The two turkey protozoa that cause significant animal welfare and economic distress include various Eimeria species of coccidia and Histomonas meleagridis. For coccidia, oral ingestion of the organism allows for colonization and replication, while fecal shedding passes the organism to another host. Once one turkey is infected it can pass Histomonas to its flock mates by cloacal contact.
Outbreaks of coccidiosis followed by Histomonosis (blackhead disease) is commonly seen in the field but the relationship between the protozoa is not understood. Turkey fecal moisture, intestinal health and behavior changes due to coccidiosis could be increasing horizontal transmission of Histomonas.
Clinical signs of coccidiosis, like macroscopic lesions in the intestines, are not necessarily evident but altered weight gain and feed conversion occur frequently. Birds can become more vocal. Depending on the infective dose, strain of coccidia, and immune response of the turkey, intestinal irritation leading to diarrhea can occur. Birds are also more susceptible to other infectious agents. This is potentially due to the damage the coccidia can cause on the mucosal lining of the intestines but studies on this interaction are limited.
Coccidia sporozoites penetrate the turkey intestinal mucosa and utilize the intestinal tract for replication and survival. Of the seven coccidia Eimeria species known for infecting turkeys, four are considered pathogenic (E. adenoeides, E. gallopavonis, E. meleagrimitis and E. dispersa). The pathogenic strains are utilized for vaccine development to aid in a non-medicated option for coccidia immunity.
Coccidia acquired immunity and epidemic outbreaks are based on coprophagy, or fecal consumption. If a flock is vaccinated, it is done on day of hatch where either a spray or gel application is given to poults for oral administration of (live) oocysts. This allows for the initial interaction between the poults gut and the oocysts so that early immunity can be developed through production. Once the oocysts reach a certain level of maturity while in the turkey gut, they are expelled from the intestinal lining and shed in the feces. Coccidia sporozoites have a protective coating known as the oocyst wall that can withstand drastic changes in the external environment so this organism can survive for extended periods outside of the host. Coprophagy and the ground pecking behavior commonly seen with turkeys allows for the ingestion of oocysts by the flock so that acquired immunity occurs. Currently, the turkey industry has limited coccidia vaccine options because: (1) there is only one commercial vaccine available; (2) producers can use an autogenous vaccine but this is not tested or well controlled; (3) turkeys do not feather peck as often, altering the dosage of a gel-administered vaccine. These three variations could cause a rolling coccidia infection instead of specific cycles.
The addition of anticoccidials (ionophores) in feed, as an alternative to vaccination, have been shown successful in controlling coccidia outbreaks. However, turkeys are highly susceptible to ionophore toxicity and a common indication of toxicity is decreased feed intake with increased litter consumption. The increased litter consumption can increase oocyst intake (leading to a rolling infection) and increases the potential of consuming other parasites, like Heterakis gallinarum.
Heterakis gallinarum, the common reservoir of Histomonas, is found in poultry litter. The Heterakis egg acts as a protection mechanism for Histomonas and the eggs can survive in the environment for multiple years.
The consumption of litter contaminated with Histomonas infected Heterakis eggs is the start of an outbreak in a turkey facility but not how Histomonas becomes epidemic. Without the natural encapsulation of the egg, it is hypothesized that very few, if any, Histomonas can survive the chemical and physical challenges presented while in the digestive tract. In an experimental model, the oral inoculation of only Histomonas failed to cause blackhead disease in poults, while a cloacal inoculation from the same culture led to 60-80% of poults showing signs of infection. This study suggests that oral consumption of only Histomonas will not propagate disease. Cloacal drinking of Histomonas will instead cause blackhead disease. Physical stimulation of the cloacal vent, heavy panting or vocalization can lead to muscle contractions of the turkey anus to draw material from the environment into the ceca. In experimental models, dropping liquid containing Histomonas on the turkey vent lead to the uptake of the culture and blackhead disease. Also, uninfected poults will contract Histomonas if they are in physical contact with an infected bird; this is because a behavior commonly seen in sick turkeys is huddling. This behavior has been noted with coccidiosis and blackhead disease and allows for one bird’s defecation to encounter more of the flock’s population.
Cloacal drinking can more easily occur if the material taken up has a fluid consistency. Wet droppings are commonly seen in turkey rearing facilities with a healthy or sick flock. With coccidia specifically, outbreaks in turkeys can lead to an increase in fecal moisture. The wet fecal droppings in combination with cloacal drinking can allow for cloacal-cloacal contact to transfer Histomonas. However, it is important to note that litter quality is not indicative of a Histomonas outbreak. In the field, dry or wet litter can be seen while a flock is suffering from blackhead disease. Huddling will make the transfer of droppings between birds more likely. Turkey vocalization stimulates the movement of the cloacal vent, leading to reverse peristalsis bringing Histomonas contaminated droppings into the ceca. Establishment in the ceca will cause blackhead disease in a turkey, leading to mortality.
Turkeys fail to have an effective immune response to Histomonas. This protozoan colonizes in the ceca then eventually migrates to the liver through the hepatic-portal vein. During the first few days of infection, Histomonas invades the cecal mucosa but there are no gross pathological lesions. By day 4, the ceca start to become inflamed. Around day 7, the ceca will have a caseous core that develops in the center and the liver will start to become necrotic. At 10 to 14 days after infection, turkeys will suffer from liver failure and succumb to the disease. Currently, the time period that turkeys are contagious with Histomonas is not well defined.
A correlation between coccidia then Histomonas outbreaks have been shown in the field while experimental inoculations of both protozoa vary. One interaction of cocci then Histomonas infections in turkeys lead to more severe lesions related to blackhead disease. Other research models have failed to increase blackhead severity following coccidiosis in turkeys, but this could be due to the timing of the inoculations. Many of the coccidiosis outbreaks are not indicative by macroscopic lesions or mortality and so it can be difficult to diagnose severity of the outbreak. Initial mortality due to blackhead disease is low so diagnosis usually does not occur until after most of the population is infected. Targeting poults suffering from blackhead disease during the initial infection, instead of after horizontal transmission occurs, will decrease the spread of this parasite. Two weeks after poults are showing signs of coccidiosis, it is suggested to post any mortality to determine if blackhead disease has started in the flock.
References are available on request
From the Proceedings of the Midwest Poultry Federation Convention – 2020
