brown intestinal worm. Bovine ostertagiosis can manifest as Type I or Type II depending on the time of year.

Type I usually occurs from mid-July in intensively-grazed calves. Clinical signs are reduced appetite, weight loss and diarrhoea. Morbidity rates are usually high, but mortality is rare provided that the animal is treated early.

Type II occurs in yearlings in late winter or spring. The parasite survives the winter after being eaten by the animal any time after September. A drop in temperature is thought to trigger a delay in its normal lifecycle — this is known as hypobiosis. The parasitic larvae then over-winters in the wall of the gut. When external temperatures increase, the larvae emerge and develop into adult worms. Their lifecycle continues with egg production, when conditions are right.

What causes PGE in sheep?

The main nematode responsible for PGE in sheep is Teladorsagia circumcincta. This behaves in a similar way to Ostertagia ostertagi, although in sheep the hypobiosis of the larvae occurs in the gastric glands.1

Another roundworm that contributes to PGE in sheep is Haemonchus contortus or the barbers pole worm. Disease is associated with the blood-feeding behaviour of the growing larvae and adult worms. Acute infection happens when animals take in a lot of the infective larvae. After about two weeks, young lambs quickly become anaemic, unthrifty, lethargic and weak.

Chronic infection is due to a more gradual intake of larvae and results in a general loss of condition, pallor and a reduction in growth rate. An infection of 5000 Haemonchus contortus can then result in an impressive loss of 250ml of blood per day.2

Control and prevention of PGE

PGE control and prevention is multifaceted, employing strategies such as: 

  • Effective quarantine procedures
  • Resting/rotational grazing
  • Alternating the species of animal on the pasture
  • Conducting regular faecal egg counts, alongside strategic worming treatments

What is liver fluke?

Liver fluke infections cost the UK farming industry around £300 million per year. This is as a result of animal death, or decreased productivity in chronically infected animals that may show no obvious signs of disease.

Liver fluke (Fasciola hepatica) can infect sheep, cattle, goats, horses, people and also many other mammals. It uses the mud snail Galba truncatula as the intermediate host. Infection of these snails occurs in the summer with the appearance of infective metacercariae on pasture from August to October. The parasite sometimes infects snails during the autumn, and over-winters in the hibernating snail. It then restarts its development when conditions are good in the spring. Metacercariae from these snails then appear on pasture in May to June.

Infected Liver fluke snail
Infected liver fluke snail (Galba truncatula)

What are the signs of liver fluke in ruminants?

Infection of the host leads to liver damage and haemorrhage. This is a result of the parasite migrating through the liver tissue, and damage when the adult fluke are in the bile ducts.

The severity of infections in the host depends on the number of metacercariae ingested. Acute fasciolosis results from ingestion of a large number of metacercariae during autumn and early winter. It can result in sudden death of the animal, although this less common.

Sub-acute fasciolosis also appears in autumn and winter. It occurs when animals ingest metacercariae over a longer period of time. It results in a marked loss of condition, severe anaemia, liver enlargement, swelling of the face and lower jaw (bottle jaw), and may eventually result in the death of untreated animals.

Chronic fasciolosis is the most common form of the disease and mainly occurs in late winter and early spring. It is a result of infection by immature free-living fluke from the previous autumn. Animals suffer from loss of appetite and condition, diarrhoea, anaemia and, eventually, bottle jaw.

Control and prevention of liver fluke

  • Management of the environment — for example, fencing off boggy areas where the mud snails are more likely to occur
  • Avoid grazing cattle and sheep together on the same pasture
  • Use faecal egg counts as a tool to monitor infections levels, and indicate when to treat
  • Use faecal egg count reduction tests to find out if resistant strains of liver fluke are present

Ridgeway Research’s parasitology laboratory routinely tests for a wide range of parasites, see our Services page for more information.


  1. Taylor, et al. (2007). Veterinary parasitology (3rd Edition). Oxford.
  2. Jackson & Coop. (2007). Diseases of Sheep (4th Edition). Wiley-Blackwell.