Uncontroversial facts and new perspectives on poultry histomonosis: a review

Abstract

SUMMARY: Since therapeutic and prophylactic compounds became unavailable in many countries, outbreaks of histomonosis in turkeys and chickens have been increasing. Turkeys are particularly susceptible to the disease, whereas chickens generally survive and become carriers of Histomonas meleagridis. Although the disease has been studied since the beginning of the last century, some epidemiological aspects remain unanswered. The caecal worm Heterakis gallinarum is the intermediate host, but mechanical vectors seem to play an important role in the survival, transport and introduction of the protozoa in Turkey facilities. In turkeys, the disease can be transmitted by direct contact, referred to as horizontal or lateral transmission. Replication of horizontal transmission in experimental conditions has not been consistent in the last years, and factors influencing the transmission are not fully understood. The presence of bacteria is necessary for the protozoa to cause disease and be cultivated in vitro; however, the influence of bacteria, in the caeca and the litter, in the pathogenicity and transmission of histomonosis remains elusive. Histomonas meleagridis has tropism for the caeca and the liver. The caecum has a dynamic environment, presenting a large bacterial population, influencing the host’s homoeostasis in several ways. Genetic variability of isolates of H. meleagridis has been reported, but the impact of this genetic variability on the transmission of the protozoa has not been studied. Considering this complex host–protozoa–bacteria interaction, the present literature review focuses on factors that could impact the outcome of histomonosis infection and transmission.