Chapter 32 The Genera Treponema and Borrelia

THE GENUS TREPONEMA

Treponema spp. are motile, helical rods with tight, regular to irregular spirals (Figure 32-1). Many species are normal flora in the oral cavities, genital tract, or rumen of animals.

FIGURE 32-1 Electron photomicrograph of Treponema pallidum on cottontail rabbit epithelial cells.

(Courtesy Public Health Image Library, PHIL #1971, Centers for Disease Control and Prevention, Atlanta, 1980, David Cox.)

Treponema pallidum ssp. pallidum is well known as the cause of human syphilis, and Treponema pallidum ssp. carateum and Treponema pallidum ssp. pertenue cause the related diseases, pinta and yaws, respectively. Although it is not an animal pathogen per se, T. pallidum ssp. pallidum is included in this chapter because of its overwhelming importance as a human pathogen. The endemic treponematoses affect at least 2.5 million persons worldwide.

Species pathogenic for domestic animals include T. paraluis-cuniculi (associated withvenereal disease in rabbits [“rabbit syphilis”]) and T. brennaborense (which causes interdigital dermatitis in cattle [“hairy foot warts”]). Many of the pathogenic treponemes have not been cultivated in vitro, and genome sequencing suggests that T. pallidum ssp. pallidum has been subject to reductive evolution through centuries of association with human hosts.

Treponema pallidum and Human Syphilis

There is substantial controversy over the origin and spread of venereal syphilis, and current thought (a term fraught with danger in this case) is that nonvenereal treponemal disease has existed worldwide since the emergence of the genus Homo and that the first epidemic outbreak of venereal syphilis in Europe occurred in the late fifteenth and the early sixteenth centuries. Examination of bone lesions has long been a common means to study historical aspects of syphilis and other treponematoses. Molecular methods that allow identification of T. pallidum ssp. pallidum in paleopathologic material may make it possible to clarify issues relating to the origin and spread of syphilis.

DISEASE AND PATHOGENESIS

The incidence of human syphilis is increasing, as is often the case with diseases having such a strong sociologic component. Primary syphilis is evidenced by the development of a circumscribed lesion, called a chancre, at the inoculation site (Figure 32-2). It is initially maculopapular, but eventually develops an inflamed, necrotic, moist center. Exudate from the chancre teems with spirochetes, and this is the most infectious stage of the disease for person-to-person transmission. The chancre may be in a location that is not readily visible and may thus go unnoticed. Treatment at this stage results in a nearly 100% cure rate, but the lesion disappears without treatment in 10 to 14 days.

FIGURE 32-2 Extragenital syphilitic chancre on the right middle finger.

(Courtesy Public Health Image Library, PHIL #4146, Centers for Disease Control and Prevention, Atlanta, date unknown.)

This return to seeming normalcy is deceptive. Spirochetes move, from their primary location on mucous membranes, to the bloodstream, which in due time leads to the development of secondary syphilis. Fever and rash are characteristic of this stage of the disease, and T. pallidum is found in the erythematous skin lesions. These clinical signs occur in most, but not all, infected people, and treatment is generally effective at this stage. However, the secondary symptoms will also resolve spontaneously, and a surprising number of cases go untreated.

Treponemes remaining after the secondary stage invade cardiac muscle, the musculoskeletal system, and the central nervous system (CNS) in tertiary syphilis. The antibody response remains strong, and this stage is relatively noninfectious. Symptoms of the tertiary stage develop slowly; cardiac damage and CNS symptoms are common, and an enormous portion of healthcare dollarsin the United States are devoted to providing care for the syphilitic insane.

A major concern is transplacental transmission during latent infections, resulting in congenital syphilis at a rate approaching 1000 U.S. cases annually. The infection manifests in malformed teeth and long bones, cardiac lesions, and CNS effects leading to learning disabilities and mental retardation.

Elucidation of virulence mechanisms of T. pallidum has been limited by failure of all attempts to cultivate it in vitro. Furthermore, there is no suitable in vivo model of human disease. Treponema pallidum can be maintained by serial passage in rabbit testicles, a process in which laboratory technicians have approximately the same risk of occupational exposure to the organism as street-based sex workers. However, symptoms (in the rabbits) are limited to rather severe orchitis in steroid nontreated animals, and no visible symptoms at all if steroidal antiinflammatory drugs are used. Chancres resembling those in human syphilis are produced by intradermal inoculation of rabbit skin, and these resolve much as would be expected in humans. However, the infection does not become systemic. Information gleaned from study of natural cases and limited work with in vivo–grown organisms has revealed that T. pallidum adheres to and invades cell monolayers (Figure 32-3).

FIGURE 32-3 Electron photomicrograph of Treponema pallidum on cottontail rabbit epithelial cells.

(Courtesy Public Health Image Library, PHIL #1975, Centers for Disease Control and Prevention, Atlanta, 1980, David Cox.)

Treponema pallidum has a surprising lack of outer-membrane proteins, and the treponemal equivalent of porin activity (to allow diffusion of nutrients through the outer membrane) remains undiscovered. There is likely some sort of active transport mechanism, because T. pallidum is susceptible to β-lactam antibiotics, and these are not thought to diffuse through membranes.

Treponema pallidum may protect itself from the immune response by cloaking with host proteins such as α₂-macroglobulin, albumin, and class I major histocompatibility complex (MHC) proteins. This strategy is obviously not completely successful because individuals with syphilis do produce antibodies against the organism. Fibronectin may mediate treponemal attachment to host cells, and in processing a treponemal antigen–fibronectin complex, the immune system may produce antibodies against fibronectin itself, with subsequent immune-mediated damage to cardiopulmonary, musculoskeletal, and central nervous systems in tertiary syphilis.

Indirect effects of T. pallidum may cause local tissue damage. Binding of the organism to cultured endothelial cells stimulates expressionof intercellular adhesion molecule 1 (ICAM-1), which in turn binds to integrins on the surfaceof neutrophils and enables their emigration into surrounding tissue. Endothelial cells are also stimulated to produce cytokines, and treponemal lipoproteins activate tumor necrosis factor-α (TNF-α) and interleukin-1 (IL-1) production by macrophages. Both may contribute to the inflammation, which plays such a prominent role in pathogenesis of primary and secondary syphilis.

DIAGNOSIS

The Wassermann test, and other tests based on anticardiolipin (rather than antitreponemal) antibodies are useful, but prone to false positives. A variety of specific tests now available are basedon bacterial immobilization by antibodies, fluorescent antibody assay, and other detection systems.

Treponema brennaborense and Hairy Footwart

Disease and Etiology.

Bovine papillomatousdigital dermatitis (PDD; digital dermatitis, hairy footwart) was first described nearly 30 years ago in Italy and shortly thereafter in the United States. It is now widely distributed in North America, affecting cattle in more than 40% of U.S. dairy herds. There is at least one report of papillomatous pastern dermatitis, of apparent spirochetal etiology, in a horse.

The incubation period of PDD is about 3 weeks. Its clinical presentation is episodic lameness of variable severity, as a result of acute or chronic ulceration of the skin on the bulbs of the heel or the interdigital cleft and often just above the perioplic horn of the heels. In intense cases, cows often walk on their toes and hooves are clubbed. Lesions are most commonly associated with plantar or palmar skin adjacent to the interdigital space (Figure 32-4). Both medial and lateral digits of individual limbs are involved in most animals.

FIGURE 32-4 Gross lesions of papillomatous digital dermatitis.

(Courtesy Deryck Read.)

Erosions can encompass the superficial layers of the epidermis, and epithelial hyperplasia and hypertrophy, hemorrhage, pain, swelling, and foul odor are hallmarks of the disease. Lesions average 4 cm in diameter, are circular to oval, and are most common on the hind feet. They have the granular, red appearance of a strawberry and are often surrounded by a ridge of hyperkeratotic skin with hypertrophied hairs. Proliferation offiliform papillae increases as lesions mature, with formation of long wartlike projections. Microscopic examination reveals epidermopoiesis and papilloma formation, with perivascular aggregationof inflammatory cells. Silver staining of sections reveals invasive spirochetes deep in lesions, invading the stratum spinosum in company with coccoid and large bacillary forms. However, spirochetes are also found, essentially alone, in deeper tissues.

A novel treponeme isolated from typical lesions was determined, based on chemotaxonomy, protein profiling, and analysis of 16S rDNA sequences, to be a new species, and was named T. brennaborense. It is small and highly motile, with two periplasmic flagella arranged in a 1-2-1 pattern (Figure 32-5). It has α-glucosidase and N-acetyl-β-glucosaminidase activity, and growth is inhibited by rabbit serum. Comparative 16S rDNA sequence analysis of multiple isolates revealed three phylotypes clustered with the saprophytic human oral (Treponema denticola and Treponema vincentii) or genital (Treponema phagedenis) treponemes. Treponema brennaborense is probably most closely related to Treponema maltophilum, from human periodontitis. Differential distribution in lesions suggests that development of deep lesions may correlate with the presence of a particular phylotype or combination of phylotypes. Immunohistochemical examination of lesion biopsies reveals that organisms from many countries are antigenically related.