In intensive farming systems, roundworm (nematode) control needs to be applied to avoid infection levels that affect production or cause disease. More often than not, infection is subclinical, with subtle effects on liveweight gain and/or milk yield. In animals with high burdens due to a lack of, or inappropriate, control measures, disease can be seen.
For many years, the regular application of broad-spectrum anthelmintics underpinned control programmes. This has led to drug resistance. Reports of resistance in cattle nematodes are increasing worldwide. It is commonly detected in the intestinal worm species, Cooperia oncophora, but there are also reports of drug resistance in the pathogenic worm, Ostertagia ostertagi, including in the UK. With new dewormer actives still on the horizon for cattle, once effectiveness of the current products is lost, significant worm-related losses could become commonplace.
To reduce selection pressure for resistance, evidence-based approaches to control need to be taken. These require a balance between treatments to reduce infection and minimise production loss, whilst preserving efficacy. Variation in worm prevalence and farm management means that plans should be designed on a case-by-case basis. Effective plans require a good knowledge of parasite epidemiology and farm history, supported by diagnostic tests to help build a picture of the parasites present, which animals are most susceptible to infection and, ideally, anthelmintic sensitivity of the resident worms.
Diagnostic-led targeted treatments can be used during the grazing season, with all-group strategic treatments to target worm stages not detectable by faecal egg count (FEC) tests. Diagnostic-informed treatments are best utilised in summer for nematode control, along with grazing strategies that break the worm life cycle to reduce challenge.
For nematodes, first and second grazing-season cattle should be the main priority in control plans. In the first season, disease risk depends on birth date and whether or not calves graze with their dam. At highest risk are weaned calves on pasture grazed by cattle in the previous 12 months. In these high-risk groups, all-group treatments can be administered strategically from three weeks after turn out, in the first half of the season, to reduce larval contamination. Frequency of applied treatments will depend on the level of “persistence” of the formulation used.
All-group treatments provide a strong selection pressure for anthelmintic resistance. For this reason, and where calves are at lower risk of disease, ie when grazing clean pasture or spring calves grazing with their dams, decisions to treat should be based on growth monitoring and FEC testing.
Liveweight gain is a very effective indicator of infection level when there is sufficient nutrition and no other disease, so routine growth monitoring is a valuable tool when assessing whether or not to treat. This can be supported by FEC testing to monitor contamination or effectiveness of treatment.
Faecal egg count (FEC) testing
From late spring to summer these tests can be used, especially in calves, to assess nematode contamination onto pasture to guide management and treatment decisions. Nematode egg shedding does not correlate well with burden or liveweight gain in young cattle. FEC tests measure egg shedding to provide information on:
- Worm infection dynamics and contamination across a season
- The differential diagnosis of diarrhoea and/or weight loss in grazing cattle
- Identifying animals that contribute most to contamination for selective treatments to reduce egg shedding within groups
- Estimating the effectiveness of dewormers
The FEC test gives an approximate value for egg numbers shed and can be used to estimate contamination, eg a 200kg calf with a 200 egg per gram (EPG) FEC that deposits 5 percent of its liveweight in dung each day will excrete approximately 2,000,000 eggs/day.
The test is also a useful tool for informing treatment decisions in dairy cows year-round to mitigate the production effects of subclinical infections. Blanket use of anthelmintics in all milking cows is not recommended as not all cows have a burden requiring treatment.
Nematode eggs are not evenly distributed in dung so representative samples from across a pat must be obtained – take a heaped dessertspoon’s worth in three to four sub-samples from across the pile, and mix well. The sample must be kept cool and the container airtight to prevent egg hatching, which will lead to underestimation of counts. The Control of Worms Sustainably (COWS) group has recently published a leaflet on how to carry out FECs, which can be viewed here.
Dung should be mixed well at the lab before taking a sub-sample for counting. It is good to use a sensitive method to reduce variability. Methods that apply a large multiplication factor, ie X100, lead to imprecise results and are not recommended for efficacy testing.
Several tests are available with good sensitivity: FLOTAC (sensitive to 5 EPG) and FECPAKG2 (sensitive to 20 EPG). These are useful for testing older cattle, which often have low FEC.
FEC tests have no value in detecting pre-patent infection and therefore are of no use in diagnosing early parasitic gastroenteritis.
Testing dewormer efficacy
It is also important to test effectiveness of dewormers. The FEC reduction test (FECRT) is used for this purpose. As a “look-see”, dung samples can be taken one to two weeks post-treatment from 10 animals/group to provide a guide as to a treatment’s effectiveness.
If this test proves positive, a more detailed FECRT should be performed. This involves testing individual samples from as many of a group as possible (15 or more). Dewormer should be administered at 100 to 110 percent dose rate and FEC analysed for each calf at treatment and 10 to 17 days later, depending on the anthelmintic used (10 days after benzimidazole, 14 to 17 days after levamisole or macrocyclic lactones).
The mean percentage egg count reduction is obtained by comparing day of treatment FEC and post-treatment FEC. Values below 90 percent (benzimidazole, levamisole) or 95 percent (macrocyclic lactones) indicate resistance may be an issue.
Before the start of the grazing season, a worm control plan should be drawn up so that anthelmintics are used as needed. To reduce selection pressure for resistance, decisions should be based on epidemiology, history, season, weather and type and age of the stock and guided by diagnostic tests. The risk of helminth-associated disease can be reduced by avoiding contaminated grazing in high-risk animals and by using mixed or alternate grazing with sheep.