Trichostrongylus, Ostertagia, Cooperia and Nematodirus species are the main nematodes that cause parasitic gastroenteritis (PGE) in susceptible cattle in the British Isles. PGE causes production losses and, if treatment is not timely, severe disease and death. Every grazing animal will be exposed to nematodes; therefore, it is important to prevent severe infestations.
The introduction of anthelmintics in the middle of the 20th century (Holden-Dye and Walker, 2014) revolutionised the treatment of PGE. But unfortunately, anthelmintic resistance is a worldwide issue. Resistance to anthelmintics is widespread and increasing as not many different groups of anthelmintic products are available. To preserve the efficacy of available products it is essential that farmers, veterinarians and the pharmaceutical industry work together to mitigate anthelmintic resistance.
To preserve the efficacy of available products it is essential that farmers, veterinarians and the pharmaceutical industry work together to mitigate anthelmintic resistance
Trichostrongylids (the collective name for the most common causes of PGE) can either cause disease in the abomasum or small intestine. Trichostrongylus and Ostertagia spp are present in the abomasum and Cooperia and Nematodirus spp are found in the small intestine. All these nematodes have direct life cycles, which means no intermediate host is needed to cause infection. After ingestion, the infective larvae either enter the gastric glands of the abomasum or the crypts of the small intestine. The prepatent period (the period from ingesting infective larvae to the appearance of egg-laying females) for the trichostrongylids is generally three weeks when hypobiosis (arrest in development) does not take place (Radostits et al., 2007).
PGE can take hold when cattle are not resistant or resilient enough to withstand the number of infective larvae ingested. It causes diarrhoea and weight and muscle loss, caused by the depletion of proteins by the nematodes. PGE in a herd of youngstock initially presents as reduced growth rates throughout the whole group. Some of the affected individuals can show profuse diarrhoea, but it is important to bear in mind that it is not unusual that only a few individuals are affected by diarrhoea.
Parasitic gastroenteritis can take hold when cattle are not resistant or resilient enough to withstand the number of infective larvae ingested
PGE is typically seen in youngstock that is grazing pasture for the first or second season. Some trichostrongylids create faster immunity in cattle than others. The development of immunity also depends on the worm burden of individual animals. Age resistance is also important (older animals cope better than younger ones). Resilience mostly depends on nutritional status, although well-fed animals can also be overwhelmed by massive infestations if the exposure is high enough (Radostits et al., 2007).
Presence of nematodes and faecal egg counts
As reduction in growth is one of the main clinical signs of PGE, it is crucial that worm burdens are assessed by faecal egg counts during the grazing season. A regular faecal egg count, carried out when the worms lay eggs in their host, provides crucial information on the extent of the worm burden. Important factors for interpreting the results are:
- Knowing the possible length of exposure to nematodes. Consider factors such as when the animals go out to graze and the farmer’s pasture management
- The time of year
- The weather conditions in the last month
As reduction in growth is one of the main clinical signs of PGE, it is crucial that worm burdens are assessed by faecal egg counts during the grazing season
When sampling for groups of cattle, it is important that faecal egg counts are carried out on a good subset of faecal samples to ensure that the results can be interpreted correctly. Control Of Worms Sustainably (COWS), a voluntary initiative with a clear website, provides some robust sampling guidelines (COWS, 2020).
The contamination of a pasture with nematodes depends on the time of the year and the previous presence of stock. Fields not stocked in the winter will be a lot cleaner in spring than pastures that are grazed all year round. Frosty dry winters also reduce contamination, but worms can survive mild wet winters. Harvesting the grass for feed is a good way to reduce the level of contamination of a pasture.
Development of resistance
Worm resistance to anthelmintics develops by exposure to anthelmintic treatment – every anthelmintic treatment increases resistance. Parasitic nematodes have many biological and genetic features that favour the development of drug resistance. Short life cycles, high reproductive rates, rapid rates of evolution and extremely large population sizes combine to give many parasitic worms an exceptionally high level of genetic diversity (Anderson et al., 1998). Implementation of refugia can reduce the speed at which anthelmintic resistance develops.
Anthelmintic resistance spreads more readily when the effectivity of an anthelmintic is reduced, when a product is used frequently and when the principles of refugia are not used
Refugia means that a part of the nematode population (both free-living and in the host) is not exposed to anthelmintics. By ensuring this, less selection for anthelmintic resistance takes place. Anthelmintic resistance spreads more readily when the effectivity of an anthelmintic is reduced, when a product is used frequently and when the principles of refugia are not used. As resistance develops slowly, it will take time before it becomes noticeable. When resistance shows as a clinical problem, it often is irreversible (Greer et al., 2020; Hildreth and McKenzie, 2020; Kaplan, 2020).
The principles of refugia
There are several strategies to ensure that the number of nematodes in refugia is sufficient to slow down the development of resistance appropriately. First of all, groups of cattle only need worming when parasitism affects their growth rate. It is important not to have the intervals of anthelmintic treatment too short. If clinical signs and faecal egg counts indicate that treatment for most of the group is needed every six weeks, then pasture measurement or housing is necessary to reduce the treatment interval.
Secondly, leaving a part of the herd untreated is a good way to ensure nematodes on pasture are in refugia. This can be done by only treating a few individual animals suffering from parasitism or by leaving 10 percent of the herd untreated. Measuring average daily gain during the grazing season can help you decide if youngstock have heavy infestations. To be able to interpret the result, you must take into consideration the feeding levels of the group as well.
Leaving a part of the herd untreated is a good way to ensure nematodes on pasture are in refugia
To reduce the number of trichostrongylids surviving treatment, the anthelmintic used needs to be highly effective. This will reduce the number of resistant larvae deposited on the pasture. Efficacy can be tested by faecal egg count reduction tests (FECRT) (Kaplan, 2020). FECRT involves taking samples before and after anthelmintic treatment and comparing the results. The interval between samples depends on the wormer used.
Lastly, to improve the efficacy of treatment, two different anthelmintic products can be used in combination. This principle is already used in available combination products, but it is possible to use two separate products at the same time, although not in the same syringe (Kaplan, 2020). This is a strategy that needs careful consideration and regular faecal egg counts are needed to assess worm burden.