The innate characteristics of pathogenic organisms play a huge role in determining modes of disease transmission. Understanding these characteristics is essential to preventing disease introduction or controlling and/or eradicating them once introduced. Here, we will discuss four general pathogen characteristics and describe how these characteristics each influence transmission.

Persistence in the environment

The persistence of the organism in the environment determines how long we can expect to find the agent. For example, anthrax (Bacillus anthracis) produces spores that persist in the environment for many years.

Immediately infectious

Some pathogenic agents are immediately infectious to an individual and do not require time for development either within the environment or within another host or vector.

For example, cysts of Giardia spp. are immediately infectious to animals when shed.

Giardia is the most common cause of water-borne diarrheal disease in humans in North America. The two forms in the lifecycle are the trophozoite, the actively feeding stage in the small intestine, and the cyst, the persistent form which can survive in the environment.

	This image is of Giardia lamblia cysts
	Here, we can see an EM of Giardia lamblia trophozoites.

Require time to develop to the infectious stage

In contrast to pathogens which are immediately infectious, some organisms require time within the environment to develop into an infectious stage.

For example, Toxocara canis, an ascarid of wild and domesticated dogs has a direct lifecycle in which unembryonated, non-infectious eggs are shed in the feces of dogs. A time period of 2-4 weeks in the environment is required for the infectious second-stage larva to develop within the egg.

In this photo, we can see two Toxocara canis eggs. The one on the right contains the infectious L2 stage. The egg on the left is not embryonated, and thus is not infectious at this time.

Indirect life cycle requiring development in vector/another host

Pathogens with an indirect life cycle require an intermediate host, which is defined as a host in which the agent develops but does not reach sexual maturity. For example, most parasite trematodes (flukes) and cestodes (tapeworms) and some nematodes have indirect lifecycles requiring intermediate hosts. Many species may act as intermediate hosts. For example, snails are usually the first intermediate host of trematodes.

Ticks may require one, two, or three hosts to complete their life cycle. The majority of hard tick species in North America have a three-host life cycle in which the larva, nymph, and adult feed on three different hosts.

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