Veterinary Epidemiology and Zoonosis

RELATIONSHIP BETWEEN DIFFERENT TYPES OF ANIMALS AND PLANTS

• A particular biome consists of different types of animals and plants. Some are common and others are scarce; Some are large, others are small.
• Reasons for these variations have been suggested by ecological studies.

Food chains

• Charles Elton noted what foxes ate in the summer and winter. In summer the foxes ate birds. The birds ate berries, tundra leaves and insects.
• The insects also ate leaves. Thus Elton noted that there was a food chain: tundra- insects-birds-foxes.
• In the winter there was a different food chain: marine animal- seal-polar bear-fox.
• Therefore in animal communities, a complex system has evolved with food chains linking animals.

The size of the animal and food chains

• Elton observed that animals fed at different levels in the food chains. These levels he termed as 'Trophic levels'.
• He also noticed that animals occupying different trophic levels generally were of different sizes.
• The foxes were the largest and the birds were smaller (one level down). Similarly further down the pyramid, the insects were even smaller.

The significance of food web to disease transmission

• The food web of an animal can determine the transmition of infectious agents. An animal acts as host and to which food poisoning toxin is risk.
• Helminthic parasites for which there are definitive and intermediate hosts, are frequently transmitted by food webs. E.g. Echinococcus granulosus. Eating sheep offal infects dogs.
• Salmon poisoning: Rickettsia parasitizes a fluke that in turn parasitizes a snail that in turn releases infected miracidia, which parasitizes the salmon. Feeding salmon to dogs transmits the rickettsia.