Acanthamoeba keratitis

Introduction to Acanthamoeba keratitis Acanthamoeba Keratitis (AK) is a new infectious keratopathy caused by Acanthamoeba protozoa. It was first discovered in 1973, and the number of cases has increased year by year in recent years. Due to the complicated clinical manifestations of the disease, it is difficult to diagnose and treat, and it is often misdiagnosed as herpes simplex keratitis or fungal keratitis. Therefore, it is necessary to strengthen the understanding and research of this disease. basic knowledge The proportion of sickness: 0.8% Susceptible people: mostly young and healthy people, most have a history of contact lens wear or eye trauma Mode of infection: contact spread Complications: glaucoma optic disc edema optic atrophy oppressive optic neuropathy retinal detachment choroiditis

The cause of Acanthamoeba keratitis

(1) Causes of the disease

Pathogenic self-born life Amoeba is a single-cell structure of protozoa, widely found in natural environments, such as fresh water, salt water, soil, dust in the air, dirt, spoilage plants and human and animal waste, in the tub, air filtration Amoeba has been isolated from the water-cooling tower, contact lens, and mirror box care solution. The pathogenic self-born life of amoeba can not only live in nature, but also proliferate in the warm-blooded host. It is called an amphibious organism and belongs to a facultative parasite.

Acanthamoeba

It is more common in soil or water source contaminated by feces, and has a trophozoite and cystic period. The trophozoite is a form of activity and infection of Acanthamoeba. It is oblong and has a diameter of 15 to 45 m. Many spinous processes, called spine-like pseudopods, are usually based on bacteria and other microbes for food, and are propagated in a two-split manner. The average reproductive cycle is about 10h (6-24h). When the environmental conditions are not suitable. The trophozoite becomes smaller and secretes a thick double-walled wall to form a cyst.

The capsule is round and has a diameter of 10 to 25 m. The inner wall is smooth and polygonal. The outer wall is often wrinkled. The inner and outer walls are connected to each other to form a spine hole. It is a channel for cyst metabolism, and the capsule is resistant to the external environment. Strong, not sensitive to general antibacterial drugs, chlorides, chemical disinfectants, etc., can survive for several years in the natural environment, the capsule can float in the air, and the cysts have been isolated from the nasopharynx of asymptomatic individuals. When the environmental conditions are appropriate, especially when the nutrients are sufficient, the cysts are converted into trophozoites within 3 days.

Acanthamoeba protozoa were previously considered to be non-pathogenic protozoa, and it was discovered in 1958 that Acanthamoeba can cause fatal infections in animals. It has been confirmed that Acanthamoeba can cause granulomatous encephalitis in humans. And keratitis.

There are currently 25 species of Acanthamoeba found, of which at least 8 species (A. castellanii, A. culbertsoni, A. hatchetti, A. lugdunensis, A. polyphaga, A. quina, A. rhysodes, A. Griffini) can cause keratitis in humans.

2. Nergen

The F. striata protozoa are bred in fresh water and can also cause human corneal infection. The active trophozoite is long amoebic, 7m × 20m in size, with a large pseudopod at one end, nourishing in an unsuitable environment. The body can be changed into a flagella type with two or even two flagella. It can also become a cyst. The flagellum does not divide and does not form an encapsulation. The capsule of the N. falciparum is smaller and has a diameter of 9 m. The wall of the capsule is smooth and porous. Under suitable conditions, the flagellar type and cyst can be converted into trophozoites.

3. Classification of Acanthamoeba

(1) Morphological classification: In 1977, Pussard and Pons were mainly divided into 18 species and 3 groups according to the cyst morphology. The pathogenic Acanthamoeba mainly belonged to group II, and A.culbertsoni in group III also had Pathogenicity.

The main characteristics of group I are: large cysts and trophozoites, the average diameter of the capsule is 18m, the distance between the inner and outer walls of the capsule is wide, the outer capsule is smooth or slightly wrinkled, the inner capsule is star-shaped, and the inner and outer capsule walls are located at the inner wall. Connected, the spine hole covers the inner capsule.

The main characteristics of group II are: the average diameter of the capsule is <18m, the distance between the inner and outer capsule walls is large or small, the outer capsule is often wavy or papillary, and the inner capsule can be star-shaped, polygonal and triangular, and sometimes circular. Or elliptical, no obvious protrusions are formed, and the spine cap is located at the junction of the inner and outer capsules, and is a depression formed by the inner capsule.

The main characteristics of group III are: the average diameter of the capsule is <18m, the outer capsule wall is thin, with or without wrinkles, the inner capsule is round, and there are 3 to 5 slightly protruding arms. It is difficult to classify group III by morphological characteristics alone. 5 species in the middle.

Since Acanthamoeba is widely distributed in nature and is divided into diseased and non-pathogenic species, it is necessary to identify Acanthamoeba at the genus and below the level, but the amorphous species of Acanthamoeba has certain limitations. For example, some external conditions can affect the morphology of the cysts; the capsules are different in different periods; the various forms in the same group are similar, especially the groups II and III. Therefore, many researchers are looking for more objective and accurate classification. method.

(2) Genotyping: It is currently believed that the analysis of DNA sequence differences is the most promising method for the exact typing of Acanthamoeba at the following levels. At present, more studies are performed on 18SrDNA gene sequencing.

According to the classification of 26 strains of Acanthamoeba in 2004, the Beijing Eye Institute showed that 25 strains were 18SrDNA genotype T4, and only 1 strain was T3.

The identification of genotypes is of great significance for the laboratory diagnosis of Acanthamoeba infection. Many laboratories have successfully applied PCR technology to diagnose Acanthamoeba keratitis. In addition to morphological and genotyping, there are laboratories. Exploring the application of isozyme zymogram, monoclonal antibody and other methods to the classification of Acanthamoeba, and then study the relationship between various types of Acanthamoeba and pathogenicity and drug efficacy.

(two) pathogenesis

Pathogenic self-living life Amoeba protozoa can survive in nature without the need to parasitize in the host. It feeds on bacteria, fungi and other protozoa. About 25% of the trophozoites carry bacteria in the throat of normal people. The intestine has also isolated the acanthamoeba, a human infection caused by the pathogenic self-produced amoeba, which is an accidental contact infection or opportunistic infection.

Amoeba protozoa first binds to the lipopolysaccharide of the corneal epithelial cell membrane, adheres to the surface of the corneal epithelium, and then releases active enzymes such as neuraminidase, which causes the corneal epithelial cells to become thinner and necrosis, causing destruction of the epithelial barrier. Invasion of the corneal stroma, recent studies have found that amoeba can damage corneal epithelial cells in three ways:

Endocytosis

Similar to phagocytic cells, direct phagocytosis of some cell membrane components.

2. Spontaneous exocytosis

In the absence of an activation process, amoeba protozoa spontaneously release lytic enzymes, resulting in epithelial membrane damage.

3. Membrane activation

When the amoeba is in contact with the corneal epithelium, the combination of the cell membrane surface and the receptor or ligand of the epithelial membrane surface activates the enzyme release process, causing damage to the epithelial cells.

Acanthamoeba keratitis prevention

For the risk factors of pathogenic spontaneous living amoebic keratitis, appropriate preventive measures should be taken. In particular, the education of lens care knowledge of contact lens wearers should be strengthened. Contact lenses should be strictly avoided during sleep, and strict avoidance should be strictly avoided. Clean the lens with tap water or your own liquid.

Complications of Acanthamoeba keratitis Complications glaucoma optic disc edema optic atrophy oppressive optic neuropathy retinal detachment choroiditis

In some severe cases, refractory glaucoma can occur, and the posterior segment of the eye is rarely involved, but optic disc edema, optic neuropathy, optic atrophy, retinal detachment, choroidal inflammation and macular scar formation are visible, and even contralateral choroidal choroiditis is even. There are reports.

Acanthamoeba keratitis symptoms common symptoms eye pain corneal ulcer abscess keratitis

Pathogenic self-born life Amoebic keratitis patients are mostly young healthy people, male and female proportions are equal, most have a history of contact lens wear or eye trauma, the vast majority of single eye involvement, individual patients can also have both eyes onset, The onset is generally slow. The early stage of inflammation is mainly the irregularity of the corneal epithelium, rough or repeated epithelial erosion, sometimes showing pseudo-dendritic changes. Patients often have obvious eye pain, and the degree often exceeds the signs, forming symptoms. The phenomenon of separation from signs.

As the disease progresses, inflammation gradually invades the stromal layer, forming a plaque, semi-annular or annular infiltration of the pre-corneal stroma. Some lesions are similar to changes in discoid keratitis, and some patients may have radial keratitis.

If not diagnosed and treated in time, corneal infiltration quickly develops into corneal ulcer, matrix abscess, and satellite formation and anterior chamber empyema. In severe cases, corneal necrosis occurs. If corneal ulcers involve the limbus, it often leads to limbal margin. Inflammation, even scleritis.

In severe cases, more than 20% of cataracts occur, especially after prolonged disease, corneal transplantation, and long-term use of glucocorticoids.

Acanthamoeba protozoa can be mixed with bacteria, fungi and viruses. The mixed infections mainly include Staphylococcus epidermidis, Staphylococcus aureus, Streptococcus and Corynebacterium.

The diagnosis of Acanthamoeba keratitis mainly depends on clinical manifestations and laboratory tests. Corneal scraping is a commonly used method for the culture of Acanthamoeba.

Examination of Acanthamoeba keratitis

1.10% KOH wet seal inspection

Take the corneal scraping tissue of the lesion area, the culture of Acanthamoeba protozoa or the surgically removed corneal material, apply or spread on a glass slide, add 1 drop of 10% KOH solution, and clearly show the double layer of the protozoa under the ordinary microscope. The shape of the wall capsule is simple and practical, and is suitable for primary hospitals. Conditional hospitals can be examined by Hemacolor staining, triple staining, Calcoflour white and other staining methods.

2. Acanthamoeba protozoa culture

Place the corneal scraper tissue on the surface of 2% nutrient-free agar. Drop 1 drop of live or dead Escherichia coli broth on the surface of the inoculum and incubate in a 35 ° C incubator. Generally, it can be grown in 3 to 7 days. The Acanthamoeba protozoa can directly observe the trophozoites and cysts of Acanthamoeba protozoa and the spine process in the warm distilled water by an inverted microscope.

3. Immunofluorescence technique check

At present, at least 8 species of Acanthamoeba can cause human keratitis, and different species of Acanthamoeba protozoa antibodies can be used to identify the species of Acanthamoeba by indirect immunofluorescence. Currently, there is no such species in China. Antibodies can be sent to the US Centers for Disease Control (CDC) for identification if necessary.

4. Pathological section staining examination

Materials were taken from keratotic lesions drilled by trephine and surgically resected keratopathy, fixed, dehydrated, dipped in wax, embedded, sectioned, and then hematoxylin-eosin (HE) or periodic acid-Schiff (PAS) staining, staining by the above two methods can clearly show the cystic amoeba encapsulation in the cornea, and the results of the scraping or protozoa culture can be verified by pathological examination, and the pathological section can be confirmed according to the needs and conditions. Dyeing methods such as Giemsa, Wright, Triple and Calcoflour white are used for diagnosis.

5. Confocal microscope direct inspection

The above-mentioned examination methods are all invasive diagnostic techniques. In order to obtain tissue, it needs to be taken at the lesion site, causing certain damage to the cornea. The recently introduced confocal microscope can directly observe AK patients, which is a non-invasive, An early rapid diagnosis method with high contrast and high magnification can detect images of Acanthamoeba in various layers of the cornea (from epithelium to endothelium), and sometimes even pseudopods protruding from Acanthamoeba protozoa, or not Regular image of nerve swelling and thick edges.

Diagnosis and differentiation of Acanthamoeba keratitis

Pathogenic spontaneous life amebic keratitis, early identification of epithelial lesions of monocystic keratitis, the rate of misdiagnosis is higher, the epithelial lesions of the initial onset, there is a tendency to prolong the unhealed, at the same time Patients with a history of trauma or contact lens wear should be highly suspicious, and timely corneal scraping cytology is helpful for differential diagnosis. When corneal stroma infiltration and ulceration occur, it should be associated with monosporous discoid keratitis. The identification of bacterial and fungal keratitis, the history of intense eye pain or the appearance of radial keratitis have all contributed to the differential diagnosis. In recent years, the application of corneal confocal microscopy has been an early rapid diagnosis of Acanthamoeba keratitis. New means are provided. Through the corneal confocal microscope, the acanthamoe cyst can be observed in the living cornea, which is helpful for clinical diagnosis, but the confocal microscopy is negative, and the clinical diagnosis cannot be completely denied.

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