Problem parasites at housing – plan, plan, plan

Parasite control is an essential part of any herd-health plan, ideally drawn up with your vet while considering all the health issues on the farm such as vaccinations, fertility and nutrition. It shouldn’t be set in stone though, and factors such as weather conditions should make you rethink the details as you go through the year. For example, this year saw a dry early summer, so very little dosing was probably needed until the heavy rains and warm weather set in. This late summer and autumn weather has provided an ideal environment for worms and flukes to thrive, and so it is expected that many farmers would have had problems with scour from gut worms, and coughing from lungworms (hoose). 

Liver fluke

The autumn is when animals are most likely to become infected with liver fluke. The parasite completes its life stage in the snail host over the summer and the infective stage attaches to the grass near waterlogged areas. These areas should be avoided for grazing where possible in autumn. After infection, it can take up to 12 weeks for the parasite to move from the gut to the liver and fully develop into adults where they will start laying eggs, which can be detected in a faecal sample. If animals are treated for liver fluke before housing, they may need a second treatment a number of weeks after housing. Also, some flukicides are only effective at killing adult liver fluke and so a re-treatment may be needed if animals are treated with a drug that is not effective against juvenile fluke.

Each farm has a different risk for liver fluke so discuss any treatment plan with your veterinary practitioner, taking the farm history into account. Previous liver fluke reports from the Beef HealthCheck programme for animals, both beef and dairy, sent to slaughter at participating factories are available online on the desktop version of the ICBF website
(bit.ly/3Rzo1iR).

Rumen (stomach) fluke

Infection with rumen fluke does not usually cause clinical disease, and adult rumen fluke seem to be tolerated fairly well. If rumen fluke eggs are detected on a faecal egg sample and the animals are healthy, they do not usually require treatment. It is the immature stage of the rumen fluke that usually causes disease, particularly if many of the parasites were picked up in late autumn. Similar to liver fluke, avoid grazing younger animals on wet or poached land. As there is only one product available in Ireland for treatment for rumen fluke and its use is considered ‘off-label’, always discuss whether treatment is necessary with your veterinary practitioner.

Lice and mites

In winter housing, the longer hair coats, warmth, humidity, and lack of UV light create a good environment for lice and mites to thrive. Closer contact between animals at housing allow lice and mites to spread easily between animals. Therefore, all animals in a group should be treated at the same time to prevent reinfection from untreated animals. Lice and mites are normally treated with an avermectin (clear drench) injection, pour-on product, or a topical pyrethroid. These medicines have no effect against lice eggs and once these hatch three to four weeks later, an animal may become re-infected. A re-treatment later in the season may also be needed if the burden of infection is very high. If a treatment does not seem to be effective, consult with a veterinary practitioner to identify the parasite involved and develop a tailored treatment plan. For example, injectable products seem to be less effective against chewing/biting lice, so a pour-on product might be a better choice if these are present. In contrast, mites usually respond better to injectable products.

Roundworms

Stomach and gut worms, as well as lungworms (collectively known as roundworms) can usually be controlled by any of the three main wormer classes. The exception to this is the larvae of the stomach worm ostertagia, which become dormant in the stomach wall over the winter period and can cause scours in the late winter/early spring (Feb-May) when they re-start their development. This particularly affects young animals that may have picked up large numbers of these worm larvae over the autumn. Products containing levamisole (yellow drench) are not effective against these inhibited larvae; instead, use a product from the clear drenches (macrocyclic lactones) or certain white drenches (benzimidazoles).

Ensure that you check the withdrawal periods for any medicines used in dairy animals, to avoid residues in the milk for the next lactation.

Resistance and the long-term viability of wormers

Anthelmintic (wormer) resistance is making us rethink our approach to dosing animals. Resistance is considered present on a farm when a product does not kill the target parasites as effectively as expected. This resistance is carried genetically in the worm populations and is not related to the cattle themselves. There are only three classes of broad-spectrum medicines used to treat gut worms for cattle. Once resistance develops for one medicine in the class, resistance is present against all the medicines in that class. This is why it is important to test if the wormers used on farm are effective. Treating too often, especially when a small number of worm eggs or larvae are expected on the pastures, can drive the development of resistance but that has to be balanced against treating when it is necessary for the health of the animal.