Histomonas meleagridis

Disease: Histomonosis, infectious enterohepatitis, blackhead.

Hosts: Chicken, turkey, peafowl, guinea fowl, pheasant, ruffed grouse, quail, chukar partridge.

Location: Ceca, liver.

Geographic Distribution: Worldwide.

Prevalence: This parasite is practically ubiquitous in chickens, altho it seldom causes disease in them. It is one of the most important causes of disease in turkeys. Before control measures were developed, it drove many turkey raisers out of the business, and even now the United States Department of Agriculture (1954) has estimated that it causes an annual loss of $3,815,000 in turkeys and $149,000 in chickens due to mortality alone.

Morphology: This parasite was first recognized by Theobald Smith in 1895. He thought it was an amoeba and named it accordingly. It was later confused with a number of other microorganisms. Some workers thought that it was one of the forms assumed by a pleomorphic Trichomonas, others that it was part of the life cycle of a coccidium, and still others confused it with the budding fungus, Blastocystis. Finally Tyzzer (1919, 1920, 1920a) showed that the organism was a flagellate and described it in detail. His observations have been confirmed by DeVolt and Davis (1936), Bishop (1938) and Wenrich (1943) among others.

Histomonas meleagridis trophozoites from cecum

H. meleagridis is pleomorphic, its appearance depending upon its location and the stage of the disease. The forms in the tissues have no discernible flagella, altho there is a basal granule near the nucleus. Tyzzer described three stages. The invasive stage is found in early cecal and liver lesions and at the periphery of older lesions. It is extracellular. It is 8 to 17 u long and is actively amoeboid, with blunt, rounded pseudopods. Its cytoplasm is basophilic with an outer zone of clear ectoplasm and finely granular endoplasm. Food vacuoles containing particles of ingested material but no bacteria are present.

The vegetative stage is found near the center of the lesions and in slightly older lesions than the invasive stage. It is larger, measuring 12 to 15 by 12 to 21 u. It is less active than the invasive stage and has few if any cytoplasmic inclusions. Its cytoplasm is basophilic, clear and transparent. The vegetative forms are often packed tightly together, and cause disruption of the tissues.

Tyzzer called the third form the resistant stage, but it is actually no more resistant than the other stages. There are no cysts. This form is 4 to 11 u. in diameter, compact, and seems to be enclosed in a dense membrane. The cytoplasm is acidophilic and filled with small granules or globules. These forms may be found singly or they may be packed together so that their outlines appear rather angular. They, too, are extracellular, but they may be taken up by phagocytes or giant cells.

A fourth form of the parasite is flagellated and occurs in the lumen of the ceca. The same form is found in cultures. Its body is amoeboid and may be 5 to 30 u in diameter. Wenrich (1943) found that 400 individuals from the ceca of 2 pheasants measured 9 to 28 u in diameter with a mean of 13.9 u, and that 400 individuals from the ceca of 2 chickens measured 5 to 18 u in diameter with a mean of 7.9 u. The cytoplasm is usually composed of a clear, outer ectosarc and a coarsely granular endosarc. It may contain bacteria, starch grains and other food particles, including an occasional red blood cell. The nucleus is often vesicular, with a single dense karyosome, or it may contain as many as 8 scattered chromatin granules. Near the nucleus is a basal granule or blepharoplast from which the flagella arise. There is typically a single, short flagellum, but as many as 4 may be present. Movement may be amoeboid, and there may be a pulsating, rhythmic, intracytoplasmic movement. The flagella produce a characteristic, jerky, oscillating movement resembling that of trichomonads, but Histomonas can be differentiated from them because it lacks an undulating membrane and axostyle. Wenrich (1943) found peculiar, cylindrical feeding tubes in about 15% of the individuals from one of the 2 pheasants he examined and also in some individuals from a chicken. These sometimes extended out as much as the body diameter and often had internal extensions as long or longer. This form is sometimes present in large numbers in the lumen of the ceca, but it is ordinarily absent or very difficult to find.

Life Cycle: Reproduction is by binary fission, and there is no evidence of a sexual cycle. Wenrich (1943) considered the larger, 4-flagellate forms in the ceca to be adult. There are no cysts. The naked trophozoites are delicate and do not survive more than a few hours when passed in the feces. Turkeys can be infected by ingesting trophozoites, and this mode of infection plays a part in transmitting the parasites once disease has appeared in a flock (Tyzzer and Collier, 1925). However, large numbers of the parasites must be ingested. Tyzzer (1934) pointed out that oral infection with infected liver tissue or cecal discharges is somewhat unreliable because of the death of the protozoa during their passage thru the alimentary tract. Lund (1956) found that oral administration of 10,000 to 100,000 protozoa in saline caused infections in about 40% of 6- to 9-week-old poults, and illness in about 20%. However, when digestible materials were added to the inoculum, the infection and morbidity rates fell sharply. The protozoa remained in the gizzard and upper intestine longer in the presence of food, and were destroyed before they reached the cecum. Horton-Smith and Long (1956a) found that infections with trophozoite suspensions could be produced only in starved chickens or, in chickens that were feeding, by giving them an alkaline mixture just before dosing them. They believed that successful in fection depends on the pH of the gizzard and possibly upper intestine. The pH of the starved gizzard is 6.3 to 7.0, that of chickens on feed is 2.9 to 3.3, and that of chickens on feed which have received alkali is 6.2 to 6.5.

By far the most important mode of transmission is in the eggs of the cecal worm, Heterakis gallinarum. Its discovery by Smith and Graybill (1920) was a milestone in the history of parasitology. This mode of transmission has been amply confirmed by many workers, and is the preferred method of producing experimental infections (Tyzzer and Fabyan, 1922; Tyzzer, 1926; Swales, 1948; McKay and Morehouse, 1948; Lund and Burtner, 1958). The parasites are carried inside the Heterakis eggs; eggs treated with disinfectants or other chemicals which do not kill them are still infective.

Infection of Heterakis eggs is so widespread that Histomonas infections can be produced with batches of eggs taken from a very high percentage of turkeys or chickens even if the hosts do not appear sick. Not every egg is infected, however. Lund and Burtner (1957) found that less than 0.5% of the embryonated eggs from experimentally infected chickens contained the protozoa, that less than half of the cecal worms they examined from these birds contained Histomonas-infected eggs, and that positive worms contained an average of only 2 infected eggs each.

The Heterakis eggs must hatch and liberate larvae in order to transmit the protozoa. Histomonas has never been seen in the infective eggs, its presence being inferred from the experimental results. However, Tyzzer (1926) found the protozoa in half-grown Heterakis from birds with histomonosis, and (1934) in the cells of the intestinal wall of 10-, 12-, and 21-day old worms from experimentally infected birds, and Kendall (1959) found them in a 4-day-old H. gallinarum larva.

The possibility that arthropods may transmit histomonosis has been considered by a number of authors. Mechanical transmission by flies and even grasshoppers is possible (Frank, 1953), but it is of minor importance.

Epidemiology: Histomonas is extremely common in Heterakis-infected chickens, and these birds constitute the principal reservoir of infection for turkeys. This accounts for the fact that it is almost impossible to raise turkeys successfully on the same farm with chickens. In addition, wild gallinaceous birds such as the wild turkey, pheasant, quail and ruffed grouse may be infected, but their role as reservoirs of infection for domestic turkeys has not been properly assessed.

Birds become infected most commonly by ingesting infected Heterakis eggs. Infective eggs can survive for one to two years or even longer in the soil. Farr (1954) infected chickens and turkeys with Histomonas from eggs which had been in the soil in Maryland for 66 weeks.

Pathogenesis: Histomonosis can affect turkeys of all ages; the course and mortality of the disease vary with age. Poults less than 3 weeks old are refractory according to Swales and Frank (1948), but from this age to about 12 weeks, the disease is acute and may cause losses averaging 50% of the flock and ranging up to 100%. The birds often die 2 or 3 days after showing the first signs of disease. In older birds, the disease is more chronic, and recovery may take place. The mortality decreases with age, and losses in these birds rarely exceed 25%. However, even birds of breeding age may be affected.

Chickens are much less susceptible than turkeys. They ordinarily show no signs of disease, but serious outbreaks may occur in young birds. Histomonosis occasionally occurs in the peafowl (Graybill, 1925; Dickinson, 1930), guinea fowl (Graybill, 1925) and quail (Graybill, 1925). Serious outbreaks may occur in captive ruffed grouse (Tyzzer and Fabyan, 1920; Graybill, 1925) and chukar partridges (Honess, 1956). Altho the parasite occurs in pheasants, it is apparently not very pathogenic for them.

When the histomonads are released in the cecum, they enter the wall and multiply, causing characteristic lesions. Later they pass by way of the blood stream to the liver.

The incubation period is 15 to 21 days. The first sign of disease is droopiness. The birds appear weak and drowsy, and stand with lowered head, ruffled feathers and drooping wings and tail. There is a sulfur-colored diarrhea. The head may or may not become darkened. This sign, which is responsible for the name blackhead, may also occur in other diseases, so the term is a misnomer.

The principal lesions of histomonosis occur in the cecum and liver. One or both ceca may be affected. Small, raised pinpoint ulcers containing the parasites are formed first. These enlarge and may involve the whole cecal mucosa. Sometimes the ulcers perforate the cecal wall and cause peritonitis or adhesions. The mucosa becomes thickened and necrotic. It may be covered with a characteristic, foulsmelling, yellowish exudate which may consolidate to form a dry, hard, cheesy plug that fills the cecum and adheres tightly to its wall. The ceca are markedly inflamed and often enlarged.

The liver lesions are pathognomonic. They are circular, depressed, yellowish to yellowish green areas of necrosis and tissue degeneration. They are not encapsulated, but merge with the healthy tissue. They vary in diameter up to a centimeter or more and extend deeply into the liver. In older birds the lesions are often confluent.

Other organs such as the kidney and lung may occasionally be affected. P. P. Levine (1947), for example, described numerous white, round areas about 1 mm in diameter in the kidneys of an affected turkey.

The parasites can be readily found on histologic examination of the lesions. Hyperemia, hemorrhage, lymphocytic infiltration, and necrosis occur, and macrophages and giant cells are present. The pathology of histomonosis in turkeys has been described by Malewitz, Runnels and Calhoun (1958) among others.

McGuire and Cavett (1952) studied the effect of histomonosis on the blood picture of experimentally infected turkeys. The non-protein nitrogen, uric acid and hemoglobin levels declined progressively, but tended to recover just before death. The blood sugar rose during the incubation period but decreased during development of the liver lesions; severe hypoglycemia was present just before death. The total leucocyte count rose as the result of proliferation of heterophils, myelocytes and monocytes.

If the birds recover, the protozoa disappear from the tissues, and repair takes place. The exudate and necrotic tissue in the ceca are incorporated into the cecal plug, which becomes smaller and is finally passed. If the lesions were not too severe, the ceca may eventually appear entirely normal, but in other cases there may be so much scarring that the lumen is obliterated. In the repair process, the lesions are invaded by blood vessels, lymphoid cells and connective tissue. The liver lesions may be completely repaired or there may be extensive scar tissue.

Immunity: Birds which recover from histomonosis are immune to reinfection. In addition, as mentioned above, susceptibility decreases with age. Lund (1959) found that infection of turkeys with a nonpathogenic strain of Histomonas did not protect the birds against subsequent infection with a pathogenic strain introduced by feeding Heterakis eggs, altho it did afford some protection against rectally introduced pathogenic histomonads.

Diagnosis: Histomonosis can be diagnosed from its lesions. Those in the liver are pathognomonic. In case of doubt and in order to differentiate the liver lesions from those caused by tumors, tuberculosis or mycotic infections, histologic examination is desirable. The cecal lesions can be distinguished from those caused by coccidia by microscopic examination of scrapings from the mucosa.

Cultivation: Histomonas was first cultivated by Drbohlav (1924) in a diphasic medium consisting of coagulated egg white slants overlaid with blood bouillon containing 1% peptoneo It has since been cultivated in a number of other media, both diphasic and monophasic (Tyzzer, 1934; DeVolt and Davis, 1936; Bishop, 1938). Delappe (1953, 1953a) found that addition of penicillin or streptomycin or both to Laidlaw’s culture medium facilitated the initial isolation of the protozoa, but he was unable to obtain axenic cultures. When the bacteria disappeared, the protozoa did likewise.

Treatment: Since histomonosis can be prevented by proper management, drug therapy should be regarded as a secondary line of defense against the disease. The chemotherapy of this disease has been reviewed by Wehr, Farr and McLoughlin (1958). While a number of phenylarsonic acid and quinoline derivatives have been used with some success in the past, the only one of them which is now used to any extent is 4-nitrophenylarsonic acid. When fed as 0.0125 to 0.075% of the mash or 0.006 to 0.04% of the drinking water for 3 days before and 21 days after experimental infection, this compound prevents death. However, there is a high relapse rate following cessation of treatment. Hence, to be effective this compound must be fed continuously until 5 days before slaughter. Mashes containing 0.01 to 0.03% of this compound stimulate growth, but 0.02% in the drinking water decreases egg production of adults and growth and livability of poults (Moreng and Bryant, 1956).

Thiazole derivatives are used most commonly against histomonosis. Three of these are enheptin, acetylenheptin, and nithiazide (Hepzide). The first is 2-amino-5-nitrothiazole, and the other two are derivatives of it. Enheptin was introduced by Waletzky, Clark and Marson (1950), and its activity was confirmed by a number of workers, including McGregor (1953), Jungherr and Winn (19 50), DeVolt, Tromba and Holst (1954), and Joyner and Kendall (1955). Acetylenheptin (2-acetylamino-5-nitrothiazole) was found by Grumbles, Boney and Turk (1952, 1952a, 1952b) to be just as effective as enheptin; it was also studied by Brander and Wood (1955) and Cooper and Skulski (1957) among others. Nithiazide (1-ethyl-3-[5-nitro-2-thiazolyl] urea) was introduced by Cuckler et al. (1956, 1957) and Cuckler and Malanga (1956).

These drugs have both prophylactic, suppressive and therapeutic value. Enheptin is usually fed continuously in the mash at the rate of 0.05% for prevention and suppression. If feeding is begun within 2 days after the infective dose of Histomonas is given in an experimental infection, it will almost completely prevent the disease. If it is begun later than this, it will suppress the disease as long as it is continued, but after it is withdrawn, histomonosis will reappear in the flock. If enheptin is to be used in treating turkeys which already show signs of disease, 0.1 to 0.2% of the drug is mixed in the feed. Not all the birds will recover, but quite a high percentage do. Acetylenheptin is used in much the same way. The preventive level of nithiazide in the feed recommended by the manufacturer in 1958 was 0.03%.

Potential hazards are often associated with feeding drugs continuously. Hudson and Pino (1952) and Pino, Rosenblatt and Hudson (1954) found that enheptin prevented or delayed sexual maturity in chickens and turkeys. When fed in the ration to chickens, it produced complete sexual involution or inhibition in both males and females. In young birds, sexual development did not take place, while in older ones the testes, ovary and oviduct atrophied. The effect was less marked in turkeys, altho 0.1% enheptin in the ration reduced the level of reproductive performance. This effect was found to be due to inhibition of gonadotropin secretion by the pituitary, and could be counteracted, at least in part, by simultaneous administration of gonadotropic hormone. Shellabarger and Schatzlein (1955) found that enheptin caused rats to have larger thyroid glands and to accumulate less iodine than normal rats. They suggested that these antithyroid properties might explain why enheptin inhibits the secretion of pituitary gonadotropin in the chicken.

Grumbles, Boney and Turk (1952) and Cooper and Skulski (1957) compared enheptin with acetylenheptin. The former found that 0.1% enheptin in the feed reduced production, fertility and hatchability in turkeys, but that acetylenheptin had no such effect. The latter found that enheptin decreased spermatogenesis and egg production and increased embryo mortality when fed to chickens at preventive levels. Acetylenheptin was less toxic. It had no effect on egg production, fertility or embryo mortality, and reduced sperm production only slightly.

According to Cuckler, Porter and Ott (1955), 0.1% nithiazide in the feed did not interfere with growth, maturation or reproduction of chickens or turkeys.

The nitrofuran, furazolidone (NF-180, Furoxone), was found by McGregor (1953a, 1954), Horton-Smith and Long (1955, 1956) and Costello and DeVolt (1956) to suppress histomonosis when fed at the rate of 0.01 to 0.04% in the feed. Even with the higher doses, however, some relapses occurred after medication was stopped, and slight lesions were found in treated birds killed during the experiments.

Cooper (1956) reported that feeding 0.02% furazolidone to pullets for 12 weeks had no effect on body weight, egg production, fertility or hatchability, but Cooper and Skulski (1955, 1956) found that feeding this drug to cockerels and roosters reduced the number of spermatozoa and decreased weight gains.

Control: Histomonosis can be prevented by good management. Turkeys should be kept separate from chickens, since chickens are carriers. Young turkeys should be kept separate from adults. The same attendants should not care for chickens and turkeys. Persons who go from one flock to another should take care not to carry the infection on contaminated shoes or equipment.

Young birds should be raised on hardware cloth, and the droppings should be removed regularly. When the poults are old enough to move onto range, they should be placed on clean ground where neither turkeys nor chickens have been kept for 2 years. The length of time infective cecal worm eggs survive in the soil depends upon soil type, weather and amount of cover provided by vegetation. They will survive only a few weeks on barren soils in warm, dry regions, but may remain alive for several years in heavy soils in moist climates.

The range should be rotated at regular intervals. Different farmers use different intervals. Many of them move the birds along every week, not returning to the same place during the same season. Another rotation system which has been recommended is to move the birds thru a series of 4 lots, allowing them to remain on each for a month. The frequency of rotation depends on the climate. In cool, damp climates the birds should be moved at least every 10 days, but in hot, dry climates they need be moved less frequently, and it is even possible to raise turkeys successfully without changing the range if the area around the feeders, waterers, roosts and shelters is kept dry. Low areas and streams that drain poultry yards should be fenced off.

The feeders and waterers should be placed on wire platforms. Most of the droppings are deposited around them, and this practice keeps the turkeys from getting at them. Wire should also be used beneath roosts and in shelters to keep the birds from their droppings.

Treating the birds with phenothiazine to prevent histomonosis by killing the cecal worms has been suggested. It is ineffective in controlling active outbreaks, but may help prevent future ones. Phenothiazine kills the cecal worms, but does not prevent their eggs from hatching and releasing the histomonads (Wehr and Olivier, 1946).

To eliminate Heterakis, 0.5% phenothiazine is mixed with the feed if the birds are not getting roughage, and 1.0% if they are on good range or getting supplementary roughage. The medicated ration is given for 5 to 7 days, the regular ration is fed for about 15 days, the medicated ration is then given again and alternated as before with regular feed until about 3 weeks before the birds are to be marketed. Phenothiazine should not be fed during these 3 weeks.