Infectious Disease and Microbiology - Afipia Species

Basics

Afipia is a genus of aerobic gram-negative bacteria that includes Afipia broomeae, A. clevelandensis, and A. felis. These organisms have been investigated as possible human pathogens, but their overall role in clinical disease remains uncertain.

Historically, A. felis attracted considerable attention because it was once proposed as the cause of cat-scratch disease. Subsequent research showed that most cases of cat-scratch disease are actually caused by Bartonella species, particularly Bartonella henselae.

Microbiologic Characteristics

Afipia species are pleomorphic, aerobic, gram-negative bacilli. Because they may stain poorly with routine methods, they can sometimes be demonstrated more clearly with silver-based stains.

Their variable morphology and difficult visualization can make laboratory recognition challenging.

Clinical Significance

The pathogenic importance of Afipia species in humans is not fully established.

Although these organisms have occasionally been isolated or detected in patients with compatible clinical syndromes, they are not considered common human pathogens.

The most important historical association is with cat-scratch disease.

Cat-Scratch Disease Association

For a period of time, Afipia felis was believed to be the primary cause of cat-scratch disease.

Current evidence indicates that this is not the case.

Most cat-scratch disease is caused by:

Bartonella henselae

Other Bartonella species may occasionally be involved.

A. felis may possibly account for a small minority of cases or may be detected in unusual clinical circumstances, but its precise role remains uncertain.

Clinical Presentation

If Afipia is involved in a cat-scratch-like syndrome, the presentation may resemble typical cat-scratch disease.

Possible manifestations include:

• Regional lymphadenopathy

• Fever

• Malaise

• Tender or enlarged lymph nodes

• A papule or lesion near the site of inoculation

However, because Bartonella henselae is much more strongly associated with this syndrome, Bartonella infection should usually be considered first.

Diagnosis

Diagnosis is not straightforward because Afipia species are rarely encountered and routine clinical testing is limited.

Histologic examination of affected lymph nodes may help when the organism is suspected.

Silver stains can improve visualization because these gram-negative bacilli may be difficult to identify with standard staining techniques.

Histopathology

Affected lymph nodes may show inflammatory changes resembling those seen in cat-scratch disease.

Depending on the stage of disease, findings may include:

• Granulomatous inflammation

• Necrosis

• Suppurative changes

• Reactive lymphoid hyperplasia

These findings are not specific for Afipia, so clinical correlation and exclusion of more common causes are necessary.

Differential Diagnosis

The major differential diagnosis is:

Bartonella henselae infection

Other causes of chronic or subacute lymphadenopathy should also be considered, including:

• Tuberculosis

• Nontuberculous mycobacteria

• Toxoplasmosis

• Tularemia

• Lymphoma

• Other bacterial lymphadenitides

The patient’s exposure history and laboratory findings help guide the diagnosis.

Treatment

There is no well-established antimicrobial regimen for Afipia infection because convincing clinical data are limited.

Macrolide antibiotics may have activity and have sometimes been used when treatment is considered necessary.

Potential agents include:

• Azithromycin

• Clarithromycin

However, evidence supporting a specific regimen or duration is insufficient.

General Management

When a patient presents with a syndrome resembling cat-scratch disease, management should primarily be directed by the more likely etiologic agent, especially Bartonella henselae.

If Afipia is specifically identified, treatment decisions should be individualized according to:

• Clinical severity

• Site of infection

• Host immune status

• Microbiologic findings

• Response to therapy

Prognosis

Because Afipia-associated disease is rare and incompletely defined, prognosis is not well characterized.

If infection is limited to a cat-scratch-like lymphadenitis syndrome, the course would generally be expected to be relatively benign, but this remains based on limited clinical experience.

High-Yield Taxonomy

Important species include:

• Afipia broomeae

• Afipia clevelandensis

• Afipia felis

High-Yield Historical Association

A. felis

was once believed to cause:

Cat-scratch disease

But most cases are now known to be caused by:

Bartonella henselae

Exam Essentials

Genus:

→ Afipia

Organism type:

→ Pleomorphic aerobic gram-negative bacillus

Useful stain:

→ Silver stain

Historical disease association:

→ Cat-scratch disease

Current major cause of cat-scratch disease:

→ Bartonella henselae

Possible role of A. felis:

→ Rare or minority contributor, but uncertain

Diagnosis:

→ Histologic examination of affected lymph nodes, supported by specialized microbiologic methods

Treatment:

→ No established standard regimen

Potentially useful drug class:

→ Macrolides

Key clinical pearl:

→ If a patient has classic cat-scratch disease, think Bartonella first, not Afipia.