Case 1 introduction

 Etiologic agent of erythema migrans: Borrelia burgdorferi.

 Primary reservoir of infection: Small rodents, primarily the white-footed mouse.

Primary vector of transmission: Ixodes tick.

Summary: A 28-year-old woman who has been recently camping in the New England area complains of fever and a skin rash consistent with erythema migrans. Confirmatory serologies are sent.

CLINICAL CORRELATION

Introduction

Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to humans by Ixodes ticks. This disease was first recognized in Old Lyme, Connecticut, with the identification of cluster cases of arthritis in children. The infection is characterized by a "bull's eye" skin lesion, which develops from the site of the tick bite, 1-4 weeks postinfection. Additional initial symptoms include fever, fatigue, headache, joint pain, or mild stiffness of the neck. Lyme disease is the most common vector-born disease in the United States, and if left undiagnosed and untreated, the infection usually progresses to involve the nervous or vascular systems and cause fluctuating or chronic arthritis.

Approach to Borrelia Species

Definitions

Erythema migrans: Skin lesion composed of redness (erythema) with central clearing (target lesion).

Spirochetes: Thin spiral bacteria of which three genera cause significant disease in humans: Leptospira, Borrelia, and Treponema, which lead to leptospirosis, lyme disease, and syphilis, respectively (see Table 3-1 for an abbreviated listing).

Objectives

1. Know the characteristics and virulence factors of B. burgdorferi.
2. Know the reservoir, vector, and host involved in the transmission of B. burgdorferi.

Discussion

B. burgdorferi belongs to the spirochete family of prokaryotes. It stains gram-negative, although spirochetes are considered neither gram positive nor gram-negative. Spirochetes consist of a flexible, multilayer outer cell membrane and a more rigid, peptidoglycan-containing cytoplasmic membrane. Between these two layers are endoflagella that insert at the ends of the spirochete. Rotation of these flagella creates the characteristic cork-screw shape of these organisms. This provides for motility of the organism and hides the normally antigenic flagella from host defenses. These organisms are microaerophilic and have a doubling time of 8-24 hours. The disease is endemic in several regions of the United States including Northeastern, Midwest, and Pacific coast states. However, most reported cases occur in New York, Connecticut, Pennsylvania, and New Jersey.

The spirochetes that cause Lyme disease have been divided into genospecies. Three genospecies, B. burgdorferi sensu stricto, B. garinii and B. afzelii, are known to cause Lyme disease and are known collectively as B. burgdorferi sensu lato. The outer membrane of B. burgdorferi contains unique outer surface proteins (Osps), which are thought to play a role in their virulence. Small rodents, particularly the white-footed mouse, are the primary reservoirs of B. burgdorferi, and the vector of transmission is the Ixodes tick. The larva of the tick is born uninfected. The ticks become infected with the spirochete on feeding on an infected animal. This usually occurs during the nymph stage of the tick's life cycle. The spirochetes multiply in the gastrointestinal (GI) tract of the tick, and then are transmitted to the animal host by regurgitation or salivation during a subsequent feeding. B. burgdorferi are next transmitted to humans via tick bite followed by dissemination through the bloodstream to the joints, heart, and central nervous system (CNS). The nymphal stage of the tick is more infective than the adult and larval stages. Most exposures to Borrelia occur between the months of May and July, when the nymphs are most active.

Clinically there are three stages of B. burgdorferi infection: stage 1, which occurs in the first 1-4 weeks postinfection, involves the initial characteristic skin lesion referred to as "erythema migrans"; stage 2 follows months postinfection with neurologic and cardiac involvement; and stage 3 results in chronic arthritis of the joints.

Diagnosis

The diagnosis of Lyme disease is made primarily by clinical presentation and patient history of exposure. Confirmation of a clinical diagnosis is made serologically via the detection of antibody by enzyme-linked immunoabsorbance or indirect immunofluorescence. However, serologic tests are most reliable 2-4 weeks post infection, because of cross-reactivity with normal flora, and Western blot analysis should be used to confirm a positive serologic test. Alternately, new PCR-based tests are available to detect B. burgdorferi DNA. B. burgdorferi is difficult to grow in culture, requiring complex culture media and a microaerophilic environment. It is also difficult to visualize under light microscopy, but can be seen under darkfield microscopy or when stained with Giemsa or silver stains.

[3.1] A 9-year-old boy presents with a migratory rash with central clearing on the back of his neck. The child had recently been on vacation with his family in Oregon and had gone hiking. The child's pediatrician observes the rash and suspects an infection with Borrelia burgdorferi. Which of the following is thought to be a virulence factor of this organism?

[3.2] If the child's infection is left untreated, which of the following symptoms would most likely appear?

[3.3] A small tick, of the genus Ixodes, most commonly transmits Borrelia burgdorferi. Which of the following diseases is also transmitted by a tick?

[3.1] E. Differential expression of outer surface proteins is thought to be involved with virulence; answers A, B, C, and D are incorrect: (A) intracellular growth in leukocytes is a virulence factor of Ehrlichia; (B) endotoxins are characteristic of gram-negative organisms, not Borrelia; (C) localization in reticuloendothelial cells occurs in infections with Francisella tularensis; (D) an antiphagocytic capsular antigen is not a virulence factor of Borrelia.

[3.2] D. Later stages of infection with Borrelia burgdorferi include arthritis, meningitis, nerve palsies, and cardiovascular abnormalities; answers A, B, C, E, are incorrect: (A) arthritis, not urethritis, is a later manifestation of infection with Borrelia burgdorferi; (B) the skin rash or erythema migrans expands centrifugally, not centripetally; (C) biphasic illness with fever and chills occurs more commonly with Leptospira infections; (E) swelling of lymph nodes is more commonly associated with Yersinia infections.

[3.3] C. Similar to Lyme disease, Ehrlichiosis is also transmitted via a tick vector; answers A, B, D, and E are incorrect: (A) Q fever is most commonly transmitted via inhalation of dried feces or urine contaminated with rickettsiae; (B) Leptospirosis is typically transmitted via ingestion of contaminated food or water; (D and E) both yellow fever and eastern equine encephalitis are transmitted by mosquitoes.

A 19-year-old male college student presents to the student health department with abdominal pain, diarrhea, and fever. He says that his symptoms started one day ago. He has had 10 stools in the past day and has noted blood mixed in with the stool on several occasions. He usually eats at home but reports having eaten chicken in the college cafeteria three days ago. He has no history of gastrointestinal (GI) disease. On examination he has a temperature of 37.8C (100F) and appears to be in pain. His abdomen has hyperactive bowel sounds and is diffusely tender but without rigidity, rebound tenderness, or guarding. A general surgeon is consulted and is considering the diagnosis of acute appendicitis versus bacterial gastroenteritis possibly related to the chicken eaten. A stool sample tests positive for blood and fecal leukocytes. Stool cultures are sent and are subsequently positive for a pathologic organism.

 What is the most likely pathologic organism?

 In what atmospheric environment does this organism grow?

 Most likely etiology of this infection: Campylobacter jejuni.

 Preferred atmospheric environment: Microaerophilic (presence of increased levels of carbon dioxide).

Summary: A 19-year-old man presents with a bacterial gastroenteritis that mimics appendicitis.

CLINICAL CORRELATION

Introduction

More than 50 serotypes of C. jejuni have been identified based on heat labile (capsular and flagellar) antigens. C. jejuni is endemic worldwide, and most cases of infection are associated with eating chicken, although milk, water and other meats have also been implicated. Human-to-human transmission is rare. C. jejuni is one of the most frequent causes of bacterial diarrhea occurring most often in the summer or early fall. The incubation period is 1-3 days followed initially by symptoms of fever, malaise, and abdominal pain. C. jejuni can cause bloody diarrhea, mucosal inflammation, and bacteremia, suggesting that it is invasive to the lining of the intestine. Most cases of Campylobacter gastroenteritis is self-limited, with symptoms resolving within 7 days; however, relapses can occur in 5-10 percent of cases which are untreated.

Complications of Campylobacter gastroenteritis include pancreatitis, peritonitis, or more uncommonly arthritis, osteomyelitis, and sepsis. A serious postinfection sequelae of Campylobacter gastroenteritis is Guillain-Barre syndrome, an acute demyelinating disease. Antigenic similarities between the lipopolysaccharides on the surface of some serotypes of C. jejuni and myelin proteins are thought to be responsible for Guillain-Barre disease.

Other Campylobacter species such as C. coli also cause gastroenteritis, which is clinically indistinguishable from C. jejuni infection. C. fetus is primarily a cause of bacteremia, septic arthritis, peritonitis, abscesses, meningitis, and endocarditis in immunocompromised patients.

Most often Campylobacter jejuni infection is self-limited and does not require specific antimicrobial therapy. Supportive care, that is, hydration, is often the only treatment needed. If specific therapy is needed for severe disease, or infection in immunocompromised patients, erythromycin is the drug of choice, because of the recent increase in resistance to fluoroquinolones.

Prevention involves care in food preparation. Foods, especially chicken, should be completely cooked, and exposure to raw or undercooked chicken or unpasteurized milk should be limited, especially in pregnant or immunocompromised persons.

COMPREHENSION QUESTIONS

[4.1] Which of the following are the special laboratory conditions needed to recover Campylobacter jejuni?

[4.2] A 21 year-old woman presents to the emergency room with shortness of breath 2 weeks after recovering from a "stomach flu." Physical exam reveals ascending muscle weakness that began in her toes. Cardiac irregularities are also notable. A review of the patient's chart revealed that a bacterial stool culture 2 weeks earlier, during the patient's "flu" episode, found comma-shaped organisms growing at 42C (107.6F). Which of the following pairs represents the causative agent of this patient's flu and the postflu condition, respectively?

[4.3] In a nonimmunocompromised patient with Campylobacter jejuni as the causative agent of their food poisoning, which of the following is the treatment most often required?

[4.4] A 20-year-old college student develops diarrhea that lasts for approximately 1 week. Stool cultures reveal a motile, microaerophilic gram-negative rod that is isolated by incubation at 41C (105.8F) on medium containing antibiotics. This organism is most likely to be which of the following?

[4.1] C. The isolation and identification of Campylobacter jejuni can be achieved using special culture characteristics. Three requirements must be met. First, a selective medium is needed. There are several selective media in widespread use: Skirrow's medium uses vancomycin, polymyxin B, and trimethoprim; other selective media contain cefoperazone, other antimicrobials, and inhibitory compounds. The selective media are suitable for isolation of C. jejuni at 42C (107.6F); when incubated at 36-37C (96.8-98.6F), other Campylobacters and bacteria may be isolated. Finally, incubation must be in an atmosphere with reduced oxygen and added carbon dioxide. The colonies appear to be colorless or gray and may be watery and spreading or round and convex.

[4.2] A. Guillain-Barre syndrome (acute idiopathic polyneuritis) is associated with infections such as herpesvirus and Campylobacter jejuni (comma-shaped bacteria that grows at 42C [107.6F]). It is believed that some C. jejuni serotypes have surface lipopolysaccharides that are antigenically similar to myelin protein leading to the inflammation and demyelination of peripheral nerves and ventral root motor fibers. Suspected Guillain-Barre in a patient is always a medical emergency because respiratory distress or failure can ensue, and the patient should always be admitted to the hospital for careful treatment and observation. The other answers contain correct matching of the causative agent with the resulting condition, but do not reflect the clinical scenario described.

[4.3] E. Most infections with C. jejuni are self-limiting and thus do not require specific antimicrobial therapy, except in cases of severe disease and infection in immunocompromised individuals. Therefore, most often the only required therapy is hydration and supportive care. When specific antimicrobial therapy is indicated, the drug of first choice is erythromycin, with alternate drugs being tetracycline, ciprofloxacin, and ofloxacin.

[4.4] D. Based on the culture characteristics indicated above, the only possible answer is Campylobacter jejuni. Please also refer to the discussion for Question 4.1.
CASE 5

INTRODUCTION

A 19-year-old woman presents for the evaluation of pelvic pain. The pain has progressively worsened over the past week. She has also been having some burning with urination and a vaginal discharge. She is sexually active, has had 4 lifetime partners, takes oral contraceptive pills, and occasionally uses condoms. On examination, she appears in no acute distress and does not have a fever. Her abdomen is soft with moderate lower abdominal tenderness. On pelvic examination, she is noted to have a yellow cervical discharge and significant cervical motion tenderness. No uterine or adnexal masses are palpated, but mild tenderness is also noted. A Gram stain of the cervical discharge reveals only multiple polymorphonuclear leukocytes. A direct DNA probe test subsequently comes back positive for Chlamydia trachomatis.

 How does C. trachomatis enter a target cell?

 What are the two stages of the C. trachomatis life cycle?

 How C. trachomatis enters a target cell: the elementary body of C. trachomatis binds to receptors on the host and induces endocytosis.

Other Chlamydial species are known to cause disease in humans. Atypical pneumonia is caused by Chlamydia pneumoniae, and presents with fever, headache, and a dry hacking cough. Additionally, psittacosis is another atypical pneumonia caused by Chlamydia psittaci. This organism is acquired by inhalation of feces from infected birds, which serve as the reservoir.