We have designated persons with the cases of A. algerae myositis described here as case-patients A and B and those with cases reported in 2004, 2012, and 2013 as case-patients 1, 2, and 3, respectively (3,5,8). We reviewed the medical records of case-patients 2 and 3 to obtain additional information.

In the cases reported here, A. algerae myositis caused fever, weight loss, fatigue, generalized muscle weakness and pain, dysphagia, glossitis, peripheral edema, and diarrhea (Table 1). The limb edema might have been multifactorial and associated with local inflammation of the muscle; hypoalbuminemia; and impaired renal function, including proteinuria (19). The urinary myoglobin was negative in case-patient A, and urinary sediment staining for microsporidia was either not performed or not documented in any of the case-patients. Three case-patients had diarrhea, but no microsporidia were observed in gut biopsy specimens or in stained fecal samples (5,8). Although oropharyngeal ulceration and mucositis in case-patients B and 3 might have been caused by medication, it also might represent A. algerae infection (4,8).

The cardiac and central nervous system disorders may be associated with the A. algerae infection, related to co-morbidities or nonspecific features of severe illness. Case-patient B had delirium and atrial fibrillation with increased troponin I. Case-patient 2 had delirium, seizures, and an increased troponin I, 1.32 μg/L (reference <0.01). Seizures were associated with Trachipleistophora anthropophthera dissemination to the brain (16,17). However, the signs of cortical vasculitis and ischaemia on magnetic resonance imaging of the brain in case-patient 2 may have been due to angio-invasive fungal infection, considering the concurrent Aspergillus lung abscess. Case-patient 1, who had type 2 diabetes mellitus, died of a cerebrovascular infarction (3).

Laboratory indicators of muscle damage and inflammation included elevated CK, aspartate aminotransferase, alanine aminotransferase, erythrocyte sedimentation rate, and C-reactive protein and decreased serum albumin (20) (Table 2). Peak CK levels were lower in the transplant patients. Lymphopenia and decreased immunoglobulin levels might have been related to preadmission medications, including corticosteroid therapy (21). Although the underlying causes are yet to be determined, anemia and proteinuria were common features. The resolution of the neuropathy on nerve conduction studies in case-patient A might indicate recovery from peripheral nerve infection.

After fixation in formalin and staining, biopsy specimens can be examined under light microscopy (3,5). Muscle biopsies show necrotic myocytes containing spores, which stain poorly with hematoxylin and eosin (Figure 1, panel A) (5). Silver stains, particularly the Warthin-Starry, clearly demonstrate 3- to 4-μm ovoid spores (Figure 1, panel B) (5,22). In case-patient 1, muscle was immunofluorescently stained by using antibodies to A. algerae (3). The differential diagnoses include the yeast forms of fungi and leishmania amastigotes.

Transmission electron microscopy of biopsies can confirm a diagnosis of microsporidial myositis and differentiate Anncaliia spp. from Trachipleistophora hominis (3,5,23). Although electron microscopy is highly specific for microsporidiosis, the number of organisms within a biopsy specimen limits sensitivity. Features that define Anncaliia spp. include binary fission; diplokaryotic nuclei; lack of a parasitophorous vacuole, with the thickened plasmalemma of meronts and sporonts in direct contact with host cytoplasm; and vesiculotubular appendages projecting from the plasmalemma (Figure 2, panel A) (5,23). Degenerate sporoblasts and spores appear crenated (Figure 2, panel B). Mature spores have a dense exospore coat, a pale endospore, and single rows of 8–11 polar tubule coils (Figure 2, panels B, C). This number of polar tubule coils and the absence of deep protoplasmic extensions are features of A. algerae; however, species identification also relies on nucleic acid sequence homology (5,23). DNA extracted from clinical specimens can be amplified by PCR (3,5). The primers target areas of the large and small subunit ribosomal RNA genes and sequence analysis enables identification of the organism.

For case-patient A, who survived, a muscle biopsy was performed as soon as it became evident that empiric corticosteroid and intravenous immunoglobulin therapy was not effective. The corticosteroid dose was minimized; etanercept, methotrexate, and leflunomide had been stopped; and leflunomide was washed out with cholestyramine (24). Measures were taken to avoid aspiration pneumonia. On histopathology, microsporidiosis was considered, and a regimen with albendazole was begun. For case-patients B and 2, in whom microsporidiosis was diagnosed postmortem, antifungal agents were used. In vitro studies indicated that amphotericin B had no activity against the tested microsporidia (25).

Albendazole targets microtubules and prevents formation of intranuclear spindles in microsporidia (25). Microsporidial myositis caused by species other than A. algerae has been responsive to albendazole alone or in combination with other agents (9,12,18). Although albendazole has not been tested for activity against A. algerae, the survival of case-patient A suggests clinical effectiveness. Side effects include headache, gastrointestinal upset, and elevated levels of transaminases (25).

In case-patient A and a case-patient with Trachipleistophora myositis, the combination of albendazole, sulfadiazine, and pyrimethamine was successful (12). However, the contribution of sulfadiazine and pyrimethamine to the regimen is unclear because in vitro testing has indicated these agents are ineffective against the microsporidian Encephalitozoon cuniculi (26). Metronidazole, itraconazole, atovaquone, and clindamycin were among the medications used for case-patient 1 (3). The activity of these agents against microsporidia varies (25,27). Fumagillin and its analogs have been used topically to treat corneal infections and orally for gastrointestinal microsporidiosis (27), but because of possible side effects, they have not been used to treat microsporidial myositis (27).

Like other microsporidia, A. algerae depends on host cells to obtain metabolites (28). Genes that code for a diversity of kinases and transport proteins are regarded as an adaptation to the broad range of insect hosts (7,28). A. algerae was previously investigated as a biologic control agent for mosquitoes because heavily infected larvae rarely survive to adulthood (7). Other than in humans, no naturally occurring infections have been identified in mammals.

Mosquitoes become infected during their aquatic immature stages, either through exposure to spores in the water or by feeding on dead insect hosts (7,29). Infection occurs when the polar tubule is extruded from the spore and sporoplasm is introduced into the host cell through the tubule (7). In the initial proliferative phase, asexual reproduction occurs (7,29). Sporoloblasts then mature into spores (7,29). Although spores survive outside the host, they become nonviable when dried at room temperature for 5 days (30,31). Information is limited about the life cycle of A. algerae in Australia. Culex sitiens Wiedemann, an abundant coastal mosquito that usually bites birds, is currently the only documented insect host (7,32).

The exact mode of transmission of A. algerae to humans is yet to be established. Contact with water that contains spores generated in insect hosts is thought to be the most likely mechanism (6,33). Mechanical inoculation of spores by crushing a feeding mosquito into a bite wound has also been proposed as means a transmission (3,7). However, spores from mosquito tissue have substantially lower germination rates in human plasma or serum than those first exposed to water (34). Direct inoculation from a mosquito bite also is considered less likely because the saliva of laboratory-infected mosquitoes did not contain A. algerae (30). The Australian case-patients lived on the eastern seaboard, within 500 km of each other, in a warm temperate environment. They were exposed to mosquito habitat through proximity to bodies of water on golf courses or coastal woodland.

All the 5 case-patients reviewed were taking immunosuppressant therapy, and 2 had coexistent diabetes mellitus. Before infection, methotrexate, leflunomide, corticosteroids, and antitumor necrosis factor α therapy were used as treatments in the case-patients with rheumatoid arthritis. Epidemiologic studies have suggested that methotrexate and leflunomide are not associated with a substantial increase in serious infection, although this lack of association does not exclude a contribution with combination immunosuppression (35–37). Antitumor necrosis factor α therapy and corticosteroids are associated with increased risk (35,38,39). The severity of rheumatoid arthritis is also a risk factor for serious infection (35). However, before the addition of corticosteroids 4 weeks before hospital admission, methotrexate was the only immunosuppressive agent used by case-patient B, and he had no erosive or extraarticular disease.

Case-patients 2 and 3 were solid-organ transplant recipients (5). The immunosuppressive agents used in the months preceding the development of A. algerae myositis were mycophenolate mofetil, tacrolimus, azathioprine, and corticosteroids. Systemic infections caused by Encephalitozoon spp. in solid-organ transplants have been associated with these therapies (40). Studies with E. cuniculi have indicated the importance of cytotoxic CD8+ T-cell immunity for protection against severe and persistent infection (1).

A. algerae myositis is uncommon infection and has a high case-fatality rate. Whether it has emerged because previous cases have gone unrecognized or because epidemiologic factors have changed is unclear. A. algerae transmission occurs possibly through contact with water that contains spores amplified by insect hosts. The signs and symptoms of A. algerae infection primarily indicate myositis. Whether other organs and tissues may be infected has yet to be confirmed. In case-patient A, who survived, microsporidiosis was diagnosed on biopsy, and clinical management was based on the reduction of immunosuppression, measures to prevent complications, and a treatment regimen based on albendazole. Genomic analysis for drug targets and susceptibility testing of A. algerae may lead to increased therapeutic options. A. algerae myositis is an additional differential diagnosis to be considered in immunocompromised persons.