The identification of Haemonchus species and diagnosis of Haemonchosis

Authors: Zarlenga, Dante; Hoberg, Eric; Tuo, Wenbin

Submitted to: Advances in Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2016
Publication Date: 3/31/2016
Citation: Zarlenga, D.S., Hoberg, E.P., Tuo, W. 2016. The identification of Haemonchus species and diagnosis of Haemonchosis. Advances in Parasitology. 93:145-180. doi.org/10.1016/bs.apar.2016.02.023.

Interpretive Summary: The differentiation of H. contortus from H. placei, two pathogenic, gastrointestinal nematode parasites of livestock ruminants, has been deemed by many as inconsequential because of morphological, biochemical and biological similarities between the organisms. Over time, two camps have emerged; those wishing to define H. contortus and H. placei as distinct species and those considering them as morphs, races, or isolates of a single, widespread species. However, since comparative morphological criteria were recognized that support their classifications as distinct species, studies of the epidemiology and population genetics of these organisms have become dependent on increasingly rapid and cost-effective protocols for accurate identification. Although there is a dearth of methods currently available that allow accurate differentiation of H. contortus from H. placei, those that are available are not routinely applied. Consequently, there is ongoing confusion about the relative importance of using morphological characters in the absence of clear criteria for specific identification. Herein we describe the benefits and pitfalls of current and developing methodologies for identifying Haemonchus species and the importance of being able to define species that have commonly been characterized only by host associations i.e. H. placei in cattle, and H. contortus in sheep and goats. Because of new transmission patterns worldwide and anthropogenic forcing, using host associations for identification are no longer valid especially as this relates to field-based epidemiological studies. Consequently, in the absence of definitive identification, the value of such studies becomes equivocal. We have taken a more guided examination of studies involving the diagnosis and identification of haemonchosis in the hope of teasing out efforts focusing on species identification. This work will be beneficial to those working in the general area of nematode diagnosis; in particular researchers, diagnosticians and veterinarians with a desire to better identify Haemonchus species infecting domestic and wild ruminants.

Technical Abstract: Diagnosis is often equated with identification or detection when discussing parasitic diseases. Unfortunately, these are not necessarily mutually exclusive activities; diseases and infections are generally diagnosed and organisms are identified. Diagnosis is commonly predicated upon some clinical signs; in an effort to determine the causative agent, identification of genera and species is subsequently performed. Both identification and diagnosis play critical roles in managing an infection, and involves the interplay of direct and indirect methods of detection, particularly in light of the complex and expanding problem of drug-resistance in parasites. Accurate and authoritative identification that is cost and time-effective, based on structural and molecular attributes of specimens, provides a foundation for defining parasite diversity and changing patterns of geographic distribution, host association and emergence of disease. Most techniques developed thus far have been grounded in assumptions based on strict host associations between Haemonchus contortus and small ruminants, i.e. sheep and goats, and between Haemonchus placei and bovids. Current research and increasing empirical evidence of natural infections in the field demonstrates that this assumption misrepresents the host associations for these species of Haemonchus. Furthermore, the capacity of H. contortus to utilize a considerably broad spectrum of ungulate hosts is reflected in our understanding of the role of anthropogenic forcing, the “break down” of ecological isolation, global introduction and host switching as determinants of distribution. Nuanced insights about distribution, host association and epidemiology have emerged over the past 30 years, coincidently with the development of increasingly robust means for parasite identification. In this review and for the sake of argument, we would like to delineate the diagnosis of haemonchosis from the identification of the specific pathogen. As a foundation for exploring host and parasite biology, we will examine the evolution of methods for distinguishing H. contortus from other common gastrointestinal nematodes of agriculturally-significant and free-ranging wild ruminants using morphological, molecular and/or immunological methods for studies at the species and genus levels.