A fungus that commonly grows on pigeon droppings can cause meningitis. In this lesson, we will look at the yeast Cryptococcus, and its ability to cause disease in humans unfortunate enough to breathe it in.
Nearly every city in America, regardless of size, is teeming with wildlife. One thing you should always remember is that wild animals can be dangerous. So, the next time you are walking down the sidewalk in Pittsburgh, L.A. or Omaha, keep your eyes peeled, your head on a swivel and try not to be the next victim of an attack by Columba livia, the devastating Rock Pigeon!
Ok, this might be a bit dramatic. No one is getting mauled by vicious packs of pigeons. Besides flocking around you begging for French fries, the worst thing you have to fear are unexpected bombs deposited on your head as the pigeon flies away. Dodging the droppings might save your clothing but that pigeon poo, if left to dry on the sidewalk, could turn dangerous. If you find that hard to believe, let me introduce you to the fungal pathogen Cryptococcus.
There are not many fungal diseases that have a major impact on human health. One of the few exceptions is Cryptococcus, an oval-shaped yeast that can cause fungal meningitis. There are two major species that cause disease in humans, C. neoformans and C. gattii. C. gattii is found primarily in tropical regions but has recently been gaining a foothold in the Pacific Northwest. C. neoformans is widespread throughout the United States, so for the rest of this lesson, I will focus on this species, referring to it simply as Cryptococcus.
Cryptococcus is classified as a yeast, meaning it is a single-celled fungus. The yeast is found in soils around the world, but is especially common in urban areas. The reason for the higher concentration in and around cites is because the yeast grows very well in bird droppings. High populations of pigeons provide a natural and continuous source of fungal growth media. The growing yeast cells can be blown into the air by wind. Cells that land on bird poo can grow and reproduce very quickly, producing even more cells. The cells themselves have a very thick, gelatinous capsule, which protects them from drying out while airborne and from being attacked by the immune system.
I want to take a minute here and make a point about Cryptococcus infection. The fungus does not infect the pigeon itself, it only grows on the feces. Cryptococcus is also not passed from person to person or animal to person. Infection requires inhaling airborne cells.
Cryptococcosis and Fungal Meningitis
As the fungus is dispersed by wind, it is inevitable that some unsuspecting person, or more realistically, many people, are breathing in these cells. If the person has a healthy immune system, nothing usually results from this exposure and symptoms rarely develop. When they do develop, the disease is called Cryptococcosis. The yeast cells penetrate the air sacs of the lungs and typically cause a very minor pneumonia-like infection. This results in brief fever, shortness of breath and chest pains.
Occasionally, Cryptococcus can move from the lungs into the bloodstream and develop into a much more dangerous disease. The Cryptococcus cells in the blood tend to localize to the meninges, which are the membranes that cover the brain and spinal cord. This can cause a potentially fatal fungal meningitis, which is simply inflammation of the brain and spinal meninges caused by fungal infection.
Meningitis can be caused by bacteria and viruses as well, but fungal meningitis is considered to be slower and chronic. The symptoms, including piercing headaches, neck stiffness, confusion, weakness and paralysis, come on very slowly. It's common for people to disregard the early symptoms and pass them off as other illnesses. Fungal meningitis without treatment is 90% fatal, as the disease causes a severe swelling of the brain.
Diagnosis and Treatment
Before the disease can be treated, a positive diagnosis is required. Most often, yeast cells are observed microscopically in samples of mucus or spinal fluid. Recently, blood tests that identify antibodies to Cryptococcus have been used to provide a more accurate diagnosis.
Armed with a diagnosis, treatment depends on the symptoms. In the case of mild lung infections, fluconazole or itraconazole can easily eliminate the fungus. If the patient has developed fungal meningitis, treatment is more involved. Amphotericin B, given intravenously, is very successful in even the worst cases. There are a couple of downsides, though. It can often take six months or more of treatment to cure the disease. Amphotericin B also causes side effects like anemia and kidney damage. The patient requires constant monitoring to ensure these side effects don't get too severe.
Infection by Cryptococcus is very rare, impacting only about one in 100,000 people in the United States. The rarity is more impressive when you consider this fungus is prevalent in cities where large populations live in very close proximity.
There is a population at greater risk of Cryptococcus: those with compromised immune systems, like AIDS patients. For this group, Cryptococcus infects about 450 in 100,000 people and can be devastating. Prompt treatment and better medications have been reducing the number of deaths in AIDS patients, but Cryptococcus is still a potentially life-threatening illness.
It's time to review. Cryptococcus is a single-celled, oval-shaped yeast that can cause fungal meningitis. C. neoformans is the most common species, but C. gattii has been increasing in prevalence recently in the Pacific Northwest.
Cryptococcus is normally found in soil, but the fungus grows very well in bird droppings. The high populations of pigeons in cities have created a natural source of growth media for Cryptococcus in urban areas. The fungal cells growing on feces can get stirred up by the wind. If these airborne cells are inhaled by a human, Cryptococcosis can develop. Most infections never produce symptoms in healthy individuals but some people can develop minor pneumonia-like symptoms of shortness of breath, fever and chest pains. If the yeast invades the blood, fungal meningitis can result. Localizing to the brain and spinal cord, Cryptococcus can cause headaches, neck stiffness, paralysis and even death.
Treatment is generally very effective. Fluconazole or itraconazole can easily eliminate the fungus in mild respiratory cases. Intravenous Amphotericin B is very successful in even the worst cases of meningitis. The treatment can take over six months to eliminate the fungus and often causes severe side effects. Fortunately, the disease is rare and usually only a concern for those with compromised immune systems.
Following this lesson, you'll be able to:
- Identify the structure of Cryptococcus
- Explain how Cryptococcus can be transmitted
- Describe symptoms of Cryptococcosis and fungal meningitis
- Summarize the treatment options when one is infected with Cryptococcus