Ciliates: Infection, Transmission & Diseases

Ciliates, or ciliates, are nocellular eukaryotes with cilia on the cell surface, which they use for locomotion and to swirl food. They are found primarily in water and soil, living as commesals and, less commonly, as parasites. The species Balantidium coli is considered the only human pathogenic species.

What are ciliates?

Eukaryotes or eukaryotes are living organisms with a nucleus. They are distinct from bacteria and archaea. Ciliates are also called ciliophores, ciliata, or ciliates and correspond to unicellular eukaryotes whose cell surface bears cilia. The cilia are used for locomotion and for swirling food. Ciliates are considered a phylum of the Alveolata, which include about 7500 different species. Eukaryotic organisms are the most highly developed and differentiated species of protists. The length of ciliates ranges from ten to up to 300 micrometers. Certain species of ciliates are even more than one millimeter long. The organisms consist of contractile filaments of the cytoskeleton, such as actinmyosin or microtubule filaments. This allows ciliates to change their surface structure and adapt to chemical or physical stimuli. They fling long protein filaments outward in response to certain stimuli. Predatory species of ciliates often carry toxicysts that puncture the membrane of their victims and secrete toxic substances for immobilization. Ciliates are characterized by nuclear dimorphism. Thus, they possess nuclei of different sizes. Their diploid small nucleus is called micronucleus and their polyploid large nucleus is called macronucleus. The macronucleus corresponds to the vegetative cell center and the micronucleus forms the germ line. Presumably, ciliates evolved from syncytial multicellularity.

Occurrence, distribution, and characteristics

Ciliates are found primarily in freshwater, marine, and soil environments. In response to changes in oxygen or carbon dioxide concentration, ciliates exhibit pure reactive changes in their direction of movement, transporting themselves to an area with more favorable conditions. In addition to free-swimming ciliates, there are sessile ciliates. Some live as commensals, such as the Enodinia in the rumen of ruminants, which decompose the cellulose of the food by cellulase. Many members of the species live symbiotically within the inner layer of green algae such as zoochlorelle. For example, a purely parasitic species is Ichthyophthirius multifiliis, which attacks freshwater fish. Many species of ciliates are distributed worldwide. Asexual reproduction is practiced by many species in the form of transverse division or, rarely, longitudinal division. The genus Colpoda forms division cysts. In sexual reproduction, DNA exchange takes place by means of a plasma bridge. This conjugation results in the dissolution of the macronucleus. The micronuclei of the two partners give rise to a diploid nucleus in each of the two partners by means of division processes of meiosis and mitosis. After the separation of the sexual partners, another mitosis takes place, which doubles the diploid nucleus in each case. One of the resulting daughter nuclei becomes the macronucleus by means of polyploidization, and the second becomes the micronucleus. The food spectrum of ciliates is broad. Besides bacteria, some ciliates feed on flagellates, algae, amoebae or fungi. Other representatives live predatorily on other ciliates. The cell membrane bears the cytostome as a mouth-like opening for food intake. Food is packed into food vacuoles upon ingestion and circulates inside the cell on a solid path throughout the cell body, where it is acidified by acidosomes and enriched by lysosomes with hydrolase. Inside the cell, the food decomposes and vital substances reach the cytoplasm, with residual substances being excreted at the cell’s juicer in the form of a cytopyge.

Diseases and ailments

Balantidium coli is the only species of ciliates that affects humans. It lives commensally to parasitically in the digestive tract and can cause diarrhea and ulcers in the intestines. Balantidium coli have a very broad spectrum of possible hosts, ranging from hollow animals to crustaceans and various mammals, so especially pigs. In many hosts, infestation with the ciliates causes no symptoms of disease. Humans are rather rarely infested, but may develop gastrointestinal tract symptoms when infected.The cysts of the ciliate are usually ingested with contaminated food or water. In the host intestine, the cysts develop into active trophozoites. They then form flask-like lesions within the submucosa, which lies between the mucosa and muscle tissue within the colon. There, the ciliates form nests. The host excretes the cysts as well as active cells in the feces. The ciliates do not form known toxins, but infestation with the ciliates may promote the formation of ulcers in the colon in humans. This association is due to the formation of hyaluronidase, which dissolves hyaluronic acid in connective tissue. In particularly severe cases, symptoms may include bloody stools, permanent imperative bowel movements and severe weight loss. In extreme cases, such a severe infestation can be fatal. However, this is rarely the case, as infection with ciliates can be treated well with tetracyclines or metronidazole. Hygiene is a preventive measure, which is particularly relevant when handling animals such as pigs. Especially in warmer climates, prophylactic hygiene in this context protects against infection with the ciliates. The pathogens are not usually ingested via pork as long as the meat is not consumed raw.