Veterinary Epidemiology and Zoonosis

ECOLOGICAL CLIMAX, ECOLOGICAL INTERFACES AND ECOLOGICAL MOSAICS

Ecological climax

• When plants, animals, microbes, soil and macroclimate have evolved to a stable, balanced relationship, an ecological climax traditionally is said to have occurred.
• When infections are present, they too are stable and therefore are usually endemic. Also, the balance between host and parasite usually results in inapparent infections. Such stable situations can be disrupted frequently by man resulting in epidemics. Example: Bluetongue, a viral disease of sheep, was recognized only after the importation of European breeds of sheep into South Africa towards the end of the 19th century. The virus, however, was present in indigenous sheep before that time, but was part of an ecological climax.
• The importation of exotic sheep represented a disturbance of stable climax.
• A climax involving endemic infectious agents indicates that all factors for maintenance and transmission of the agent are present.
• Sometimes changes in local ecology may tip the balance in favour of disease incidence. Example: The seasonal periodicity of FMD in South America may be as a result of seasonal increases in the size of susceptible cattle population when animals are brought into endemic area for fattening.

Ecological interfaces

• An ecological interface is a junction of two ecosystems.
• Infectious diseases can be transmitted across the interfaces.
  • Example is the transmission of yellow fever, an arbovirus disease of man. The virus is maintained in apes in Africa in autochthonous ecosystem in the forest. The mosquito, Aedes Africans transmits the virus between apes.
• The mosquito Aedes simpsoni, bridges the interface between the autochthonous forest ecosystem and the anthropurgic-cultivated areas.
• The mosquito therefore maintains a plantation cycle in which man and apes may be infected.
• Finally the urban mosquito Aedes aegypti can maintain an urban cycle in man.

Ecological mosaics

• An ecological mosaic is a modified patch of vegetation created by man, within a biome that has reached a climax.
• Infection may spread from wild animals to man in such circumstances.
  • Example: Clearing of the forest encourages a close cover of weeds on the ground creating conditions that are favourable for the incursion of field rats with mites infected with scrub typhus which form mite islands and that local areas become endemic for scrub typhus.