Free-living amoebae
Free-living amoebae are protozoa that normally live in the environment and only occasionally infect human or animal hosts. Acanthamoeba spp. and Naegleria fowleri are the most commonly seen species, both causing central nervous system infection and disease.
Overview
Free-living amoebae are protozoa that normally live in the environment and only occasionally infect human or animal hosts. Acanthamoeba spp. and Naegleria fowleri are the most commonly seen opportunistic species, both causing central nervous system infection and disease. Other opportunistic amoebas that may be involved in human or veterinary cases include Balamuthia mandrillaris and Sappinia diploidea. Most infections with these amoebae have been reported in people, but cases of Acanthamoeba and Balamuthia have been reported in dogs, and rarely other animals. Both N. fowleri andor Acanthamoeba are commonly associated with swimming in warm, naturally occuring freshwater in hotter regions of the world, such as Texas and Florida. A thorough travel and activity history are key to diagnosis.
Acanthamoeba spp. has a two stage life cycle: a trophozoite stage, that feeds on bacteria and reproduces by binary fission; and a double-walled cyst stage, in which the parasite is dormant, but resistant in the environment. Acanthamoeba spp. can cause Granulomatous Amoebic Encephalitis. This infection of the central nervous system is caused by inhalation of the amoebae from the environment, typically from soil or freshwater, and causes disease predominantly in immune-compromised individuals. Acanthamoeba trophozoites travel up the nasal passages, through the sinuses and up into the brain causing necrotizing granulomatous lesions in various parts of the brain. The organism can also spread elsewhere in the body causing chronic granulomatous lesions (disseminated form). The onset of CNS signs can take several months to a year to appear after infection, and the signs include headaches, stiff neck, behavioural changes, coma, and often death. Diagnosis can be made by detection of amoebae, DNA, or antigen in CSF or brain tissue. Often diagnosis is only made post-mortemat necropsy. Treatment is seldom used because of lack of effectiveness, but combinations of antibiotics and antifungals have met with guarded success in people. An experimental drug called miltefosine may hold promise for treatment of Acanthamoeba infections.brain tissue. Often diagnosis is only made post-mortemat necropsy. Treatment is seldom used because of lack of effectiveness, but combinations of antibiotics and antifungals have met with guarded success in people. An experimental drug called miltefosine may hold promise for treatment of Acanthamoeba infections.
A second disease caused by Acanthamoeba spp. is amoebic keratitis, usually as a sequel to corneal trauma or contaminated contact lenses. Infection of the cornea causes intense pain, photophobia and tearing and, if left untreated, can lead to corneal ulceration, eventual blindness and enucleation. Diagnosis can be made by finding the organisms in corneal scrapings or biopsy or using confocal microscopy. Several drugs have proved useful in treating amoebic keratitis and prognosis is good if caught early. Infection can be prevented by avoiding swimming while wearing contact lenses and by using sterile lens solutions. The veterinary significance of amoebic keratitis is largely unknown, although cases have been described.
Naegleria fowleri has a flagellate stage as well as the trophozoite and cyst stages. The motile trophozoite of N. fowleri feeds on gram negative bacteria and reproduces by binary fission. The trophozoite becomes flagellate as a result of changes in the ionic concentration of their environment and only remains in this form for an hour or two. The flagellate usually has 2 flagella and does not divide or feed. The trophozoite becomes the double-walled cyst when environmental conditions become unfavorable.
The source of infection with N. fowleri is usually a warm natural water source or moist soil. The parasite causes Primary Amoebic Meningoencephalitis, where trophozoites infect nasal tissues and sinuses of immunocompetent hosts and travel up the olfactory nerve into the brain. Symptoms appear within 3-6 days and include headache, stiff neck, seizures, coma and death. Death occurs within 1-2 weeks of infection without treatment. Diagnosis is made through detection of amoebae, DNA, or antigen in CSF or in the brain tissue. Previously, the treatment of choice for N. fowleri infection was amphotericin B, but more recently use of an experimental drug called miltefosine has met with success. If diagnosed within a few days of infection and treated aggressively prognosis is fair, otherwise, survival is unlikely. Cases of Naegleria in dogs have not been described.
References
Chia-Ching Chien R et al. (2018) Canine amoebic meningoencephalitis due to Balamuthia mandrillaris, Veterinary Parasitology: Regional Studies and Reports 13: 156-159, https://doi.org/10.1016/j.vprsr.2018.06.003
Cooper E et al. (2021) Should Veterinary Practitioners Be Concerned about Acanthamoeba Keratitis? Parasitologia 2021, 1, 12–19. https://doi.org/10.3390/ parasitologia1010002
Dubey JP et al. (2005) Disseminated Acanthamoeba sp. infection in a dog. Veterinary Parasitology 128: 183-187.
Kent M et al. (2011) Multisystemic infection with an Acanthamoeba sp. in a dog. Journal of the American Veterinary Medical Association 238: 1476-1481.
https://www.wormsandgermsblog.com/2014/07/articles/animals/dogs/brain-eating-amoeba-and-dogs/