Establishment of Histomonas meleagridis Challenge Model in Turkeys: An Industry Perspective

Histomoniasis (blackhead disease, histomonosis, enterohepatitis) in turkeys, is caused by a protozoan parasite, Histomonas meleagridis. Currently, there are no commercial vaccines or prophylactic/therapeutic treatments available for blackhead disease. Due to the lack of vaccines, treatment and efficacious drugs, the turkey industry faces a huge economic impact due to mortality, morbidity and condemnations. In order to evaluate any vaccine or efficacious prophylactic/ therapeutic treatment, the establishment of a robust challenge model is essential. This manuscript describes the establishment of a robust challenge model against recently isolated wild-type H. meleagridis isolates from the USA.

Histomonas meleagridis is the causative organism of histomoniasis (blackhead disease, histomonosis, enterohepatitis). H. meleagridis affects all gallinaceous birds including turkeys and chickens. Turkeys are highly susceptible to H. meleagridis compared to other gallinaceous birds. H. meleagridis can be directly introduced into the flock or may be carried by vectors such as Heterakis gallinarum, earthworms and insects. Fomites, sharing equipment and movement between farms may facilitate disease transmission. Once H. meleagridis enters the flock it is transmitted between the birds by the cloacal route and fecal-oral transmission. Histomoniasis causes general clinical signs such as dullness, depression, ruffled feathers and drooping of wings. H. meleagridis affects ceca, induces typhlitis and cecal core formation (Figure 1) and then reaches the liver by systemic circulation. It causes liver degeneration and necrotic foci (Figure 1). In later stages of the disease, sulfur-yellow feces results in yellow staining around the vent (Figure 2). The clinical signs are usually seen one to two weeks after infection and increased mortalities may be noticed from the second week onwards. Histomoniasis can cause severe mortality in turkeys up to 80–100%. However, some field outbreaks of histomoniasis have been documented with low mortalities and morbidity. In some cases, the turkeys may not show any clinical signs but have higher condemnations in the processing plants due to visceral lesions. Thus, the severity of histomoniasis outbreaks varies between flocks. Many factors, such as the virulence of H. meleagridis isolate, management method, age of the bird, season, immune status, concurrent infections and gut health, play a role in histomoniasis outbreaks.

Recently, increased incidences of histomoniasis have been documented in many countries after the withdrawal/ ban of previously licensed prophylactic/therapeutic products. Currently, neither commercial vaccines nor prophylactic/ therapeutic treatments are available in USA for histomoniasis. In order to find a solution for histomoniasis, a robust challenge model needs to be established to screen the vaccine candidates or test the efficacy of prophylactic/therapeutic products. Thus, choosing an ideal challenge isolate and administering by an appropriate challenge route plays a critical role in establishing the challenge model. The aim of our studies was to develop a robust challenge model for histomoniasis with recently isolated wild-type H. meleagridis isolates from the USA.

Materials and Methods

Histomonas meleagridis challenge isolates Three H. meleagridis field isolates (HMA, HMB and HMK) were evaluated. HMA was isolated from a histomoniasis outbreak in turkeys (2020) in Midwest, USA. HMB was isolated from a histomoniasis outbreak in turkeys (2020) in the Southeastern, USA. HMK was isolated from a broiler breeder flock (2021) with histomoniasis incidences in the Southeastern, USA. All isolates were confirmed by PCR and sequencing. Cecal samples were collected and transported in plug-sealed cap flasks with modified Dwyer’s media. The flasks were shipped in an insulated container at a warm temperature. The culture was incubated at 41°C for a period of 2–3 days and observed under the inverted microscope. After visual observation of histomonads, the culture was further subcultured and stored in LN2 containers with 10% DMSO. A few days before the challenge, the culture was retrieved back from LN2 containers and incubated at 41°C. After subculturing, the culture was examined under the inverted microscope and histomonads were enumerated.