Trichomonas and Free living Amoebas

TRICHOMONAS AND FREE LIVING AMOEBAS

Trichomonas vaginalis

Trichomonas vaginalis is a common sexually transmitted parasite. Infection rates vary from 10% to 50%, with the highest reported rates found in the USA. Infections are usually asymptomatic or mild although symptomatic infection is most common in women. Trichomonads are all anaerobes and contain hydrogenosomes. This organelle is found in very few other anaerobic eukaryotes and is often termed the ‘anerobic mitochondrion’. A number of functions have been assigned to it, and it has been shown to function in the generation of ATP. This organism does not exhibit a life cycle as only the motile (flagellate/amoeboid) trophozoite (Figure 6.2g) has been seen and division is by binary fission. Trichomonads have a pearshaped body 7–15 μm long, a single nucleus, three to five forward directed flagella, and a single posterior flagellum that forms the outer border of an undulating membrane.

Trichomoniasis in women is frequently chronic and is characterized by vaginal discharge and dysuria. The inflammation of the vagina is usually diffuse and is characterized by reddening of the vaginal wall and migration of polymorphonuclear leucocytes into the vaginal lumen (these form part of the vaginal discharge).

Because there is no resistant cyst, transmission from host to host must be direct. The inflammatory response in trichomoniasis is the major pathology associated with this organism; however, the mechanisms of induction are not known. It is likely that mechanical irritation resulting from contact between the parasite and vaginal epithelium is a major cause of this response but the organism produces high concentrations of acidic endproducts and polyamines, both of which would also irritate local tissues.

Freeliving opportunist amoebas

The freeliving opportunist amoebas are an oftenforgotten group of protozoans. The two major groups, Naegleria and Acanthamoeba, infect humans and both can cause fatal encephalitis. Both types of infections are rare, with less than 200 cases of Naegleria fowleri infection recorded worldwide and approximately 100–200 cases of Acanthamoeba ulcerative keratitis per year. This disease is commonly associated with contact lens use and it is thought that infection is caused by a combination of corneal trauma and dirty contact lenses. Both types of amoeba produce resistant cysts and Naegleria also exhibits a flagellate form. Both Acanthamoeba and Naegleria are freeliving inhabitants of fresh water and soil, but Naegleria fowleri (the human pathogen) reproduces faster in warm waters up to 46 °C. Treatment of water by chlorination or ozonolysis does not entirely eliminate cysts and both amoebae have been isolated from air-conditioning units.

Naegleria fowleri is the causative agent of primary amoebic meningo-encephalitis, a rapidly fatal disease that usually affects children and young adults. In all cases, contact with amoebae occurs as a result of swimming in infected fresh water. The organisms enter the brain via the olfactory tract after amoebae are inhaled or splashed into the olfactory epithelium. The incubation period ranges from 2 to 15 days and depends both on the size of the inoculum and the virulence of the strain. The disease appears with the sudden onset of severe frontal headache, fever, nausea, vomiting and stiff neck. Symptoms develop rapidly to lethargy, confusion and coma and in all cases to date the patient died within 48–72 hours.

Acanthamoeba castellanii, A. culbertsoni and other pathogenic Acanthamoeba species can cause opportunist lung and skin infections in immuno-compromised individuals. Where amoebae spread from such lesions to the brain, they can cause a slowly progressive and usually fatal encephalitis. In addition, Acanthamoeba can cause an ulcerating keratitis in healthy individuals, usually in association with improperly sterilized contact lenses. The presence of cysts and trophozoites in alveoli or in multiple nodules or ulcerations of the skin characterizes acanthamoebic pneumonitis and dermatitis. Spread of amoebae to the brain produces an encephalitis, characterized by neurological changes, drowsiness, personality changes and seizures in the early stages of infection, which progress to altered mental status, lethargy and cerebellar ataxia. The end point of infection is usually coma followed by death of the patient. Acanthamoeba keratitis is characterized by painful corneal ulcerations that fail to respond to the usual anti-infective treatments. The infected and damaged corneal tissue may show a characteristic annular infiltrate and congested conjunctivae. If not successfully treated, the disease progresses to corneal perforation and loss of the eye or to a vascularized scar over thinned cornea, with impaired vision.