Ciliates: Definition, Characteristics, Transmission of Balantidiasis and Its Causes, Symptoms, Diagnosis and Treatment

They are a Phylum of the kingdom Protistas, which includes single-celled organisms.

The group of protozoan parasites of ciliates (Ciliophora), are commonly known as ciliates.

The term cilia refers to hair-like structures that are present in large numbers on the surface of some parasites and involved in their motility.

The total number of existing ciliated parasites is estimated to be up to 30,000 species, but Balantidium coli is the only known species capable of parasitizing humans.

Characteristics of the ciliates

Ciliates have three characteristics that distinguish them:

  1. At some stage of their life they present short extensions called cilia, which provide mobility in a coordinated way, in the form of waves, which allow them to trap particles.
  2. They have two nuclei, one is a large and very characteristic macronucleus and the other is a micronucleus, which is reserved for sexual reproduction, which they perform sporadically. It has the genetic information necessary for the regulation of the parasite’s functions, it also stores genetic information for its reproduction.
  3. Another characteristic is the presence of a specialized, cleft area called a cytostome, a cellular mouth or primitive mouth. This cytostome located at the anterior end of the body, presents a cytopharynx that continues to a primitive anal pore, called the cytopygium. They carry out phagocytosis feeding.

Balantidium coli

It is considered the largest protozoan parasite capable of infecting humans.

Therefore, humans are considered to be an accidental host for Balantidium coli, which means that they are not a normal host for this parasite.

This is a common ciliated parasite of man, lower primates, and pigs, but also other wild animals, such as wild boars.

The main reservoir is the large intestine, especially in the ileocecal and reectosigmoid region; extraintestinal involvement is rare.

Reproduction is asexual and is carried out by transverse binary fission, although conjugation also occurs.

The cyst has a rounded shape with a diameter of 40 to 60 μm, it has a double wall and under this wall are the cilia.

In the cytoplasm it is hyaline and one or two contractile vacuoles are observed, unique among protozoan parasites.

There are also numerous digestive vacuoles containing many phagocytosed particles.

These particles are preserved from the trophozoite stage, because they usually do not expel food products that are not digested, and that will remain there during the encysting process.

In the posterior end and in rare occasions the primitive cytopygium or anus is observed.


The parasite is present throughout the world, but it is especially a problem in developing countries with poor water sanitation.

In some endemic areas, water contaminated with feces from infected people can also contribute to the transmission of the disease.

It is most common in tropical regions of the world with an infection rate of approximately 1% in humans and 20 to 100% in pigs.

This disease is particularly common in the Philippines, Bolivia, and Papua New Guinea, particularly in areas where people almost cohabit with pigs.

Many free-living and commensal ciliates are also parasites of ruminants and horses, as well as free-living aquatic species and must be differentiated from B. coli.

As such, the human infection, called balantidiasis, is considered a zoonotic disease.

Causes of balantidiasis

Human infections are most common where malnutrition occurs, where pigs share a room with humans.

The transmission of the disease is fundamentally by fecalism.

In humans it is mainly due to ingestion of parasite cysts from pig feces. The parasite appears to be non-pathogenic in pigs.

This disease is also more common in people with a weakened immune system or who suffer from malnutrition, since their stomach is usually less acidic than others.

Actually, normal heartburn in healthy people can prevent disease by destroying ingested cysts.

Symptoms of Balantidiasis

Balantidiasis is generally asymptomatic due to a certain balance established between the parasite and normal human flora during the time the parasite adapts to its unusual host.

The results of human infection range from asymptomatic to severe with invasion of the mucosal lining followed by bleeding and ulceration, including explosive diarrhea that occasionally leads to dysentery.

It can also be life-threatening if the colon is perforated during the acute phase of the disease.

Most human cases disappear spontaneously, while some become asymptomatic, resulting in a carrier state.


The diagnosis is made from the presence of parasites in the stool or in the tissues collected during a colonoscopy.

However, as this parasite is rare in humans, it may take time to obtain a proper diagnosis.

The disease, when it occurs, is clinically indistinguishable from other causes of hemorrhagic colitis.

This ciliate can behave like a commensal, and can sometimes cause symptoms similar to those produced by E. histolytica


This disease can usually be treated with antiparasitic drugs.

Treatment usually involves oxytetracycline therapy , which usually lasts 10 days.