Physical signs
Almost all grazing animals have internal parasites. They often don’t cause clinical disease and stock can look normal or even in good condition. However, if animals ingest large numbers of worm larvae and infections get out of control, they can suffer from both subclinical disease (signs that can be measured but not seen visually) or clinical disease (e.g., scours, weight loss or even death).
Factors such as animal species, age, farming systems, pasture availability, and seasonality all contribute to the varying impacts of parasites on livestock.
A sheep with a zero worm burden has no compromise to its production, wool, lamb or meat as a result. There is an impact on production from the lowest worm burden through reduced appetite, refocus of immune system to the parasite, damage to the gastrointestinal system and potential protein and blood loss.
The visual effects (clinical signs) are apparent when the worm burden becomes high and there is substantial reduction in production and a negative impact on the health of the animal. It is the non-visual (sub-clinical) effects which accounts for the majority of production loss in a sheep enterprise as it can go un-noticed over a long period of time.
Basic worm lifecycle
Step: 1
All roundworms begin as eggs. These eggs are produced by female adult roundworms living in the gut of the animal.
Step: 2
These eggs are passed out in the animal’s dung onto the ground.
Step: 3
The eggs hatch into larvae, given the right environmental conditions (sufficient moisture levels and temperature).
Step: 4
The larvae develop and become infective. They move up blades of grass in water droplets, where they can be eaten by the grazing animal.
Step: 5
Once ingested, the larvae enter the gut of the animal and either develop into adults quickly, or they may stay in an “inhibited” stage until a later time (for example, during a summer drought or winter cold). When the external conditions are favourable for worm survival, the inhibited larvae develop into adults.
Main parasites affecting sheep
Trichostrongylus, Teladorsagia, Cooperia, Nematodirus
Haemonchus contortus (Barbers pole)
Parasites are a major contributor to poor performance in young stock, second only to poor feed quality1.
In New Zealand we invariably see mixed infections of parasites in animals. However, at certain times of the year and under certain conditions some parasite species can predominate. Haemonchus in summer and Trichostrongylus in winter can be examples of this. The body’s immune response to the presence of the adult worms and L4 larvae leads to the outpouring of fluid and protein into the gastrointestinal tract, and results in the clinical signs of scouring and loss of weight.
Haemonchus is not regarded as a scour worm since both the L4 larvae and the adult parasites suck blood and can cause severe anaemia and even death of lambs before any scouring is seen. Haemonchus are prolific egg producers, and under the right conditions numbers can build up quickly on pasture. If numbers continue to rise, it does not take long for severe impacts to occur.
Main parasites affecting cattle
Ostertagia ostertagi
This is the most important parasite affecting cattle in New Zealand2. Ostertagia live in the abomasum (4th stomach) and affect calves and young stock, and can even cause clinical parasitism in adult cattle. They also have the ability to hibernate (become inhibited) over winter and emerge en masse in the spring, causing massive gut disturbances resulting in death. This syndrome is known as Type II Ostertagiosis.
Immunity takes 18 months or more to develop to this parasite. There is significant drench resistance among Ostertagia to white (benzimidazole), clear (levamisole) and now emerging resistance to mectin (macrocyclic lactone) drenches as well as combinations of these.
Trichostrongylus axei
These parasites also live in the abomasum and are regarded as less pathogenic than Ostertagia, however mixed infections with Ostertagia are fairly common and are likely to compound the effects on the animal.
Cooperia oncophora
This is mainly a parasite of younger cattle, affecting those less than 12 months of age. Immunity develops early on and readily to this parasite. Cooperia produce many eggs (1000s a day vs the few hundred that Ostertagia produce) so contaminated pastures and infections with this parasite can build up readily. It is still reasonably sensitive to levamisole, however there is widespread resistance to the mectins and white drenches.
Dictyocaulus viviparus
This is also mainly a parasite of young stock, however older animals whose immunity has waned due to lack of exposure can also be affected. Lungworm are very large parasites (~10cm) and can physically block the trachea and lower airways preventing the animal from breathing. Coughing is an early sign as they try to clear mucus and parasites from their lungs, but animals may also be found dead. Lungworms are still susceptible to the mectin family.
Liver Fluke
Liver fluke are flat, leech-looking parasites that feed on blood and bile in the liver. They damage the liver and cause protein losses that depending on the severity of the infection, can come on acutely or be more chronic. They are not controlled by the standard round worm drench actives, so need to be targeted with actives specific to liver fluke. They have a long, complicated lifecycle and need still or slow-moving water and a certain type of water snail (Lymnaeid snails) to complete their lifecycle. Consequently, unlike round worm parasites, liver fluke are not on every farm.
Lice
Cattle can also be afflicted by biting and/or sucking lice. Generally, lice do not cause severe disease and are more of an annoyance. A heavy infection will make cattle rub on gates and fences which may cause damage. One of the sucking lice Haematopinus eurysternus, can cause anaemia in calves but this is very rare and needs to be a very heavy infection. Dairy breeds tend to have more sucking lice and beef breeds tend to have more biting lice but they can have either or both.
Lice numbers build up slowly over winter when they are protected by the longer winter coat and humidity. Numbers peak in Spring then will tail off again as animal immunity develops and winter coats are shed. In general, if cattle are in good condition and well fed, lice should not be an issue and they should not need routine treatment3.
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Effective management
Every farm is unique, and effective worm management needs to be tailored to take this into account.
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Drenching best practices
If drenching is to be effective, a drench must be given correctly. Check out our tips for best practice drench administration.
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Sheep drenches and drench usage
There are a variety of drench options available for sheep. Understanding how and when we use them is important.
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Pre-lamb management
Pre-lambing is a crucial time in the Ewe’s production cycle, so giving them the be start to lambing is essential.
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Cattle drenches and drench usage
There are a variety of drench options available for cattle. Understanding how and when we use them is important.
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Drench products
When it’s time to reach for the drench gun, Zoetis provide a range of solutions. Check to see if these will suit your farm.
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