Haemonchus contortus is an abomasal blood sucking roundworm. Adult haemonchus worms, and worms approaching adulthood (L5), have the ability to remove large quantities of blood (0.05ml/day) from their host. That means a sheep carrying 5,000 H. contortus may lose 250ml of blood per day. This, combined with blood loss from the mucosa, produces acute disease caused by rapid anaemia with the risk of death. As few as 500 adult worms can cause disease.
Haemonchus contortus differs from other pathogenic strongyle worms in other ways to:-
Haemonchosis is the disease caused by Haemonchus contortus. It can occur in adult and young sheep, causing reduced milk production in lactating ewes, with knock-on consequences for their lambs. It has been reported to be a cause of death in the ewes, likely due to large numbers of overwintered larvae emerging, either inside the animal or on pasture.
Acute infections, resulting from the ingestion of many infective larvae over a short period of time,
The onset of clinical signs may be so sudden that affected animals are still in good body condition.
Sub-acute infections are characterised by bottle jaw (sub-mandibular oedema), which means it can resemble liver fluke.
Chronic infections are characterised by a more general failure to thrive, showing signs of weight loss, poor body condition, bottle jaw, lethargy and weakness. The chronic nature of the blood loss leads to an exhaustion of iron reserves and the development of anaemia.
Historically, haemonchus was considered only to be a problem in South East England, but in recent years, the incidence, frequency of reports and geographical range has anecdotally increased. The season where animals were affected also appears to be extending, with more outbreaks later in the autumn. The reason for this is likely to be linked to climatic climate change, coupled with sheep movements. Data on the actual prevalence is relatively sparse and, from a practical perspective, it is important to remember the presence of haemonchus in a faecel sample does not mean there is or will be clinical disease. Outbreaks of haemonchosis are still relatively uncommon in the UK, even on farms with a history of the disease. It continues to be more typically associated with tropical areas that have summer rainfall (e.g eastern Australia). And given that there are still a significant proportion of sheep farms in the UK that do not have Haemonchus contortus present, or farms without or limited anthelmintic resistance in their H.contortus population, the importance of quarantine treatments to prevent important of the parasite should not be overlooked.
See SCOPS Technical Manual. Chapter 3.1.4 for more about diagnostics, monitoring, immunity in adult sheep, treatment options, how the parasite overwinters, factors involved in an outbreak, and vaccination
There is very little data available on the prevalence of anthelmintic resistant strains of Haemonchus contortus in the UK. It is also very difficult to take messages from other countries, due to their difference to the UK and lack of data. This means it is likely options remain for individual flocks, providing we monitor and investigate. We must not make assumptions regarding the anthelmintic resistance status on a farm. The implications for treatment options are huge. Even if the anthelmintic resistance status is well documented for other roundworms, it is not advised to assume the same status applies to haemonchus. It is imperative drench checks and speciation are carried out to establish if treatments have been effective, and if survivors are haemonchus only and/or other worm species. Currently there are no confirmed cases of anthelmintic resistance in haemonchus to closantel in the UK.