Monitoring parasite resistance and guiding sustainable control strategies in South Africa
Text: Ilsemarie Greyvenstein
Abstract
Equine parasitic infections present a substantial challenge to the health of horses and the equine industry in South Africa. The rising prevalence of resistance of internal parasites to anthelmintic drugs indicates a critical need for sustainable and scientifically guided parasite control strategies. Worm egg counts (WECs) are a diagnostic tool for assessing parasite burdens and identifying resistance to certain active ingredients in dewormers that are available in South Africa. This overview examines the role of determining WECs, focusing on the methodology, interpretation, and applications in targeted treatment and integrated parasite management (IPM). Additionally, it explores the prevalence of anthelmintic resistance, highlighting challenges and proposing future research for sustainable control.
Introduction
Worm egg counts are essential for monitoring and maintaining your horses’ health, enabling targeted deworming and reducing the risk of resistance. By regular testing, you can ensure your horse receives the correct treatment at the right time.
Equine parasitic infections continue to pose a significant threat to equine health worldwide, leading to weight loss, colic, reduced performance and, in severe cases, even fatality. In South Africa, where the equine sector plays an important economic role, parasite infections result in considerable productivity losses, highlighting the need for effective management strategies.
Common equine parasites in South Africa include small strongyles, large strongyles, ascarids and tapeworms. Strongyles are the most common and infest horses of all ages. Ascarids predominantly afflict young foals.
The extensive use of anthelmintic drugs (known as dewormers) has induced the development of parasite resistance, causing parasites to be less susceptible to pharmacological agents. This jeopardizes the effectiveness of conventional dewormers. Globally, this phenomenon highlights the need for the implementation of control strategies.
WECs serve as a tool that enables the quantification of parasite burdens, the identification of infected horses needing particular attention, and the detection of anthelmintic resistance. Guided by WECs, veterinarians and para-veterinarians can implement treatment protocols, reduce the overuse of dewormers, and in this way, slow down the development of parasite resistance. Dewormers are only administered in cases where the WECs are high instead of routinely.
Equine parasites: Prevalence and contributing factors
In South Africa, the key equine gastrointestinal parasites include:
- Small strongyles (Cyathostomins): Most prevalent in adult horses; these parasites induce clinical manifestations such as weight loss, diarrhoea, and colic.
- Large strongyles (Strongylinae): Though their prevalence has declined due to the use of ivermectin, they still cause arterial damage and colic.
- Ascarids (Parascaris equorum): Primarily affecting young foals, ascarids manifest as weight loss, sub-optimal growth and respiratory issues.
- Tapeworms (Anoplocephala spp.): These parasites are implicated in intestinal irritation and colic.
- Equine gastrodiscus: Caused by (Gastrodiscus aegyptiacus), found in water sources such as marshlands and dams.
Factors that increase parasitic transmission in South Africa include the warm, humid climate, improper pasture management, and high horse densities.
Methodology and interpretation of worm egg counts
The McMaster technique remains the most effective for WEC determination. It entails the following steps:
- Collection of faecal samples
- Preparation of flotation solutions
- Quantification of eggs using McMaster chambers
- Calculation of eggs per gram of faeces (EPG)
Factors influencing WEC accuracy include faecal integrity, sample freshness and observer proficiency. The interpretation of WEC results involves the identification of parasite types and WEC thresholds, beyond which treatment becomes necessary. However, variables such as the age, health, and prior anthelmintic exposure of the horse must be considered. The type of parasite detected determines the treatment prescribed. Follow-up, involving re-testing, is necessary to obtain treatment success.
Sedimentation test for gastrodiscus
Equine gastrodiscus caused by (Gastrodiscus aegyptiacus) is a condition that affects horses, particularly in areas where there are marshy pastures or access to water sources such as rivers and dams. This parasite has a complex life cycle involving freshwater snails as intermediate hosts. Equines become infected by grazing on vegetation where the parasite’s infective stage, metacercariae, is present. After leaving the snail, the parasite finds suitable vegetation or a surface in water. It then encysts itself as metacercariae to protect against environmental factors such as UV radiation.
The metacercariae remain dormant, until they are ingested by an animal grazing on the vegetation or drinking the contaminated water. Once inside the digestive system, these parasites shed their outer layer and develop into mature flukes. They migrate to the caecum and colon to complete their life cycle. The eggs are shed back into the fields or waterbody, where the cycle starts again. Controlling access to contaminated pastures and water sources is key to preventing infections.
While many infections do not show any clinical symptoms, and some horses can live with these flukes for years, others will exhibit colic, lethargy, diarrhoea, loss of appetite and weight loss. These parasite eggs can be found through a sedimentation test, where gravity is used to collect the heavy eggs in the sediment that drop to the bottom of the collection cup. This sediment is rinsed a few times to increase visibility. The liquid is placed under the microscope where the eggs can be observed. It is important to test horses once a year for gastrodiscus. Horses that have access to water sources or that have a history of previous infections should be tested more often.
Anthelmintic resistance: Detection and scope
WECs facilitate resistance detection through the Faecal Egg Count Reduction Test (FECRT). Reduction analysis pre- and post-treatment is used to evaluate drug efficacy, with insufficient EPG reduction indicative of resistance. However, confounding factors, such as inconsistent egg shedding and inhibited larval stages, may complicate the accuracy of the FECRT. Treated horses should be re-tested within 14 days to establish whether follow-up treatment is necessary to break the parasite cycle.
Resistance to benzimidazoles, pyrantel and abamectin has been documented in South African equine parasites. Contributory factors include:
- Excessive anthelmintic use without WEC guidance.
- Improper dosing, leading to survival of resistant phenotypes.
- Poor pasture management, thereby sustaining transmission cycles.
Integrated sustainable strategies for parasite control
Targeted treatment via WEC-guided results and consultation minimises drug usage, thereby preserving the efficacy of anthelmintic drugs.
Strategies should be followed to integrate WECs with practices such as:
- Pasture management: Rotational grazing and manure collection.
- Hygiene practices: Routine cleaning of stables and paddocks.
- Holistic controls: The use of supplements to assist in parasite control.
Such approaches reduce drug dependency, prolong anthelmintic drug efficacy, and promote equine health.
Challenges and future research
WEC-based programmes face challenges, including diagnostic costs, a lack of education and resistance to behavioural change among horse owners, where routine administration of dewormers is easier than having faeces tested.
Future research imperatives include:
- Development of new anthelmintics to counteract resistance.
- Advancement of diagnostic methods to improve results and affordability.
- Test-based results to monitor resistance.
Sending samples
To collect and send faecal samples, place two manure balls into a sealable plastic bag, ideally taking one ball from each of two separate manure piles. Clearly label the bag with the animal’s name, age, and any relevant details, such as deworming history or symptoms. When preparing for transportation, place the sealed sample in a cooler or container with ice packs. If immediate transport is not possible, refrigerate the sample, but avoid freezing or overheating it, as this may compromise its suitability for testing.
Conclusion
Equine worm egg counts are pivotal to parasite control and to delay the onset of parasite resistance in South Africa. Sustained research and education are important for refining and adopting practices that prevent parasite proliferation, ensuring equine health and productivity.
Contact Para-Vet Plus
Para-Vet Plus is a registered facility that offers Worm Egg Counts at affordable rates. We are also equipped to conduct tests for identifying the presence of gastrodiscus. Contact Ilsemarie Greyvenstein on 0720896248 or visit Para-Vet Plus on Facebook.