Coccidioides

Two forms of Coccidioides

• in soil, forms conidia within the mycelium, not externally like most other fungi

• when these are broken up and inhaled, they begin to round up (forming a spherule) and divide internally until they are full off endospores and release them (~96 hours after inhalation)

SEM and TEM of arthroconidia of Coccidioides

• these mycelia contain arthroconidia, which are fungal spores produced by segmentation of pre-existing hyphae (Wikipedia)

• every other cell in the hyphae is an empty cell that can be broken by wind to form one infectious particle

• then, these round up and form spherules after inhalation

micrograph

• internal division inside the spherule occurs; the endospores get smaller and smaller until the spherule ruptures and pours them all out

• each endospore has the capacity to grow into a completely mature spherule or, if coughed up, to form more mycelia in the soil

• so it can go either way, and what makes the decision is the environment

silver stain micrograph

• polys are attacking the endospores released from the spherule

Coccidioides is geographically localized

• first identified in the San Joaquin Valley, particularly in Bakersfield

• also, Phoenix and Tuscon, West Texas, and Mexico

• the Sonoran desert is pretty much endemic for coccidioidomycosis

• it’s found in the soil in PB

C. immitis and C. posadasii

• the former is found primarily in California

• the latter is found primarily elsewhere—US but not CA, Mexico, South America

• the two are distinguished by their DNA structure

• they separated 12 million years ago

• however, they are clinically identical

epidemiology

• found in desert regions in the southwestern US and in Mexico; you can indeed get cocci in SD or LA

• risk factors involve disturbing soil

• most common in the fall

• variable year-to-year

pathogenesis

• organism is inhaled

• sx in 35-60%

• 95% of infections resolve spontaneously in the immunocompetent

• about 5% either linger for months or disseminate beyond the lungs

• there is a much higher rate of dissemination in the immunocompromised

most common sites of dissemination

• skin

• bones and joints

• meninges: coccimeningitis without treatment has a 1-year mortality rate of 100%

T cells are important for immunity

• when it was available, positive skin test was a good prognostic sign. the test is no longer performed

• transplant, chemo rx, and AIDS predispose to disseminated disease

• in mice, T cells transfer immunity

is a Th1 immune response required?

• high titers of antibody (higher than 1:16) are associated with poor prognosis in people

• exogenous IL-12 improves outcomes in mice

• IL-10 knockout mice are more resistant to infection

coccidioidomycosis at Lemoore US Naval Air Station, 1961 to 1977

• African-Americans and Filipinos are at a high risk for disseminated cocci

• similarly, some mice strains are more susceptible

diagnosis

• antibody titer is sensitive and specific

• complement fixation titer > 1:16 is associated with disseminated disease

• culture is the gold standard

Category: Microbiology Notes