Impact of helminth infections on clinical and epidemiological features of microbial infections, and vaccine responses in ruminants: a systematic review.

Abstract

Abstract Helminth infections are ubiquitous in animal populations, and their interactions with concurrent or subsequent microbial infections can alter clinical and epidemiological features of infectious diseases and outcomes of vaccines. We used the PRISMA 2009 guidelines to conduct a systematic review of helminth-microbial-pathogen interactions and their impact on epidemiology and clinical outcomes of microbial infections in ruminants. Of the 1,375 probable studies identified through Google Scholar and PubMed databases, 20 studies met our inclusion criteria and were qualitatively analysed. Fourteen studies were on domestic ruminants, while six were on African buffaloes. Individual helminths investigated were Fasciola hepatica (six studies) Haemonchus contortus (three studies), Ostertagia ostertagi (one study) and Dicrocoelium dendriticum (one study), while the rest looked at gastrointestinal nematodes in general. Thirteen studies evaluated interactions between helminths and bovine tuberculosis (BTB), trypanosomes and theileria pathogens while four focused on responses to vaccination against rabies, bovine respiratory pathogens and Bacillus of Calmette-Guérin. Helminths interacted with Trypanosoma spp , Mycobacterium bovis , Theileria parva synergistically to aggravate clinical anaemia as measured by PCV and mortality. Furthermore, concurrent helminth infections were reported to lower the sensitivity of BTB diagnostic tests, but they did not appear to affect antibody responses to vaccines. We envision that helminths may interact with microbial pathogens in a number of ways, but primarily through immune pathways to modify clinical and epidemiological features of infections in ruminants. The implications of these interactions include missed diagnosis, underestimation of disease burden and increased severity of disease outcomes.