By Roelof Bezuidenhout
The cost of wireworm management in South Africa is estimated to be more than R600 million per year. And that’s for only one species of internal parasite.
Parasite management can range from straightforward to nearly impossible, depending on various factors, including region, farming system, type of livestock and climatic conditions. Some farms are heavily infested with parasites and diseases, whereas others are almost free of them. Much depends on how resistant the animals are to worms – and on how resistant the worms are to treatments. As a result, veterinarians won’t readily offer specific advice without knowing the exact parasite species and farm conditions. Incorrect advice can lead to disaster.
Wireworm is a small red blood-sucking parasite that pierces the lining of the abomasum (the sheep’s fourth, or “true” stomach), causing blood and protein loss. Sheep become infected when they ingest infective larvae while grazing. Once inside the stomach, the larvae mature into adult worms that lay eggs in the digestive tract. These eggs are then excreted onto the pasture through the sheep’s droppings, where they hatch and develop into infective larvae, continuing the cycle.
The infestation persists unless sheep are treated with an effective dewormer (anthelmintic) and moved to clean pasture. Genetics also play a crucial role in wireworm resistance, as some sheep are naturally more susceptible to infection than others.
Other common livestock parasites are tapeworm, nasal worm, brown stomach worm and liver fluke, to name just a few.
Natural resistance
Farmers often have a general idea of the common parasites in their area, although new parasites can be introduced when animals are brought in from outside. This highlights the importance of biosecurity vigilance.
Have you heard the saying that a healthy dog should have a few fleas? The same principle applies to livestock and worms. Livestock should not be entirely worm-free as some exposure helps them build natural resistance.
Farmers can actively select for worm-resistant animals by avoiding unnecessary dosing. Overusing anthelmintics can weaken natural resistance. Personally, I have managed to do this by deworming only lambs – sometime just once, and not even all of them. I haven’t treated an adult sheep or goat in years.
A wise approach is to cull animals that lack resistance to parasites. Ideally, over time, you should select for animals with natural resistance or tolerance. This process will take time, but it can improve the herd’s overall health in the long term.
The issue is not as straightforward for every farmer, particularly in high-rainfall areas with lush pastures and dense stocking rates. But even stock remedy companies now advocate for judicious treatment rather than the indiscriminate machine-gun approach some farmers use – treating every animal equally and dosing the entire flock or herd.
Regardless of your approach, always remember to administer a pulpy kidney vaccination at least 10 days before deworming.
How to decide which animals to treat
Every flock or herd has animals that can tolerate a light parasite load without ill effects. It makes sense, then, to treat only those that genuinely need it – a practice known as selective targeted treatment.
How do you determine which animals to treat and which to leave untreated? You can use individual worm egg counts from the dung, assess an animal for anaemia using a clinical evaluation such as FAMACHA scoring (checking the colour of the mucus membranes inside the lower eyelid), or simply observe the body condition of the animal. Some animals may appear less healthy or fail to thrive as well as others.
Bottle jaw – a swelling under the lower jaw due to fluid accumulation – is a clear and typical sign of wireworm (Haemonchus contortus). However, other nematodes can also cause anaemia without severe symptoms or high worm egg counts.
You can choose to treat the entire herd or focus on individual animals that show signs of suffering. Always be extra vigilant under abnormal conditions, such as increased rainfall and heat, or sudden changes in nutrition. These factors can stress animals, making them more vulnerable to parasite infestations.
Don’t overdose, but don’t underdose either
When dosing, be careful not to underdose, as this allows some parasites to survive and develop resistance to the treatment. To prevent this, it’s essential to rotate anthelmintics rather than using the same active ingredient every time. On the other hand, switching anthelmintics too frequently can contribute to resistance building up across all available treatments.
When it comes to rotating active ingredients, there is a theory that suggests you could persevere with one anthelmintic until it no longer appears effective, then switch to another and doing the same. And after that you use a combination of the two for the next round of treatment. However, this approach will require expert guidance.
Resistance to an anthelmintic can be measured by conducting a worm egg count 14 days after treatment. That said, worm egg count analysis is not necessarily the best indicator of when to dose. While the counts provide insight into how contaminated your pasture is, they don’t directly reflect the parasite burden in individual animals. Regular counts can, however, help you develop a strategy to manage worm levels in the pasture more effectively.
You can also use dung samples to identify and cull animals with high parasite loads. Additionally, egg counts can help assess the effectiveness of an anthelmintic – an effective treatment should reduce the egg count to zero, with no reappearance of eggs within three weeks.
Keep in mind that goats have a different metabolism than sheep and cattle, which means they may require a dose twice as high as the recommended amount per body mass. For example, if the label on the container indicates 4 ml/25 kg, goats should receive 8 ml/25 kg.
Rotational grazing
Incorporating clean grazing in your rotational system can also help control parasites. For this method to be effective, the new grazing area must contain a certain population of parasites that have not been exposed to the anthelmintic currently in use. These parasites, known as refugia, help slow the development of resistance to treatment. To increase refugia in the pasture, a portion of the herd should be left untreated.
A camp or paddock first grazed by cattle or horses is generally safe for small stock, as these species do not share the same internal parasites. However, rotational grazing will only be effective against internal parasites if the camps are rested for at least 70 days. Additionally, it is advisable to give the flock another pulpy kidney vaccination before moving them to fresh pasture.
It’s also worth noting that browsers, such as goats, typically experience fewer parasite issues. Because they feed higher above the ground, they are less exposed to moist soil conditions, where parasites thrive.
Vaccines offer hope
A holistic approach to wireworm control is the only sustainable long-term solution, but vaccination offers an additional layer of immunity to protect flocks and safeguard farmers’ profits. Relying solely on anthelmintics is becoming increasingly ineffective as anthelmintic resistance continues to rise.
A single female wireworm can lay up to 10 000 eggs per day for 21 days. Each egg develops into an infective larva, eventually maturing into a blood-sucking parasite. A singe wireworm can cause a sheep to lose 0,05 ml of blood daily. Over a 90-day period, an infestation can collectively drain up to 90 litres of blood – devastating when considering that an average 60 kg sheep has only 4–5 litres of blood. This highlights the potentially fatal impact wireworms can have on a flock.
Fortunately, a vaccine has been developed to combat the problem. Once administered, it triggers an immune response in the sheep, causing the animal to produce antibodies that bind to the worm’s gut lining. This prevents the parasite from digesting its blood meal, ultimately starving it. As a result, egg production declines.
Research shows that by reducing egg output, the vaccine lowers pasture contamination by 80% to 90%. This makes grazing safer over time and significantly reduces the need for deworming specifically for wireworm. Unlike anthelmintics, which directly target the parasite, the vaccine works by stimulating the sheep’s immune system to fight off the infestation.
*Future articles will cover the control of ectoparasites: ticks, lice, sheep keds and flies.
Also read:
• Feedlot cattle have fewer worms, study finds
• Biosecurity measures: You cannot be careful enough
• How to decide what diseases your animals have
 | Roelof Bezuidenhout is a fourth-generation wool, mohair, mutton and game farmer and freelance journalist. Attended Free State University, majoring in animal husbandry and pasture science. Other interests include golf, photography and geology. |
