Tropical and Unusual Infections of the Musculoskeletal System

The tropics are usually defined as the part of the world that is bounded by the tropics of the Capricorn and Cancer. Many infections that we now consider “tropical” or “exotic” were in fact previously found in the temperate regions of Europe and North America in the past. These include the plague epidemic (“black death”) of medieval Europe, and widespread malaria and hookworm infestations in the United States during the early 20th century. Eradication of these diseases from Europe and North America was attributed to rise in living standards, improved sanitation, public health education, hygiene, vector-control, and accessible medications.

Most types of infectious diseases can be found in the tropics. Some even regard tropical infections to be a list of problematic infectious diseases in developing countries, since most of the world's poorest nations are located in the tropics. For example, although tuberculosis and human immunodeficiency virus (HIV) are found throughout the world, they are often included in the list of tropical infections, as they cause severe morbidity and mortality in these countries. Currently, there are still a large number of infections that are predominant in tropical regions, and a few that occur almost exclusively in the tropics.

Pathogens causing tropical infections can be classified into bacterial, fungal, parasitic, and viral etiologies. The year-long hot climate in the tropics contributes to the formation of breeding grounds; large number of possible insect vectors; and the large number and variety of natural reservoirs and animal diseases than can be transmitted to humans (zoonosis). Many zoonotic diseases, such as bovine tuberculosis, brucellosis, and hydatid disease, are endemic in developing countries of Asia, Africa, and Central and South America.

Of the estimated 15 million deaths per year due to infections occurring in developing countries, more than 90% can be attributed to six diseases, namely: pneumonia, tuberculosis, diarrheal diseases, malaria, HIV/acquired immunodeficiency syndrome (AIDS), and neonatal infections.[1] Putting things into perspective, the musculoskeletal system is affected in only a very small proportion of these infections, and is usually not a major contributor to mortality. However, tropical and other unusual infections affecting the musculoskeletal system remain a source of morbidity, particularly in developed and non-tropical countries, due to delay in recognition, diagnosis, and hence timely and adequate management.

Causes of the increased incidence of the spread of tropical infections to “non-traditional” regions include: human exploration of tropical rainforests and deforestation; rising immigration; increased international air travel and tourism; and global warming, with the latter possibly allowing tropical diseases and vectors to spread to higher altitudes and latitudes, such as the Mediterranean area and the southern United States. In this issue of Seminars in Musculoskeletal Radiology, focusing on tropical and unusual infections of the musculoskeletal system, it is significant that in addition to articles written by distinguished colleagues from various parts of tropical Asia, there are contributions from the Middle East, eastern Mediterranean, and southwestern United States.

In the first article, Ng and Mclean highlight problems related to running a diagnostic radiology facility in the tropics. Areas of concern include equipment maintenance, lack of expertise, insufficient infrastructure, and issues with personnel, radiation protection, and safety.

As the most common cause of death from infectious diseases worldwide, it is appropriate that tuberculosis affecting the musculoskeletal system warranted coverage in two separate articles. The first, by Shikhare et al, deals with tuberculous osteomyelitis and spondylodiscitis, while the second, by Pattamapaspong et al, describes the imaging features of tuberculous arthritis and tenosynovitis. Although musculoskeletal involvement remains a relatively rare manifestation of tuberculous infection, lack of awareness, particularly of atypical patterns of involvement, leads to delayed diagnosis and treatment of this curable disease. The varied imaging appearances of musculoskeletal tuberculosis often mimic other infections and, at times, neoplasia.

Arkun and Mete comprehensively cover the clinical and imaging features of brucellosis, a zoonosis caused by gram-negative coccobacilli, affecting the musculoskeletal system. Its range of manifestations includes spondylitis, sacroiliitis, osteomyelitis, peripheral arthritis, bursitis, and tenosynovitis. There are differences in affected sites between children and adults.

Melioidosis, although endemic in Southeast Asia and northern Australia, is another type of bacterial infection that has been increasingly recognized worldwide due to increased travel and migration. Who better to address the imaging features of this infection than Pattamapaspong and Muttarak from Thailand, a country with extensive experience in diagnosis and management of melioidosis. Musculoskeletal melioidosis is usually seen as part of multi-organ involvement, with the commonest manifestations being septic arthritis, osteomyelitis, pyomyositis, and soft tissue abscesses.

Rounding out the bacterial causes of tropical infections, Chou et al describe the clinical and imaging features of tropical pyomyositis and necrotizing fasciitis. The former is a purulent infection of skeletal muscle that arises from hematogeneous spread, usually with abscess formation. Necrotizing fasciitis is a severe, rapidly progressive infection, involving the superficial and deep fascia, that is potentially life-threatening. Both these conditions are associated with predisposing conditions such as HIV infection and diabetes mellitus. The most frequent causative organism in pyomyositis is Staphylococcus aureus, with others including Streptoccocal infection, E. coli, and gram-negative organisms. Necrotizing fasciitis is polymicrobial in the majority of cases, with Streptococcus being the most common organism.

Corr provides an overview of the unusual entity of fungal infections of the musculoskeletal system. These infections often present with either multifocal chronic osteomyelitis or chronic mono- or polyarthritis resembling osteoarticular tuberculosis. Fungal infections discussed include aspergillosis, blastomycosis, candidiasis, cryptococcosis, mycetoma, sporotricosis, and histoplasmosis.

Taljanovic and Adam share their extensive experience of coccidioidomycosis (or valley fever), a systemic infection caused by soil fungi that is endemic in the southwestern United States. Skeletal involvement is frequently multicentric, with predilection for the long bone metaphyses, bone protuberances, relative intervertebral disc space sparing, vertebral posterior element involvement, and lesion symmetry.

The topic of musculoskeletal hydatid disease, an infectious disease caused by the parasitic tapeworm Echinococcosis granulosis, is elegantly dealt with by Arkun and Mete from Turkey. Musculoskeletal involvement is rare and often mimics tumors and other inflammatory conditions. Hydatid disease needs to be considered in a patient presenting with a multiseptated expansile osteolytic lesion with an adjacent soft tissue mass. Disc preservation and lack of sclerosis are important differentiating findings in spinal infections.

Musculoskeletal infection is one of the common manifestations of patients with AIDS. Its prevalence is particularly high in Thailand (30%), making Pattamapaspong and Louthrenoo eminently qualified to address this topic. HIV-infected patients are susceptible to a variety of opportunistic and non-opportunistic infections, which may result in diverse range of infections including septic arthritis, osteomyelitis, pyomyositis, and soft tissue and skin infections.

In 2003, a highly infectious pneumonia called severe acute respiratory syndrome (SARS) appeared in southern China. It rapidly spread worldwide, mainly due to international air travel. Very little was known about this disease initially, and establishing a diagnosis was difficult. Later on, the causative agent was found to be a novel coronavirus. Many patients were treated with high-dose steroids, and developed osteonecrosis and reduced bone mineral density as a result. Although the SARS outbreak has been over for more than eight years, its musculoskeletal complications remain, and these are discussed by Griffith from Hong Kong.

As many musculoskeletal infections have non-specific appearances, biopsy may be required to confirm the diagnosis and to identify the causative organism. Srinivasan and Peh review the planning, indications, technique, and complications of imaging-guided biopsy for patients suspected of having musculoskeletal infections.

In summary, tropical and unusual infections of the musculoskeletal system may be difficult to diagnose, clinically and radiologically, as their manifestations are often non-specific. Musculoskeletal involvement is often part of disseminated multi-organ disease in infections such as tuberculosis, brucellosis, melioidosis, and coccidioidomycosis. Having a high index of suspicion, particularly for patients living in endemic areas or with a history of recent travel to endemic areas, is required. Although some imaging findings are diagnostic or highly suggestive for certain entities, the role of imaging is often to detect, evaluate disease extent, and guide treatment planning. Prompt treatment, by institution of the appropriate antibiotics and/or surgical drainage, and long-term follow-up contribute to reduction of mortality and morbidity of these infections, particularly for potentially life-threatening conditions such as melioidosis, tropical pyomyositis, and necrotizing fasciitis. Imaging-guided biopsy has a useful role in the common scenario where confirmatory histology and/or culture are required for diagnosis and management.

REFERENCE

• 1 World Health Organization .WHO global burden of disease: 2004 update. 2008. Available at: http://www.who.int/healthinfo/global_burden_disease/2004_report_update/en/index.html Accessed October 10, 2011