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Pneumocystis Pneumonia: Infections Caused by Pneumocystis jiroveci

  • 0:06 Silent Infant Pneumonia
  • 1:06 Pneumocystis jiroveci
  • 2:57 Pneumocystis Pneumonia
  • 4:45 Diagnosis and Treatment
  • 5:31 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

AIDS patients have a lot to worry about. In this lesson we examine Pneumocystis jiroveci, the fungal pathogen that can cause life-threatening pneumonia in AIDS patients due to their compromised immune function.

Silent Infant Pneumonia

You might not believe me, but there is an excellent chance that you have already been infected by an organism that causes a potentially deadly form of pneumonia, a pneumonia so virulent that most people that develop symptoms die regardless of treatment. To make matters worse, you were probably infected before the age of four, before your immune system was even fully developed. Have your parents ever told you the story of your harrowing infant battle with pneumonia? No? Maybe it's because you caught it from them and they don't want you to guilt trip them.

In truth, unless you were born with limited immune function, you never got outwardly sick or showed any symptoms of pneumonia. But, you probably carry antibodies to the causative organism that serve as evidence of past infection. It is even likely that you are still carrying this organism as a passenger in your body. So, the next baby to be infected might just get it from you.

Pneumocystis Jiroveci

We are talking about Pneumocystis jiroveci, a deadly fungal pathogen that causes pneumonia in immunocompromised patients. Pneumocystis is an odd and unique fungus. For years, cancer patients were dying from a lung infection caused by what doctors concluded was a protozoan pathogen. No one was sure how the patients acquired the disease, but most of those who got it died. Postmortem exams showed an organism that exists in life stages like a protozoan.

They found resistant cysts that would grow and burst, releasing trophozoites that further spread the infection in patients' lungs. This was so reminiscent of the life cycle of protozoan pathogens like Giardia and Trypanosoma, that they classified the organisms as a protozoan with the name Pneumocystis carinii.

The advent of DNA sequencing yielded some interesting results. Now, doctors could take samples from the organism infecting patients and have its DNA sequenced. They compared this DNA sequence to the sequences of known protozoa. Unexpectedly, they didn't find a match. Instead, the sequences were similar to fungal sequences in the database.

There was no other conclusion to make; Pneumocystis carinii was actually a fungus. Admittedly, it is a unique fungus with a life cycle closer to that of a protozoan. Still, it is more closely related evolutionarily to traditional fungi. The reclassification required a name change from Pneumocystis carinii to Pneumocystis jiroveci, but still very little was known about this elusive fungus. That is, until a new, emerging disease would change all that, motivating renewed interest and research into Pneumocystis.

Pneumocystis Pneumonia

In the 1980s, HIV and AIDS exploded into worldwide prominence. The once obscure Pneumocystis jiroveci was suddenly thrust into the spotlight when it was determined that about 50% of all AIDS patients were dying of Pneumocystis pneumonia, a lung infection caused by the fungal pathogen P. jiroveci. Symptoms of the disease include fever, nonproductive cough, and difficulty breathing. Eventually, the fungus forms large, firm masses in the lungs leading to respiratory failure and death.

It now became more important to determine the source of this once rare infection. Studies showed that the vast majority of children are infected, but never show any signs of disease, before they reach the age of four. This suggests that the fungus is acquired sometime early in life. Later research determined that Pneumocystis is passed directly from person to person through aerosolized respiratory droplets. The infection could occur simply from parents interacting with their infants.

So, why doesn't everyone get sick? It turns out a healthy human immune system, even in young children, is more than capable of keeping the fungus in check. The organism is not eliminated, simply prevented from growing fast enough to cause any disease. Problems arise when the immune system is weakened in some way. Cancer patients undergoing chemotherapy, people on corticosteroids, severely malnourished infants, and AIDS patients typically have compromised immune function. This is enough for the Pneumocystis already present in the lungs to get ahead of the immune system and cause symptoms. Once symptoms develop, the disease is considered life threatening. If left untreated for too long, most patients will die.

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