3.1 Objectives:

• 1) To understand the concept of infection dynamics as it affects populations
• 2) To examine transmission dynamics and its implication in the spread of infections
• 3) To study the consequences of exposure of populations to single or multiple risk events
• 4) To define important terminologies in disease transmission
• 5) To compare individual versus herd or population immunity 

3.2 INFECTION DYNAMICS 

3.2.1 Introduction

Infection occurs when a biologic agent invades the host and multiplies, producing a change within that host. This change may be structural, functional, or biochemical, is not necessarily harmful, and could be limited to something as benign as a serologic response. The consequences to exposure to an infectious agent would be:

• a. No infection or disease
• b. Infection (unapparent, latent, subclinical)
• c. Disease (mild to severe to death)
• d. Immunity 

Carrier:

Is an individual shedding the infectious agent without showing clinical illness. There are many different types of carriers including:

Healthy	Incubationary	Convalescent
Intermittent	Chronic	-

Healthy carriers are probably the most important from the standpoint of disease transmission. These animals shed the agent but are never recognized as being clinically ill. In some instances, the agent may persist within the body for a long period of time with intermittent shedding at periods of stress. As an example, in Salmonella typhimurium infections in swine, the agent can be isolated from the mesenteric lymph nodes for as long as 6 months with intermittent shedding following stress situations over this Period of time.

3.3 Transmission Dynamics and Spread of Infections:

Epidemiology is concerned with the evolution of a disease in a population of animals more than with the course of that disease in a population, i.e. on its dynamics, in its duration, on what proportion of the animals will be affected are the modes by which disease agents are transmitted from animal to animal and by which they spread within a population. 

3.3.1 Development of Infection or the Infectious Disease Processes

Six essential factors:

• a. An agent must be present
• b. There must be a source or reservoir for the infectious agent
• c. Mode of escape from b.
• d. Mode of transmission - factors that bring the new susceptible host and an agent must be present
• f. There next must be a new susceptible host (at risk) 

3.3.2 Source or Reservoir of Infections

A reservoir - is any animate or inanimate substance in which an infectious agent multiplies or develops and upon which it depends as a species for its survival in nature.

3.3.2.1 Sources of Infections

• a. Cases - actually suffering from a disease and shed pathogens.
• b. Carriers - infected and shedding pathogens without showing any

3.3.2.2 clinical signs.

These could be healthy, incubating, convalescent, chronic, transient or latent carriers. 

Carriers are more important in populations than are cases because the infectious agent is maintained for a long time and it is not as easy to identify carriers in order to eliminate them from a herd and thereby control a disease. In crowded conditions though, such as in feedlots, cases become of great concern. Two examples on one disease with a zoonotic significance that has different characteristics is rabies. It is of great concern when cases are seen in foxes and skunks (or other animals). On the other hand, since bats act as carriers of rabies, these bats are of great concern in areas of the world where rabies carrier bats exist, e.g. South America.

(Note: Read the Rabies articles in JAVMA)

3.3.2.3 Modes of Escape:

A pathogen must escape from a reservoir or source of infection if a disease is to occur.

3.3.2.4 Modes of Transmission:

An infectious agent must find a way to a new host from its source or reservoir. The mode of transmission partly determines whether a disease will be sporadic, epidemic or endemic. It also largely determines the dosage reaching a new host. 

3.3.3 Modes of transmission:

• 1. Direct or contract transmission. The agent is transferred from animal to animal via physical contact.
• 2. Indirect or vehicle transmission. The agent is transported actively or passively by some inanimate medium.
• 3. Vector transmission. This term is usually reserved for the event where the agent is transmitted by an invertebrate animal 

 

 

3.4 Epidemic spread of Diseases:

3.4.1 Single exposure to a common source:

an example of this might be a feed or waterborne disease agent such as a chemical, a toxin, or microorganisms. A large number of animals are exposed at one time and the resulting disease episode is likely to be of sudden onset and of explosive nature. Such episodes are often referred to as outbreaks and give rise to outbreak investigations.

3.4.2 Multiple exposure to a common source:

similar to the above but groups of animals are exposed at different moments resulting in a number of disease episodes with a longer overall duration which therefore appears to be of less explosive nature.

3.4.3 Propagated exposure:

a large number of animals may get exposed to a disease agent (particularly an infectious agent) when the agent is transmitted from one animal to a number of others. The resulting build-up of cases therefore depends on such factors as the rate of contact between animals, the length of the incubation period, etc.

3.5 Important terminologies in disease transmission 

3.5.1 Cycle of transmission:

The cycle of transmission includes the reservoir, how the agent leaves the reservoir, the mode of transfer to the new susceptible host, and how it enters the new host.

3.5.2 Vehicle and vector:

The essential difference between a vehicle and a vector is that a vehicle is an inanimate object whereas a vector is a living animal.

3.5.3 Mechanical or biologic vector:

A mechanical vector might be thought of as no more than a living vehicle, for the agent does not undergo any essential change during its association with the vector. The agent may be carried externally or internally. In biologic vectors the agent undergoes some type of change.

3.5.4 Transovarial and transstadial:

vertical transmission is termed transovarial if the agent is transferred from the female to her eggs. Survival of the agent through stages of tick development from the nymph to the adult is termed transstadial transmission.

3.5.5 Serial interval:

the time required to complete a cycle of transmission varies with each agent. The period from the appearance of signs in one case to the appearance of signs in a second case infected from the first, is known as the serial interval. This is an observable epidemiologic unit reflecting to some extent the life cycle of the parasite.

3.5.6 Speed of transmission:

varies according to disease, route of transmission and level of herd immunity. The rate of spread is determined by such things as the number of sources, infectivity of the agent, and number of susceptible hosts. 

3.6 Herd Immunity:

As there are individual differences among animals (and humans) with respect to immunity, populations also differ in immunity. Resistance in populations or a community is referred to as Herd Immunity. This represents the proportion of resistance animals in the population.

Either due to natural exposure to an infectious agent or immunization, acquired herd immunity could result. In cases of mass immunizations, the level of herd immunity depends upon:

• a) The efficacy of the vaccine
• b) The proportion of animals actually vaccinated
• c) The proportion of immunologically competent animals vaccinated
• d) Proportion of vaccines maintained under optimal storage conditions
• e) Proportion of vaccines properly administered.

To establish and maintain an adequate herd immunity, it is not necessary to have every member of the population protected. Whether an epidemic or the spread of disease in the population is a function of:

• a) Proportion of resistant animals in the population (herd immunity)
• b) The probability of contact (rate of contact) between an infected and an uninfected individual in the population.

The level of herd immunity may change depending on whether new susceptible individuals are introduced into the population (via birth, purchase, immigration, etc.). Such changes could gradually lead to lowering of herd immunity. In general, the dynamics of the infectious process is such that if only 70-80% the population can be made resistant, epidemic outbreaks are minimized or prevented. 

3.7 Topic : Review Questions: