Parasites at grass

Worm burdens on pasture are at their highest in the second half of the grazing season, so be vigilant, warns Rebecca Carroll, programme manager with Animal Health Ireland.

The careful monitoring of stock and grazing management to reduce exposure of vulnerable cattle to parasites and judicious use of anthelmintics are all vital to manage parasites at grass.

The main parasites of importance during the grazing season are gut worms, lungworms and liver fluke. Gut worms are present on all cattle farms in Ireland; lungworms and liver fluke are present on the majority of farms, with the prevalence of the latter highest in the west.

Gut worms There are over 20 species of gut worms that affect cattle, but only two important species are commonly encountered in Ireland – a stomach worm (Ostertagia) and one that lives in the small. intestines (Cooperia).

Parasitic gastroenteritis

Infection with these worms causes the condition known as parasitic gastroenteritis (PGE). Dairy calves in their first grazing season are at greatest risk from severe gut worm infection and can experience serious clinical disease with signs such as poor appetite, diarrhoea and reduced weight gain.

Gut worms can also cause loss of performance in older cattle including reduced milk yields, lower conception rates, delayed onset of puberty and decreased daily liveweight gain (DLWG). Monitoring calves for gut worm infection depends on:

  • Regular weighing of animals
  • Regular dung sampling for parasite eggs (faecal egg count (FEC))

Research has shown that the rate of DLWG of calves at grass is correlated with their level of exposure to parasitic worms (assuming that there are no other obvious causes of poor growth such as inadequate grazing).

Ideally use scales to monitor weight gain but alternatives such as weigh bands are useful. Use of FECs should be discussed with the farm’s veterinary practitioner.

But, in general, groups of animals with an average of more than 200 gut worm eggs per gram (epg) in the first half of the grazing season warrant treatment as high levels of pasture contamination and consequent poor growth is possible. Lungworm or hoose is one of the most important respiratory diseases of cattle in Ireland.

Clinical disease is most commonly seen in first season grazing calves but can also be seen in older cattle who have failed to develop or maintain immunity or cattle with partial immunity that are exposed to a high lungworm challenge.

The clinical signs of infection are coughing and difficulty in breathing (especially when animals are being moved). It is also important to note that animals can acquire a dangerous level of infection even after only one day of grazing a contaminated pasture.

Close monitoring of all cattle at grass for respiratory disease, particularly coughing, is the best approach for the detection of lungworm infection.

Grass parasites

Lungworm larvae may be found in faecal samples, but deaths from acute hoose can occur before larvae appear in dung. If coughing is detected in a group of cattle at grass and hoose is suspected, all cattle in the group should be dosed.

Cattle should be moved to a ‘clean’ pasture or an anthelmintic with persistent action may be used to prevent re-infection. As silage aftergrass becomes available, this ground, which is a lower risk for parasites, should be used for youngstock to reduce their exposure to gut worms and lungworms.

Liver fluke is a flatworm that affects cattle of all ages, including cows. Following ingestion, the parasite penetrates the gut wall and moves to the liver where it can cause significant damage.

Clinical signs in cattle include reduced appetite, failure to thrive, lower milk yield and poor fertility, but fluke rarely causes fatalities in cattle.

Liver fluke has an intermediate host, the mud snail, in which it completes part of its lifecycle; as its name suggests, the snail is generally found in wet, muddy habitats.

Fluke burdens can be monitored by using a combination of dung samples, bulk milk antibody tests and the information provided from meat factories through Animal Health Ireland’s “Beef HealthCheck” programme, which provides a detailed report for each animal slaughtered of the results of liver and lung findings at post mortem inspection.

Farmers can reduce the burden of liver fluke on pastures by improving drainage and fencing off wetland where possible.

Preventative measures

Measures to reduce poaching will also reduce snail habitats on farms. A strategic fluke dose in early to mid-summer, designed to reduce the number of eggs being shed onto pasture, can be useful on farms with a high fluke risk.

However, it should be noted that there are no flukicides with a zero-day milk withdrawal on the market in Ireland, so this option is not available for dairy cows.

The necessity of dosing groups of cattle (apart from dairy cows) for fluke at grass will vary depending on the farm fluke risk and rainfall. Farmers can assess their farms' fluke risk by looking at:

  • the suitability of the land for snail habitats – wet farms are at the highest risk
  • the history of clinical cases of fluke on the farm
  • previous FEC results and liver results from meat factories
  • weather; mild wet weather increases the risk of fluke
  • fluke forecasts

Table 1 provides a useful risk assessment guide but final decisions on a parasite control programme should be reached in conjunction with the farm’s veterinary practitioner.

Risk assessment matrix for disease and production losses from parasites in cattle

Risk factorHighMediumLow
Age (grazing seasons, GS)<1 year (first GS) #1-2 years (second GS)>2 years (adult) ##
Weight gain (<2 years old) 2 months after turn out<0.7 kg/day0.7-0.8 kg/day0.8 kg/day
Faecal worm egg counts (FGS) ### 2 months after turnout (epg)>20050-200<50
Herbage mass kg DM/ha<1,0001,000-2,000>2,000
Sward height<4 cm4-8 cm>8 cm
Field typePermanent PastureSilage / hay after grassNewly sown, ungrazed leys
Grazing history within the last yearGrazed by cattle <1 year oldGrazed by cattle 1-2 years oldGrazed by adult cows, sheep #### or other species
Condition score (adults)<2.02.0-3.0>3.0
Bulk milk tank O.ostertagi antibodies (ODR) (dairy herd)>0.80.5-0.8<0.5
Snail habitats Faecal fluke head count (epg)Widespread > 20Patchy 1-20Fenced off / none 0
Fasciolosis diagnosedPrevious year>5 years previouslyNever
Lungworm diagnosedPrevious year>5 years previouslyNever

# beef suckler calves are at low risk of PGE before weaning; ## adult cattle rarely suffer from clinical PGE, but are susceptible to lungworm (if immunity low) and to liver fluke; ### weaned dairy calves; #### if sheep are infected with liver fluke, they can increase the risk of liver fluke in cattle.

Detailed information on gutworms, lungworm, liverfluke and other more occasional parasites affecting cattle at grass can be found in Animal Health Ireland’s parasite control leaflets at:

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