Parasite Forecast

Issue: November

Weather report

 

Figure 1: Temperature and rainfall by region for previous months. Egg count data shows the most recent counts for roundworms in sheep at each location between the sample dates stated.

The first few days of September saw rainfall across much of the country, becoming more settled and warm with particularly high temperatures around 19th-24th. The provisional UK mean temperature was 13.1 °C, 0.5 °C above the long-term average (1981-2010).  Regionally, temperatures were above the long-term average in September, and for the preceding 3 months (July-September) temperatures were around 0.5-1oC above average across all regions of the UK.

Rainfall was below average up until the 20th after which a more unsettled spell of weather set in, with 25-50 mm of rain in 24 hours reported in some areas. By the end of the month the UK had easily exceeded its September average rainfall at 127% of the long-term average. Regionally, rainfall was well above average across most of England and Wales with the exception of East Anglia. Rainfall in Scotland and Northern Ireland was more comparable to the long-term average for these regions in September, with rainfall in northern Scotland below the average amount expected for the month. Regional rainfall over the previous 3 months (July-September) was generally still above average for this time period in most UK regions with the exception of East Anglia and southeast England.

Provisional autumn fluke forecast

The provisional fluke forecast for autumn 2019 is based on temperature and rainfall from May – September. Due to the very high levels of rainfall and above average temperatures observed, this year’s forecast is currently predicting high risk in Scotland, northwest England & north Wales, moderate risk in Northern Ireland and low risk everywhere else (Figure 2).

Figure 2: Provisional 2019 UK autumn fluke risk forecast by region.

Liver fluke infection in livestock occurs through ingestion of grass and other herbage with infective “cyst” stages attached (Figure 3). These cysts are shed from mud snails that have been infected earlier in the grazing season. Both of these stages are highly dependent upon temperature and presence of water, with warm wet conditions optimal. It is therefore strongly advised that farmers with livestock grazing in high and medium risk regions are vigilant for signs of disease in the coming months, with cases of acute disease already confirmed in some areas (Figure 4).

Figure 3: Infective “cyst” stages of liver fluke (metacercariae) accumulate on “flukey” pastures as they are shed with increasing frequency from their snail intermediate hosts.

Whilst the autumn fluke forecast is predicting low risk in some parts of the UK, it should be noted that local conditions are also very important when determining on-farm fluke risk. Since temperatures have consistently been above average across most regions of the UK this grazing season, any farms with permanently wet pastures and/or permanent water bodies where mud snails may reside are likely to be at increased risk from liver fluke. This particularly applies to farms with a previous history of fluke infection. If in doubt, please seek veterinary advice.

Advised actions include:

  • Monitoring for signs of disease. Both sheep and cattle are susceptible to infection with liver fluke, although acute outbreaks are more common in sheep:
    • Sudden death in heavy infections (Figure 4)
    • General dullness, anaemia and shortness of breath
    • Rapid weight loss, fluid accumulation (e.g. bottle jaw)
  • Routine diagnostic testing to give a greater insight into current infection status:
    • Antibody ELISAs are useful in detecting infection in the early stages, and are available for sheep and cattle through blood sampling, or to monitor herd-level infection status in dairy cattle through testing bulk milk tank samples.
    • Faecal antigen testing and worm egg counts can also be used to diagnose infection in individuals, although it is important to note these may give a negative result in the early stages of infection.
    • Post-mortem of lost stock in acute outbreak allows for a definitive diagnosis (Figure 4).

Figure 4: A case of acute fasciolosis in a sheep diagnosed at post-mortem from Northumberland, 8th October 2019. Acute infections can result in severe liver damage with heavy blood loss and sudden death (Photo credit: Ben Strugnell, Farm Post Mortems).

  • Where acute disease occurs, treatment with triclabendazole is recommended as this is the only product effective against both adult and early immature stages of the parasite. However, due to growing concerns over drug resistance it is essential such treatments are carried out correctly:
    • Always ensure correct dosing by:
      • Following the manufacturer’s recommendations,
      • Checking equipment is correctly calibrated prior to use
      • Dosing by weight of the animal
    • It is also strongly advised such that any treatments given are accompanied by resistance testing at 21 days post-treatment to monitor efficacy.
    • Where drug failure is present or resistance suspected, please seek veterinary advice for alternative treatment options.
  • Risk of infection can be reduced by identifying high risk “flukey” pastures and avoiding grazing these during peak risk periods.
    • Mud snails are generally found in damp, muddy areas such as the borders of permanent water bodies, wet flushes (often identifiable through presence of rushes and other water loving plant species), ditches, boggy areas etc. (Figure 5).
    • Pastures previously grazed by fluke infected sheep should be considered a risk to cattle and vice versa.

Figure 5: Areas of permanently wet pasture can provide optimal conditions for mud snails, and by extension liver fluke. Large numbers of mud snails were recovered from the wet boggy areas pictured above, which were created by a constant supply of flowing surface water and poaching by cattle.

SHEEP

Sheep: Parasitic Gastroenteritis (PGE)

This year’s warm, wet grazing season is likely to have extended risk of PGE-causing roundworm infections including Trichostrongylus, Teladorsagia circumcincta and Haemonchus contortus later into the grazing season than would normally be expected by enabling larger than usual numbers of larvae to survive on pastures to the end of the season. Data from Parasite Watch from July – October 2019 still show medium and high egg counts in sheep flocks across Great Britain (Figure 1), with severe clinical disease still being detected in many areas (Figure 6).

 

 

Figure 6: A case of haemonchosis (barber’s pole worm) in a lamb diagnosed in northeast England, October 2019 (Photo credit: Katie Waine, Farm Post Mortems)

Specific PGE risks at this time include:

  • Issues with haemonchosis may persist in infected flocks: The prolific egg production of female worms means pastures can become contaminated rapidly under optimal conditions.
  • Nematodirosis can also be observed at this time due to an autumn egg hatch.
  • Outbreaks resulting from trichostrongylosis can commonly be seen through the winter months in store and replacement lambs and, sometimes, yearlings.
  • As the weather becomes colder, some newly acquired larvae will begin to encyst in the stomach wall, resuming development in the spring if left untreated. It should be noted that in such infections FECs may be negative.
  • Consider the infection status of rams post breeding. The combination of a high workload and parasite burdens may contribute to a significant loss in body condition in these animals just ahead of winter (Figure 7).

Figure 7: Rams are generally more susceptible to roundworm infections than ewes. High parasite burdens in combination with the high workload during tupping can have a profound impact on health and body condition ahead of the winter months.

Advised actions include:

  • Monitoring for signs of disease.
    • Young animals and rams are at greatest risk from infection with Teladorsagia, Trichostrongylus and Nematodirus
    • Haemonchus infections can cause disease in animals of all ages. Signs of chronic haemonchosis can be very similar to liver fluke infection. If you are in any doubt over which parasite is causing disease, please seek veterinary advice.
  • Consider worm egg counts if infection status is unknown
  • Where anthelmintic treatments are required:
    • Move to safe pasture (eg. silage aftermath) if available.
      • Avoid dosing with long-acting group 3-ML products
      • Leave treated animals on dirty pasture for 2-3 days prior to moving.
      • Aim to leave at least 10% of the flock untreated
    • If anthelmintic treatments are administered, it is advised to check efficacy through worm egg counts:
      • Re-test 10-12 individuals at 7-14 days post treatment depending upon the product used.
      • If anthelmintic resistance is suspected, strategic use of group 4-AD or 5-SI wormers may be indicated under veterinary guidance. For more information see the SCOPS guidelines, and seek veterinary advice.

CATTLE

Cattle: Lungworm

Lungworm infection can continue to be a risk into November. Outbreaks are difficult to predict, but may be associated with wetter summers and directly following periods of rainfall. High-risk animals include unvaccinated calves in their first season and cattle bought-in from farms with no history of the disease. Disease onset can be rapid and severe, with early signs including widespread coughing in the group, initially after exercise then at rest, increased respiratory rate and difficulty breathing, rapid loss of weight and body condition, milk drop in lactating cattle and death in heavy infections.

Advised actions include:

  • Monitoring for signs of disease
  • If infection status is in doubt, consider diagnostics:
    • Faecal samples to detect lungworm larvae
    • Post-mortems (Figure 8)
    • Antibody ELISA testing
  • In the event of an outbreak of clinical disease:
    • Treat all animals within the affected group
    • Remove affected animals from contaminated pasture to safe grazing (e.g. silage aftermath), or house in a well-ventilated building.
  • In severe cases affected animals may require additional treatments (e.g. anti-inflammatories and antibiotics).
  • For more information, discuss this with your vet or SQP, see the COWS group guidelines and NADIS lungworm webinar.

Figure 8: Adult lungworm are easily identifiable at post-mortem. Here, they can be seen blocking the airways of a 4-6 month-old calf from Cheshire, September 2019 (Photo credit: Ann Courtenay, University of Liverpool).

Dosing at housing

Housing presents both opportunities and challenges for control of important parasitic disease on farm.

Worm burdens acquired over the grazing season may be targeted effectively at this time without risk of animals becoming re-infected, with the added benefit of improved feed efficiency and growth rates. As is the case during the grazing season, any treatments administered should be based on evidence of infection, such as diagnostic testing (e.g. worm egg counts), grazing and management history.

  • For growing cattle housed after their first or second grazing season, treatment with products containing either a Group 3-ML or Group 1-BZ anthelmintic is recommended at housing. These products are effective against encysted stage larvae acquired in the latter stages of the grazing season. If untreated, heavy burdens of encysted larvae can cause type-2 ostertagiosis later into the housing period due to triggered mass emergence. It is important to note that encysted worm burdens cannot be assessed by worm egg count.
  • Whilst housing prevents further infection with pasture-associated parasites, risk of louse and mite infestations may increase at this time (Figure 9). These parasites can spread directly from animal to animal very easily once they are penned in close proximity to one another in what are relatively dry, warm conditions. It is worth bearing this in mind when selecting worming treatments at this time of year: In addition to their activity against gut worms selected products (some injectable and spot-on, pour-on group 3-ML preparations) are also effective against lice. For more information, please speak to your vet or SQP.
  • Provided animals are not suffering any obvious effects, cattle exposed to liver fluke infection during the grazing season can be dosed during winter housing with a product other than triclabendazole. This allows triclabendazole to be used more sparingly, reducing selection for resistance and preserving its efficacy on farm for when it is most needed.
    • Due to their lower activity against juvenile stages of liver fluke, if using a product other than triclabendazole at housing it is necessary to either repeat or delay treatment to ensure all flukes are treated effectively. For example, if treating with closantel COWS recommend delaying treatment for 6-7 weeks post-housing before treating.
    • Some of these alternative products, such as albendazole,  oxyclozanide also have the added benefit of being licenced for use in lactating animals provided milk withhold periods are observed. It is important to check labelling of individual products.
  • In all cases of treatment at or during housing, efficacy testing should also be considered since ineffective treatments may result in a residual burden of potentially resistant parasites which will contribute to pasture contamination at turn-out the following grazing season.
  • For more information please speak to your vet, or visit the COWS website.

 

Figure 9: Pour-on preparations of 3-ML wormers are also effective in controlling sucking and chewing lice.

 

John Graham-Brown BVSc (hons) MSc (hons) PhD MRCVS
November 2019

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