Haemonchus contortus, a rising threat
The intestinal parasite Haemonchus contortus, also known as the "barber’s pole worm", has long been associated with small ruminant production in tropical and subtropical regions. However, in recent times, this highly pathogenic parasite is increasingly becoming a major threat to dairy sheep flocks in temperate, non-tropical regions such as Europe. Changes in climate, farming practices, and rising anthelmintic resistance are driving this shift, challenging traditional parasite control strategies.
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3rd stage larvae (L3) of H. contortus
The parasite
Haemonchus contortus is a blood-feeding gastrointestinal nematode that lives in the abomasum of sheep and goats. It is notorious for its rapid reproductive cycle (~20 days), prolific egg production (5 000 – 15 000 eggs per day), and ability to cause severe anemia [1].
- A moderate infection of 500 female worms can result in 2.5 to 5 million eggs per day being deposited on pasture
- A single adult worm can ingest up to 50 µL of blood daily
Anemia can lead to rapid death of host – within 24 hours after first clinical signs – mostly in young hosts whose immune system are not fully competent. Blood loss can lead to protein loss, and oedema can form, usually in the ventral or submandibular region. This latter clinical sign is known as “bottleneck”.
Clinical anemia due to H. contortus
©J. Petermann
Although traditionally considered a warm-climate parasite, H. contortus has demonstrated remarkable adaptability. It is now well established in parts of Europe previously considered unsuitable for its life cycle [2].
The climate connection
One of the key drivers of H. contortus expanding distribution is climate change. Warmer temperatures and shifting precipitation patterns are creating microclimates within temperate regions that support the development and survival of infective larvae on pasture [3]. In northern Europe, for instance, milder winters now allow larval survival through the year, extending the transmission season by up to 2–3 months [2]. In the future, this trend is expected to continue and to lead to an overall increase in H. contortus burden. In southern Europe, hotter and drier summers may reduce larval survival during peak heat, but favor bimodal seasonal transmission patterns similar to those now emerging in the north.
Importantly, microclimate – the immediate environment on pasture – may play an even more significant role than regional temperature averages. Shaded, moist areas of pasture can extend larval survival and migration, even when macroclimatic conditions appear unfavorable [3].
The rise of resistance
The increasing global prevalence of anthelmintic resistance is a growing concern for H. contortus control. Resistance to benzimidazoles (BZs), macrocyclic lactones (MLs), and other drug classes has been widely reported, including in non-tropical countries such as France, Greece, Hungary, and Bosnia and Herzegovina [4–7].
In the Pyrénées-Atlantiques (France), recent studies identified eprinomectin-resistant H. contortus isolates on five dairy sheep farms [5]. In some cases, reduced drug efficacy was linked to low eprinomectin serum levels, particularly for topical formulations. Similar resistance to benzimidazoles has been confirmed in Greece and the Balkans, where allele-specific mutations (F200Y) in the β-tubulin gene have become widespread in field populations [4,6].
This resistance is not limited to sheep. Cross-infection and resistance gene flow between sheep, goats, and even cattle have been observed, facilitated by shared grazing and transhumance practices [6].
Toward sustainable solutions
Dairy sheep flocks face a unique challenge. Unlike meat-producing systems, milk withdrawal periods following anthelmintic treatment restrict the use of many effective drugs. Eprinomectin, the only macrocyclic lactone with a zero-day withdrawal period in milk, is now compromised by emerging resistance [5]. This limitation makes it increasingly difficult for dairy producers to treat effectively without risking production losses. Given the growing threat and the limitations of current treatments, alternative and integrated control strategies are becoming essential.- Parasite monitoring: tools like FAMACHA scoring and faecal egg counts (FEC) can help identify heavily infected individuals and reduce blanket treatments [8].
- Targeted Selective Treatment (TST): rather than treating whole flocks, TST focuses on individuals most in need, preserving drug-sensitive worm populations in refugia and slowing resistance development [8]. Insights about a recent TST field trial in French dairy sheep flocks can be found in this blog.
- Grazing management: rotational grazing and resting pastures can disrupt the life cycle of H. contortus, especially if timed with knowledge of larval survival and climate forecasts [8].
- Nutritional support: proper nutrition enhances the host's resilience and immune response, reducing the impact of infection even when parasites are present.
- Breeding for resistance: genetic selection for animals with lower FEC or greater resilience to infection is a long-term strategy showing promise in several breeds [8].
- Vaccination: vaccination is considered a central piece of the sustainable control strategies, but the current vaccination offer suffers from a limited geographical presence and a cumbersome and not very cost-effective procedure [1,8].
Conclusion
For many dairy sheep farmers, small ruminant veterinarians, and researchers in temperate regions, Haemonchus contortus is no longer a distant threat. The now wide distribution of this parasite, its deleterious impacts on the health of lambs and ewes and its growing resistance against our best anthelmintics implies a drastic change in our control strategies.
© S. Jouffroy
Bibliography
[1]. Adduci I. et al. (2022). Haemonchosis in Sheep and Goats: Control Strategies and Development of Vaccines against Haemonchus contortus. Animals.
[2]. Rose H. et al. (2016). Climate-driven changes to the spatio-temporal distribution of the parasitic nematode, Haemonchus contortus, in sheep in Europe. Global Change Biology.
[3]. Wang T. et al. (2018). Microclimate has a greater influence than macroclimate on the availability of infective Haemonchus contortus larvae on herbage in a warmed temperate environment. Agriculture, Ecosystems and Environment.
[4]. Arsenopoulos K. et al. (2020). Frequency of Resistance to Benzimidazoles of Haemonchus contortus Helminths from Dairy Sheep, Goats, Cattle and Buffaloes in Greece. Pathogens.
[5]. Jouffroy S. et al. (2023). First report of eprinomectin-resistant isolates of Haemonchus contortus in 5 dairy sheep farms from the Pyrénées Atlantiques département in France. Parasitology.
[6]. Kapo N. et al. (2024). First Report of Benzimidazole Resistance in Field Population of Haemonchus contortus from Sheep, Goats and Cattle in Bosnia and Herzegovina. Pathogens.
[7]. Khangembam R. (2023). Qualitative Assessment of Haemonchus Contortus Infection in Small Ruminant Flocks in Hungary: Prevalence and Diagnostic Study. PhD Dissertation, University of Debrecen.
[8]. Besier R.B. et al. (2016). Diagnosis, Treatment and Management of Haemonchus contortus in Small Ruminants. In: Advances in Parasitology, Vol. 93.
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