Defence mechanisms of the host
The healthy animal is protected from pathogenic organisms present in the environment by virtue of the effective immune mechanism present from birth. This innate resistance is non-specific in nature i.e. it is effective against most of the pathogens. The intact skin and mucous membranes of the body afford effective protection against pathogenic organism. Acidic pH of the gastro intestinal tract, lacrimation, ciliated mucous membrane of the nasal passage and sphincters of the natural orifices are some of the natural defence mechanisms. The reticuloendothelial system (RES) of an animal body provides an effective defence mechanism by engulfing and destroying the foreign particles like bacteria. Macrophages of Metchnikoff constitute the main cellular scavenging network of the body. Besides this, genetically a particular species could be resistant to a specific pathogen. This mechanism depends on many factors such as the absence of specific microbes binding receptors, body temperature and the animal habitat. Other type of resistance is acquired resistance which is acquired when an animal comes in contact with a particular organisms. It is of two types, viz. humoral immunity and cell-mediated immunity. The first one largely depends on the presence of antibodies in the tissue fluids and the later type is attributed largely by the phagocytic cells The acquired immunity also develops with the introduction of vaccines or toxoids in animal body.
Carriers: Occasionally, a situation arises after recovery from a disease or because of the partial resistance of the host or the inability of the causal agent to produce the disease syndrome in the host, the organism lives in the host without showing any symptoms of the disease, and the host itself is unable to completely get rid of the organism and cause disease in susceptible animals. The bacteria sometimes actively multiply and bring about a fresh outbreak of the disease as a result of breakdown in the balance between the host and the parasite under certain stress factors. The carriers could be classified as contact carriers, convalescent carriers and immune carriers.