Instituto Valenciano de Microbiología

Masía El Romeral
Ctra. de Bétera a San Antonio Km. 0.3
46117 Bétera (Valencia)
Phone. 96 169 17 02
Fax 96 169 16 37
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Histomonas meleagridis: Causal agent of histomoniasis in gallinaceous birds - Molecular diagnosis (PCR).


Information 21-10-2018.


Histomonas meleagridis is a flagellated protozoan parasite that infects poultry, mainly turkeys, but also chickens, pheasants, partridges, peacocks and quails, producing histomoniasis, a disease also known as infectious enterohepatitis or black head disease. H. meleagridis is almost always mortal in turkeys, and it reduces the performance of the chickens and the production of eggs, causing serious economic losses for the poultry industry.

            Histomoniasis is a globally distributed poultry disease that was responsible for the devastation of the turkey industry in the United States at the end of the 19th century. With the introduction of the widespread use of antiparasitic food additives, through the therapeutic or prophylactic application of nitroimidazoles and nitrofurans, histomoniasis was not considered a relevant disease for many years. However, the banning of the latest feed additives in the last two decades in the European Union, North America and other countries to ensure the safety of food has led to the reappearance of histomoniasis and the resulting economic losses, especially in turkeys raised in these regions.

            Histomonas meleagridis is a single-celled, flagellated microaerophilic protozoan of the order Tritrichomonadide. H. meleagridis can infect poultry in the order Galliformes: turkeys, chickens, pheasants, partridges, peacocks and quails. Histomoniasis, infectious enterohepatitis or blackhead disease is characterized by swelling of the cecum and diarrhea with yellow-colored stools and liver necrosis. The old name of "black head disease" is a misnomer, since the heads of birds infected with histomoniasis do not turn black. The pathogenesis of the disease can vary between the different species of galliform birds, whose susceptibility to clinical histomoniasis varies widely. The course of the infection in the different host species covers the entire spectrum, from a total tolerance without lesions to a serious disease with a very high mortality rate. At one end of the spectrum, infection in turkeys causes high morbidity, mortality and serious economic losses of more than two million dollars per year in the United States. At the opposite end of the spectrum are other birds, such as certain species of pheasants, which manifest few effects of the disease and serve as carriers of the parasite for the most susceptible species.

            Histomonas meleagridis lives in the caecum of birds, where it triggers intense inflammation, thickening of the cecal wall and formation of fibrinous exudates. From the cecum, the protozoan parasite can migrate to the liver, through the hepatic portal vein, leading to multifocal areas of inflammation and necrosis, which are frequently seen in infected turkeys. Occasionally, the trophozoites of H. meleagridis reach other organs, especially the kidneys. In these organs, it may or may not cause inflammation, depending on the number of parasites that reach the organ.

            In turkeys, clinical signs may include sulfur-colored excrement, lethargy, drooping wings, closed eyes, head held close to the body, weakness or wasting. The lesions are characterized by thickening and ulceration of the lining of the cecum and by focal necrosis in the liver, which can lead to a high mortality of up to 100%. The combination of enlarged and inflamed cecum with yellow cecal nuclei and discrete patches of necrosis in the liver is considered indicative of histomoniasis. However, variations in the severity and appearance of the lesions are frequent and, occasionally cecal lesions occur without liver involvement. The second most critical case is that of chickens, with a mortality rate of 10 to 20%. Histomoniasis in chickens is less severe and the infection is usually limited to the caecum, although the infection leads to poor performance in the chickens. In laying hens, the parasite is responsible for a substantial decrease in egg production, which leads to serious economic losses.

            Histomonas meleagridis presents short life outside the host organism, disintegrating in a few hours. Therefore, the main form of transmission is through the eggs of the cecal worm Heterakis gallinarum, known as a reservoir and protector of parasites. Inside the egg, H. meleagridis remains viable for months, while unprotected trophozoites die quickly outside their host. Birds develop H. meleagridis infection by eating soil with embryonated H. gallinarum eggs containing H.meleagridis, or by eating worms infected with H. gallinarum that contain the previous parasite. Despite being the main route of transmission, histomoniasis also occurs in the absence of these nematodes. Within the poultry flocks, H. meleagridis is disseminated by cloacal drinking, when small amounts of newly spilled stool containing trophozoites of H. meleagridis are collected through the cloaca of an animal and from there they are quickly transported to the caecum. The reflexive intake of liquids through the cloaca is done in order to inoculate the immune system of young birds with the microbial flora of the surrounding environment. Histomoniasis can also occur in other ways, including the fecal-oral route, without the intervention of H. gallinarum as a vector. Recently, a cyst-like structure was described, which is believed to be another transmission mechanism of the parasite, though which it could become resistant to unfavorable environmental conditions. However, this mode of transmission has not been tested.

            Due to the prohibition of antiparasitic additives and to the fact that there are no vaccines available in the market, the measures for the control of this disease are mainly preventive. Overcrowding and contamination of drinking water by carrier birds should be avoided, and routine deworming should be performed in order to avoid the presence of its vector, Heterakis spp.

            Specific and rapid diagnostic tools are required to study the epidemiology of the parasite, estimate the risks and prevent outbreaks. Traditionally, laboratory detection of H. meleagridis in poultry has been based on microscopy, clinical symptoms and culture. However, numerous studies have shown that the morphology of this protozoan resembles other protozoa that can also be found in the caecum of birds, such as the pseudocysts of Tetratrichomonas gallinarum and Blastocystis spp. On the other hand, diagnosis based in early signs it can be ambiguous due to the similarity of the symptoms with those of other avian diseases, such as coccidiosis. Finally, protozoan culture as a diagnostic tool is extremely difficult due to the presence of other non-pathogenic organisms in feces and the caecum of poultry, which can inhibit the growth of H. meleagridis. Therefore, molecular detection (PCR) has gained importance as a sensitive and simple method for the diagnosis of histomoniasis. Although by PCR, the presence of parasites is demonstrated in most tissues of infected birds, the caecum and liver have proven to be the most important sites of predisposition. However, intestinal material and faeces samples are the samples used preferably in epidemiological studies of histomoniasis.

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