Novel Hybrid Livestock Schistosomes Can Infect Humans

In a recent letter published in the Emerging Infectious Diseases journal of the US Centers for Disease Control and Prevention (CDC), scientists discussed the case of a child in Niger who was infected by livestock-specific schistosomes through the hybridization and introgression of Schistosoma bovis with Schistosoma curassoni.

“The detection of multiple introgressed hybrids with mixed ancestry in a single child suggests that Schistosoma species may be adapting to recent anthropogenic changes,” wrote Elsa Léger, PhD, from the Royal Veterinary College, University of London, United Kingdom, and colleagues.

Schistosomes are parasitic flatworms that live in the blood vessels of their mammalian definitive hosts. They cause schistosomiasis, a neglected tropical disease of medical and veterinary importance in many parts of the world. Schistosomes undergo a sexual reproductive stage in their mammalian host, allowing interactions between different schistosome species that may result in hybridization. Introgression refers to the transfer of genetic material from one species to another as a result of hybridization and repeated backcrossing.

Therefore, hybridization and introgression can have significant consequences for the epidemiology of schistosomes and are important public health concerns.

According to the authors, scientists have long suspected that natural hybridization within and between human and animal schistosome species occurs in definitive and intermediate hosts. “In western Africa, hybrids, predominantly between S. haematobium (human schistosome) and S. bovis or S. curassoni (livestock schistosomes), have been isolated from children and snails,” they wrote. And hybrids between these livestock-only species have also been reported in cattle and sheep, but not in other hosts.

Dr Léger and colleagues shared case details from a 10-year-old girl in Kokourou, Tillaberi region, Niger, who had schistosome infection. The researchers collected clinical samples from the child as part of longitudinal monitoring and evaluation of national disease control interventions in this area which has a high prevalence and infection intensity of schistosomiasis, despite more than a decade of high-coverage mass treatment with praziquantel.

The child’s urine contained Schistosoma eggs. After the eggs hatched, the researchers were able to perform noninvasive molecular analyses on schistosome larvae. The results of these analyses identified miracidia with a livestock S. bovis × S. curassoni hybrid profile with no genetic evidence of human-specific S. haematobium.

The researchers also analyzed internal transcribed spacer (ITS) sequences—these are regions of ribosomal DNA that have been used to detect hybridization and introgression across the Schistosoma genus. Although one miracidium from the young girl had a pure S. haematobium profile, the researchers found that 18 miracidia had profiles that were hybrids of S. bovis × S. haematobium, and of S. bovis × S. haematobium × S. curassoni. This highlights the potential for repeated interactions and cross-pairings among these 3 schistosome species.

The authors noted, however, that the molecular data indicate that these hybrids were not first generation, but a result of parental and/or hybrid backcrosses. “The data confirm the occurrence of bidirectional introgression between Schistosoma species that infect livestock and those that infect humans in Niger,” they wrote. “Our results strongly indicate that hybrid livestock: livestock schistosomes can infect a human definitive host, even when neither of its single parental livestock species appears compatible.”

Therefore, Dr Léger and colleagues stressed the need to identify and understand the transmission dynamics of introgressed Schistosoma species combinations.

“If novel zoonotic hybrid species are playing a role in maintaining and exacerbating schistosome transmission, illness caused by infection, or both, treatments for humans and livestock may have to be adjusted accordingly within a One Health framework,” they concluded.

​Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.

Feature Picture Source: Yale Rosen / flickr / Creative Commons.