Parasitic myositis in tropical Australia

Although H. perplexum myopathy has previously been reported, in two people from Tasmania,1,2 the patients we report had spent most or all of the past few years in tropical northern Australia. We contacted the physicans caring for the earlier patients, who confirmed that they, too, had been exposed to tropical Australia (Box 4). One had travelled extensively in Cape York, Far North Qld, and Kakadu National Park, Northern Territory, 5 years before diagnosis. Around that time, he developed a non-specific illness with raised liver transaminase levels, and liver biopsy showed mild reactive hepatitis with eosinophils. The other was a botanist who had visited the Northern Territory, Tasmania, Europe, Kenya and Indonesia on fieldwork before developing muscle weakness.

Clinical features common to H. perplexum myositis patients were the insidious onset of diffuse limb weakness and dysphagia. The documentation of persistent eosinophilia up to 7 years before diagnosis suggests that infection may be subclinical for years. Consistent clinical signs have been diffuse limb wasting and weakness, including bilateral foot drop. Uniform abnormalities included an elevated serum CK, blood eosinophilia and mild elevations of liver transaminases. Although muscle biopsy is required to confirm diagnosis, in one reported case the initial biopsy did not reveal parasites, but a second biopsy after steroid treatment showed H. perplexum.

H. perplexum is a minute nematode measuring about 350 m by 20 m. The parasite appears to be able to complete its life cycle within human muscle, as adult nematodes are found within the sarcoplasm of muscle cells, while larvae are found both inside and outside myofibres.3 Electron microscopy shows the characteristic features of H. perplexum: a cuticularised triradiated oesophagus–intestine terminating in dark refringent granules (trophosomes).1 The outer cuticle is thin and has exterior corrugations. The male is shorter and narrower than the female, with the testes occupying 50%–60% of its body. The gravid female has paired uteri where 12 to 24 ova develop into larvae. Auto-reinfection is thought to occur when third-stage larvae escape by bursting through the female nematode’s body, a phenomenon known as endotokia matricida. The resulting damaged myofibres incite an intense inflammatory reaction resulting in myophagia.

H. perplexum was first described in the late 1990s, having been identified as the cause of a case of human myositis.2 The parasite is a member of the Robertdollfusidae family of Muspiceoidea nematodes. Distinct species of Muspiceoidea nematodes have been found in the tissues of various Australian vertebrates,4 including mice,3 bats,5 kangaroos and wallabies,6 and koalas.7 In animals, there is some evidence for cutaneous penetration as the mechanism of infection.3

The mechanism by which humans acquire H. perplexum is unknown. Other parasitic myopathies such as trichinosis and cysticercosis are acquired through consumption of poorly cooked meats. Intestinal parasites, including Strongyloides, may be acquired via direct cutaneous penetration, such as when walking barefoot, a common practice in tropical Australia. Mosquitoes, which are very common in tropical Australia during the wet season, could also be a vector for infection.

Treatment with albendazole improved muscle strength in all three cases of H. perplexum myopathy, although recovery was slow and incomplete. A biopsy after 4 weeks of treatment showed live nematodes. After 9 weeks of treatment, no live nematodes were seen on biopsy, correlating with a reduction in CK levels and peripheral eosinophilia. Therefore, treatment for at least 8 weeks is suggested to prevent auto-reinfection. Treatment solely with albendazole, without concomitant steroid use, was not associated with adverse reactions from nematode death. Furthermore, steroid therapy resulted in deteriorating muscle function and delayed diagnosis by falsely normalising blood eosinophilia.

1 A 23-year-old woman with progressive muscle weakness (Patient 1)

The patient had diffuse wasting of major lower limb muscle groups, including the quadriceps. The white arrow shows wasting of the tibialis anterior muscle, which caused foot drop. The black arrow shows the well preserved digitorum brevis muscle, accounting for preserved toe extension.

2 Micrographs of skeletal muscle (Patient 1)

A: Transverse section of quadriceps muscle fibres (M) shows a nematode, Haycocknema perplexum (HP), in cytoplasm, surrounded by interstitial chronic inflammatory cells (INF) (haematoxylin and eosin stain; original magnification, × 400). B: Longitudinal section shows parasitic nematode lying within the muscle fibre (toluidine blue stain; original magnification, × 1000).

3 Electron micrographs of Haycocknema perplexum nematode

A: Cross-section through nematode in skeletal muscle (M), showing cuticle (Cu) and oesophagus (O) (original magnification, × 2500). B: Enlargement shows cuticularised triradiated lumen of the oesophagus (original magnification, × 20 000).