An introduction to infectious diseases

DEFINITIONS:

Infectious diseases: diseases caused by infectious pathogens.
Infectious pathogens: pathogenic microorganisms, such as bacteria, viruses, aberrant proteins known as prions, parasites or fungi.

The diseases can be spread directly or indirectly, from one person or animal to another.

Zoonotic diseases are infectious diseases of animals that can cause diseases when transmitted to humans.

In infectious diseases, there are:

1. Clinically evident illness, that is, characteristic medical signs and/or symptoms of the disease, resulting from the infection;
2. Presence and growth of pathogenic biological agents in an individual host organism.

Infectious diseases may be asymptomatic for much or even all of their course in a given host.

NOTE: An infection is NOT synonymous with an infectious disease

   INFECTION versus CONTAMINATION

Contamination: the mere presence of microbes in or on the body
Infection results when the organism has evaded the body’s external defenses, multiplied, and become established in the body

In infection, there is an active interaction between the host and the pathogen. Pathogenic microorganisms invade and multiply in a bodily part or tissue, which may produce subsequent tissue injury.

INFECTION / INFECTIOUS DISEASE

Infection is the invasion of the host by a pathogen
Disease results only if the invading pathogen alters the normal functions of the body

Classification of infectious diseases by:

1. Nature: viral/ bacterial/ fungal/ prionic
2. Clinical signs and morphological and functional alterations: typical/ atypical; clinical (apparent)/ inapparent (asymptomatic, without clinical signs)/ latent = appears a long time after infection (long incubation period)
3. Number of etiological agents: monofactorial / polifactorial
4. Severity or Duration of a Disease: peracute/ acute/ subacute/ chronic/ latent

• Peracute - develops rapidly and lasts a very short time (<24 hours, sudden death) 
• Acute – develops rapidly and lasts a short time (<7-10 days) (Pasteurellosis) 
• Subacute – intermediate between acute and chronic (EIA= Equine Infectious Anemia)
• Chronic – develops more slowly, reactions to the disease are less severe, likely to be continual or recurrent for long periods (TB= Tuberculosis, Paratubercullosis)
• Latent – the causative agent remains inactive for a time but then becomes active to produce symptoms (Enzootic bovine leukosis, EIA)

5. Frequency of occurrence:

• Sporadic – occurs occasionally (Anthrax)
• Endemic – constantly present in a population (Common cold)
• Epidemic – many cases in a given area in short period (Influenza)
• Pandemic – a world wide epidemic (Foot and mouth disease)

6. Spread level:

• Communicable disease = transmitted from one host to another
• Contagious disease = communicable disease that is easily spread

7. Host response: 

• Local 
• Systemic

Primary infection = initial infection

Secondary infection = infection that follows a primary infection – often the case of opportunistic pathogens

Description of Infectious diseases

1. Definition
2. Importance and geographical distribution
3. Etiology
4. Epidemiology
5. Pathogenesis + Immunity
6. Clinical signs
7. Gross lesions
8. Diagnosis
9. Differential diagnosis
10. Control and preventive measures

The definition of a disease consists of the following:
1. Etiology
2. Epidemiology
3. Clinical signs
4. Gross lesions.

While describing the disease, its importance, such as its economical impacts, and zoonotic risks should be discussed, together with the geographical distribution of the disease, for e.g. if it has a worldwide pattern, or is restricted to an area.
The etiology is very important for the diagnosis and control and preventive measures.

Epidemiology should comprise of:
1. Susceptibility of the host
2. The source(s) of infection
3. The route(s) of transmission; and
4. The course of the disease, i.e. its occurrence.

Epidemiology is important to be able to control and take preventive measures.  While discussing about epidemiology, the epidemiological triangle should be taken into account, as a disease is the result of forces within a dynamic system consisting of the following triad:
1. The host;
2. The agent;
3. The environment.

1. Susceptibility of host

Particularities are influenced by:

• Animal species
• Age, Immune status, breed, other diseases, stress factors including immunosuppressive medication

2. Source(s) of infection

• Infected animals (with or without clinical signs), carriers, animal products, contaminated food, water, soil
• Reservoir(s) of infection (ecological niche where the infectious agent survives and multiplies: animal, arthropod, soil, natural FOCALITY 

3. Route(s) of transmission

• Direct: Skin-to-skin, mucous-to-mucous, across placenta, maternal milk, sneeze, cough
• Indirect: foodborne, waterborne, airborne, vectorborne, objects/ fomites

NOTE: transmission can be vertical as well as horizontal.

4. Course of the disease

• Sporadic level: occasional cases occurring at irregular intervals 
• Endemic level: persistent occurrence with a low to moderate level 
• Hyperendemic level: persistently high level of occurrence
• Epidemic or outbreak: occurrence clearly in excess of the expected level for a given time period
• Pandemic: epidemic spread over several countries or continents, affecting a large number of individuals

To consider controlling measures, the chain of transmission should be established.  This is known as the epidemiological inquest.

Chain of infection:

Possible outcomes after exposure to an infectious agent: