| Penicillium marneffei | ||||||||||||||
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| ''Penicillium marneffei |
Penicillium species are usually regarded as unimportant in terms of causing disease. Penicillium marneffei, discovered in 1956, is different. This is the only thermally dimorphic Penicillium, and it can cause systemic infection, creating a fever and anaemia similar to disseminated cryptococcosis.
Epidemiology
Discovered in bamboo rats in Vietnam, it is still associated with both the rats, and the area. Penicillium marneffei is endemic in Burma (Myanmar), Cambodia, Southern China, Indonesia, Laos, Malaysia, Thailand and Vietnam.
Cases have also been reported in HIV positive patients in Australia, Europe, Japan, the UK and the US. All the patients had visited Southeast Asia previously.
Although both the immunocompetent and the immunocompromised can be infected, it is extremely rare to see systemic infection in HIV negative patients.
There is a high incidence of penicilliosis in AIDS patients in SE Asia, 10% of patients in Hong Kong get penicillosis as a AIDS related illness. The incidence of P. marneffei is increasing as HIV spreads throughout Asia. An increase in global travel and migration means it will be of increased importance as an infection in AIDS sufferers.
Penicillium marneffei has been found in bamboo rat faeces, liver, lungs and spleen. It has been suggested these animals are a reservoir for the fungus. It is not clear whether the rats are affected by P. marneffei or are merely asymptomatic carriers of the disease.
One study of 550 AIDS patients showed that the incidence was highest during the rainy season, which is when the rats breed.
Another stuffy could not establish contact with bamboo rats as a risk factor, but exposure to the soil was the critical risk factor. However soil samples failed to find much of the fungus.
It is not known whether people get the disease by eating infected rats, or by inhaling fungi from their faeces.
There is an example of and HIV positive physician who was infected while attending a course on tropical microbiology. He did not handle the organism, though students in the same laboratory did. It is presumed he contracted the infection by inhaling aerosol containing P. marneffei conidia. This shows that airborne infections are possible.
To summarise, Penicillium marneffei is a thermally dimorphic fungus endemic to Southeast Asia, and a common infection in AIDS patients there. It is also found in the bamboo rat, and has a higher prevalence in the rainy season. It can be contracted by aerosol, and also through consumption.
Plating it out onto two SAB platess, then incubating one at 30°C and the other at 37°C, should result in two different morphologies. A mold-form will grow at 30°C, and a yeast-form at 37°C.
On the 30°C plate, after two days, mycelial colonies will be visible. The growth starts off fluffy and white and eventually turns green and granular. A soluble red pigment is produced, which diffuses into the agar, causing the reverse side of the mold to appear red or pink. The periphery of the mold may appear orange colour, and radial sulcate folds will develop.
Through a microscope, the mold phase will look like a typical Penicillium. Short, hyaline, septate and branched hyphae. With conidiophores located both laterally and terminally. Each conidiophore gives rise to three to five phialides, with chains of “lemon-shaped” conidia are formed.
On the 37°C plate, the colonies grow as a yeast. These colonies can be cebriform, convoluted, or smooth. There is a decreased production in pigment, the colonies appearing cream/light-tan/light-pink in colour. Microscopically, the ‘sausage-shaped’ cells are mixed with hyphae-like structures. As the culture ages, segments begin to form. The cells divide by binary fission, rather than budding. The cells are not yeast cells, they are arthroconidia.
Culturing isn’t the only method of diagnosis. A skin scraping can be prepared, and stained with Wright’s stain. Many intracellular and extracellular yeast cells with crosswalls are diagnostic of P. marneffei infection. Smears from bone marrow aspirates may also be taken; this is regarded as the most sensitive method, this can be stained with the Giemsa stain. Histological examination can also be done on skin, bone marrow or lymph nodes.
The patient’s history also is a diagnostic help. If they have traveled to South-East Asian and are HIV positive, then there is an increased risk of them having penicilliosis.
Antigen testing in the urine and serum have been trialed, with high sensitivities and specificities.
PCR amplification of specific nucleotide sequences has been achieved.
Rapid identification of penicilliosis is sought, as delays in treatment are unwanted. Treatment should be given as soon as penicilliosis is suspected.Laboratory diagnosis
The fact Penicillium marneffei is thermally dimorphic, is a big clue when trying to identify it. Culture should be done on bone marrow, skin, blood and sputum.