Biology of Disease

TYPES OF PATHOGENS

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Figure 2.11 Infections of the toenails (onychomycosis) with (A) Trichophyton tonsurans

and (B) Trichophyton rubrum with secondary infections with the fungus, Scopulariopsis

brevicaulis and the yeast, Candida guilliermondii.Courtesy of Dr Pavel Dubin, Israeli Board

Certified Dermatologist.

Figure 2.12 The ‘head’ of the pork

tapeworm,Taenia solium. Note the

hooks and suckers that allow it to

remain attached to the intestine

wall.Courtesy of Public Health Image

Library, Centers for Disease Control and

Prevention, USA.

aspergillosis and cryptococcosis. Systemic fungal infections are, in general,
much more common in immunocompromized individuals, for example those
with AIDS (Chapter 3), those undergoing cancer chemotherapy (Chapter 17)
or those being treated with immunosuppressive drugs to prevent rejection of
a transplant (Chapter 6).

The clinically important helminths (worms) can be divided into three main
groups: nematodes (roundworms), cestodes (flatworms) and trematodes
(flukes). Helminths often have complex life cycles that may involve other
hosts in addition to humans. In humans, they may infect the alimentary canal,
blood vessels, lymphatics or other tissues such as skeletal muscle. Helminths
are significant parasites in tropical climates. Examples of disease-causing
helminths are the pork tapeworm Taenia solium (Figure 2.12) that can live in
the gut and Schistosoma the cause of bilharzia.

Arthropods and Vertebrates

Arthropods may be directly parasitic, but many are also relevant to infectious
diseases as vectors of pathogens. The mite Sarcoptes scabiei lives in the
outer layers of the skin and can cause scabies while fleas (Pulex) and head
and pubic lice, Phthirus capitans and pubis respectively, are blood sucking
parasites (Figure 2.13). Houseflies and cockroaches are noted carriers of food
poisoning organisms. More specific vectors include ticks that transmit Borrelia
burgdorferi, the cause of Lyme disease. Yersinia pestis, the bubonic plague
organism is spread by the fleas on black rats (Box 2.1). Malarial parasites are
spread by female Anopheles mosquitos (Box 2.2) and tsetse flies are vectors for
Trypanosoma brucei which causes sleeping sickness. Several disease-causing
organisms use mammals as vectors with perhaps the best known being the
rabies virus.

x 75

A)

B)

C)

D)

x 10

x 30

x 25

Figure 2.13 Drawings of (A) the scabies mite, (B) flea, (C) head and (D) pubic lice.