Encyclopedia of Parasitology

2016 Edition
| Editors: Heinz Mehlhorn

Opportunistic Agents, Man

Reference work entry
DOI: https://doi.org/10.1007/978-3-662-43978-4_2235

Opportunistic infectious agents do not cause obvious symptoms by their presence in immunocompetent individuals, but can proliferate into fulminant infections in immunodeficient or immunocompromised hosts.

 Immune suppression reactions have increased recently worldwide in humans and animals. This occurs not only due to spreading of virus transmission (inclusive HIV), but also as a result of increasing use of drugs (e.g., cortisone) with immune-decreasing side effects. This deficiency of the immune system enables a broad series of agents – including viruses, bacteria, fungi and parasites – to reproduce in a much higher degree than it occurs under normal (i.e., immunocompetent) conditions. Such infections are due to the fact that  opportunistic agents – especially the parasites   Pneumocystis carinii, Cryptosporidium species and   Toxoplasma gondii are the three main reasons for deaths in AIDS-patients. The most common parasites from the recently enlarging group of opportunistic agents are listed in Table 1.
Opportunistic Agents, Man, Table 1

Opportunistic agents in immunocompromised humans

Species

Infected organ

Transmission/stage

Symptoms of disease

Infected humans/deaths per year

Blastocystis hominis

Intestine

Oral – cysts

Diarrhoea

30–40 millions/thousands

Pneumocystis carinii

Lung

Inhalation – cysts

Pneumonia

400 millions/100 thousands

Giardia lamblia

Intestine

Oral – cysts

Diarrhoea

450 millions/thousands

Leishmania spp.

Skin, inner organs, generalizing

Bite of sand flies

General destruction of organs

15 millions/thousands

Entamoeba histolytica

Intestine, liver

Oral – cysts

Diarrhoea, abscess

500 millions/thousands

Naegleria spp.

Liquor, ZNS

Via nose – at bathing

Encephalitis, PAME

Thousands/few

Acanthamoeba spp.

ZNS, liquor

Via nose – at bathing

Encephalitis, destruction of cornea

100 thousands/thousands

Isospora belli

Intestine

Oral – oocysts

Diarrhoea

l00 thousands/thousands

Cryptosporidium parvum

Intestine

Oral – oocysts

Diarrhoea

40 millions/100 thousands

Toxoplasma gondii

ZNS, generalizing

Oral – oocysts, meat

Cerebral destruction

50–60 millions/100 thousands

Cyclospora spp.

Intestine, generalizing

Oral – oocysts

Diarrhoea

Thousands/few

Microspora (e.g., Encephalitozoon)

ZNS, kidney, intestine, generalizing

Oral – spores

Destruction of organs

100 thousands/thousands

Balantidium coli

Intestine

Oral – cysts

Diarrhoea

Thousands/few

Hymenolepis nana

Intestine

Oral – larvae

Diarrhoea

Thousands/none

Strongyloides stercoralis

Intestine

Percutan/oral – larvae

Diarrhoea

100 millions/thousands

Sarcoptes scabiei

Skin

Body contact

Scabies

20 millions/thousands

Demodex folliculorum

Skin

Body contact

Rosacea

Thousands/none

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© Springer-Verlag Berlin Heidelberg 2016