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Histoplasmosis: Causes, symptoms and treatment

pigeon, carrier of histoplasmosis

What is histoplasmosis? 

Histoplasmosis is a lung infection caused by a fungus found in bird and bat droppings. The fungal spores are airborne, so they can enter the lungs if a person inhales air in an area where they are present. Histoplasmosis does not cause serious illness for most people. But in rare cases, it can spread to other areas, such as the eyes. 

When the infection spreads to the eyes, doctors call it presumed ocular histoplasmosis syndrome (POHS), or ocular histoplasmosis. POHS can lead to vision loss.

What is presumed ocular histoplasmosis syndrome (POHS)?

Many scientists believe that ocular histoplasmosis may be a long-term complication from a prior histoplasmosis lung infection. They believe the infection may travel through the bloodstream to the eye, causing damage to the retina. This has not been directly proven, which is why it is called presumed ocular histoplasmosis syndrome. 

In its early phases, POHS produces no symptoms. The infection causes inflammation of the retina, leaving behind tiny scars called “histo spots.” These spots do not usually affect vision, but they are the only way an eye doctor can tell that you had the infection.

Complications from histoplasmosis can occur even decades after the initial infection.  One of these complications is the growth of abnormal blood vessels in the choroid under the retina. These abnormal blood vessels can cause distortion and blank spots in your vision. The visual symptoms are similar to macular degeneration

Where is histoplasmosis usually found?

The fungal spores that cause histoplasmosis are most commonly found in the Ohio and Mississippi River valley. Areas like farms, chicken coops, construction sites and caves may have higher levels of spores. A person is more likely to inhale the spores in areas like these. 

What are the symptoms of a histoplasmosis lung infection?

Most people who have histoplasmosis do not have symptoms. Some people may have mild cold or flu-like symptoms for a week or so. 

Short-term symptoms from histoplasmosis infection may include:

  • Fever

  • Body aches

  • Cough (dry)

  • Exhaustion

  • Headache

  • Chest pain

More serious illness can occur in people with weakened immune systems. If this occurs, histoplasmosis infection can travel to other organs. People whose immune system is intact have a very low (but not zero) risk of serious illness.

Serious complications from histoplasmosis infection may include:

  • Difficulty breathing – due to lung damage causing air sacs to fill with fluid

  • Rash or red bumps – due to skin infection

  • Chest pain – due to fluid and inflammation around the heart

  • Meningitis – due to inflammation of membranes around the brain and spinal cord

Individuals at higher risk for serious complications from histoplasmosis infection are:

  • Those younger than age 2 and older than age 55

  • Cancer patients

  • Those who are HIV positive 

  • Those taking certain medications that affect their immune system, such as:

    • Corticosteroids

    • Medications used for cancer chemotherapy

    • Rheumatoid arthritis

    • Anti-rejection medicines for people who have organ transplants

How is histoplasmosis infection related to POHS?

When people disturb soil where histoplasmosis spores are present, the spores become airborne. If inhaled, they may enter the lungs and cause a lung infection. Many doctors suspect that even a mild lung infection may later spread to the eyes or to other organs.

How often does histoplasmosis cause POHS?

Around two-thirds of people test positive for exposure to histoplasmosis at some point. But only about 1.5% of people have evidence of scarring or damage from POHS during an eye exam.

Is histoplasmosis or POHS contagious?

No, neither histoplasmosis nor POHS is contagious. People cannot spread the infection to each other.

How does POHS damage the eye?

Scientists believe the infection spreads through the bloodstream from the lungs into the eyes. It then enters the blood vessels in the choroid of the eye. Researchers have not found any traces of the histoplasmosis fungus in a POHS affected eye. This suggests that the infection, not the fungus, causes the damage to the blood vessels.

Growth of abnormal blood vessels

POHS occurs when abnormal blood vessels begin to grow underneath the retina. This growth of new, abnormal blood vessels is called choroidal neovascularization, or CNV. 

These abnormal blood vessels cause scar tissue in the retina. This scar tissue replaces the healthy retina tissue. When scar tissue replaces healthy tissue in the macula, vision distortion and vision loss can occur. The macula is the central part of the retina and provides our central, most clear vision.

Leaking of abnormal blood vessels

The abnormal blood vessels growing underneath the retina not only cause scarring but can leak blood and fluid into the macula. 

The fovea, located in the central macula, gives us our clearest vision. It can also become damaged from the leaking. This can cause severe loss of central vision, which we rely on for reading and seeing fine details.

What are the early symptoms of POHS?

POHS has no symptoms in the eye in the early stage. After the early-stage inflammation is gone, scars called “histo spots” form on the retina. Histo spots do not affect vision. 

What are the later symptoms of POHS?

Abnormal blood vessels can begin to grow and cause vision issues even years after a mild case of histoplasmosis. Doctors do not know if there is a relationship between the histo spots and the abnormal blood vessels. POHS can cause central vision loss but does not affect peripheral vision.

Later symptoms of POHS include:

  • Distorted vision causing straight lines to look wavy

  • Blank or blurry spots in or around central vision

  • Loss of vibrancy of color vision

  • Colors looking different in each eye

  • Differences in the appearance of the size of an object seen by each eye

How do eye doctors diagnose POHS?

The only way to get an accurate diagnosis of POHS is to see an eye doctor for an examination.  Most people infected with histoplasmosis will not develop POHS.  But anyone who has been exposed and notices any changes in vision should consult an eye doctor. See your eye doctor right away if you notice wavy lines, blurred areas or blank spots in your vision.

An eye doctor will perform a dilated eye exam to look for:

  • Histo spots - which are the only indicators of prior histoplasmosis exposure

  • Retinal swelling - which is caused by abnormal and leaking blood vessels underneath the retina in the choroid

  • Atrophy around the optic disc -  which is damage around where the optic nerve enters the eyeball

Eye doctors may also perform:

  • Fluorescein angiography - retinal images that allow a clearer view of blood vessels in the retina. A dye injected in the arm travels to the retina and makes abnormal blood vessels easier to see

  • OCT angiography - a technique that provides high-quality 3D images of blood vessels without a dye injection

  • Amsler grid test - a grid that checks for wavy, dark or blurred areas of vision

What are the treatments for POHS?

POHS is one of the leading causes of irreversible vision loss in middle-aged adults in the Ohio and Mississippi River Valley. It is important to get treatment early, before the retina is severely damaged.

Treatments for POHS may include:

  • Anti-VEGF treatments - reduce the growth and leakage of abnormal blood vessels

  • Photodynamic therapy - seals off leaky blood vessels using a combination of laser light and special medicine

  • Laser treatment - destroys abnormal blood vessels using a high energy light beam 

  • Steroid injections - reduce swelling

What if POHS has already caused vision damage?

It can be very challenging to perform activities of daily life with central vision damage. There are many resources and technologies to help people who have a visual impairment. Magnifiers, special electronic devices, and other assistive technology, often referred to as low vision aids, are available. Low vision specialists can provide devices and resources to help those affected by POHS vision loss.

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Histoplasmosis. Centers for Disease Control and Prevention. December 2020.

Histoplasmosis. Merck Manual Consumer Version. April 2021.

Histoplasmosis: a clinical and laboratory update. Clinical Microbiology Reviews. December 2020.

Presumed ocular histoplasmosis syndrome. American Academy of Ophthalmology. October 2020.

Presumed ocular histoplasmosis syndrome. American Academy of Ophthalmology. August 2019.

Ocular histoplasmosis syndrome (OHS). National Eye Institute. November 2020.

Presumed ocular histoplasmosis syndrome (POHS). Centers for Disease Control and Prevention. May 2020.

Presumed ocular histoplasmosis syndrome in a commercially insured population. PLoS One. March 2020.

Histoplasmosis patient education information series. American Thoracic Society, Patient Education Information Series. October 2019.

Epidemiology of histoplasmosis outbreaks, United States, 1938-2013. Emerging Infectious Diseases. March 2016.

Histoplasmosis. Merck Manuals Professional Edition. April 2021.

Kanski's Clinical Ophthalmology. Bowling, Brad. 8th ed. London, England: WB Saunders. 2015.

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