HC19: Pathology of infectious diseases

Infectious diseases

In 80% of cases, the pathologist finds infectious disease patterns while trying to exclude malignancy. In 20% of cases, they get a biopsy intended to prove infectious origin. Pathologists try to think in patterns.

Spectrum of inflammatory responses

There is a spectrum of inflammatory responses to infection, which consists of 5 forms of inflammation:

• Suppurative/(purulent) acute inflammation → gram-positive or gram-negative bacteria
  • Most common
  • Acute inflammation
  • Many neutrophils are present
  • Pus is visible
• Mononuclear/granulomatous inflammation → mycobacteria, spirochetes, parasites, viruses
  • Chronic inflammation
• Cytopathic-cytoproliferative inflammation → HSV, adenovirus, herpes virus, HPV
  • Infected cells start proliferating under influence of viruses → necrotic responses
    • Can lead to death very quickly
• Chronic inflammation and scarring → hepatitis B
  • Mainly lymphocytes and specific antibody-producing cells are present
• Tissue necrosis → clostridium, hepatitis B
• No reaction → immune deficient host

Lung infections

If, on a CT scan, there are spots visible in the lung, its either malignancy or an infection. If there is a mononuclear or granulomatous infection in the lung, it most definitely is a case of TBC. Caseous necrosis are cavities in the lung made of dead cells, which is very common for TBC.

Testis

The testis is very difficult to biopsy, but the probability of malignancy in the testis is very high. It can be confused with TBC, which can also occur in the testis → it enters the blood stream and lymph nodes via the lungs. However, this is super rare.

Caveating/cystic lesion in the lung

An infection can lead to a caveating or cystic lesion in the lung. This is done by the echinococcus parasite, which can make cystic walls around itself and create holes in the lungs → echinococcus cysts. If the cyst is ruptured, new larvae can be made everywhere.

Graft versus host disease

Graft versus host disease can happen after a bone marrow transplant. It is a form of immunodeficiency. It can cause cytopathic or cyto-proliferative patterns → virus infections, for instance HPV or CMV associated pneumonitis. Usually, this virus can be destroyed by a well-functioning immune system. However, this can be dangerous in case of bone marrow transplantation → it can cause Graft versus host disease.

POX-virus

POX-virus can cause molluscum contagiosum ("waterwratten"). This is very contagious, but usually goes away by itself.

Herpes simplex

In case of herpes simplex ("koortslip") there is an excessive amount of cells, paired with blisters. It is cyto-proliferative.

Syphilis

Ulcerating skin lesions can be caused by plasma cell infiltrations → mononuclear infiltrations. This disease is called syphilis, which is caused by spirochetes.

Hyphen invading vessel walls

Aspergillus fumigatus is a fungus that not only can spread through the body via blood vessels, but also can destroy the vessel wall → hyphen invading vessel walls. This way, the fungus can spread through the entire human body. It is very rare and only happens when one is immunodeficient.