Veterinary Epidemiology and Zoonosis

DISTRIBUTION AND CONTROL OF POPULATIONS

• The distribution, the home range of animals and other behavioural activities of the hosts of the infectious agents affect the transmission of infectious agents. Example: The vulpine rabies.
• The maintenance host, fox’s behaviour alters the association between foxes.
• The animals may be solitary, paired or part of the family unit.
• Increased home range may also increase the spread of infection.

Niche

• Logistic equation for each species to find the relative size of each population produces pairs of equations that were derived independently in US by Lotka (1925) and in Italy by Volterra (1931). These equations are therefore called Lotka-Volterra equations. They can be derived for varying degree of competition. The conclusion drawn from these equations is of fundamental importance in ecology.
• It is that the coexistence of two strongly competing species is impossible. Coexistence is possible if the competition is weak. This led to the principle of competitive exclusion. That the competition will exclude all but one species from a particular position defined by an animal’s feeding habits, physiology, mechanical habits and behaviour. This position is 'animal’s niche'. The principle of competitive exclusion can be summarized as “one species, one niche”.

Some examples of niche relating to disease

• Louse infestation
  • Lice tend to be host specific. Pig lice do not live on man or dog or vice versa. By being host specific, species of lice avoid competition, i.e, they have their own niche.
  • Two types of lice live on man; the head louse and the body louse each parasitizising the two different parts of the body.
• Intracellular parasites occupy a niche in the cells
  • They include all viruses, bacteria, and some protozoa.
  • There are several advantages to this type of existence such as safety from humoral antibodies and extra cellular agents.
• Epidemiological interference
  • Studies in India have shown that the presence in a human community of one type of respiratory adenovirus prevents infection with other types even though the latter are common in surrounding communities.
  • This is because the first type occupies a niche (the lower respiratory tract), which therefore cannot be filled by other agents. This phenomenon is caled 'epidemiological interference'.