Factors Affecting Host Specificity

The main factor which affecting host specificity and organ specifity are.

1.      Physiology : Some of the parasites are morphologically indistinguishable found in different hosts but essentially differ physiologically. For example: Iodamoeba biitschlii of man and I. Suis of pigs are morphologically similar but essentially differ physiologically and cross transmission infection does not occur.

2.      Geographical distribution: During considering the host specificity another interesting fact is observed that morphologically similar species of the parasite utilize different intermediate host in different areas. For example : the blood fluke, Schistosoma mansoni uses Biomphalaria snails as intermediate hosts in Africa, while in the West Indies and South America, Tropicorbis centimetralis and Australorbis glabrous are intermediate hosts, in contrast same species do not infect B. alexandrina snails in Egypt.

3.      Compatibility of the parasite to its host: It is an important mechanism, which governs the host specificity. In this, the host specificity depends on the host response to the invading parasites (cellular or humeral). If this host response is poor, indicates that the parasites body proteins are less antagonistic.

4.      Biochemical agents/factors: There are number of chemical compounds or agents which play important role in determining the host specificity. For example, the composition of the bile of different mammalian hosts varies greatly in respect to bile salts, conjugation of bile salts, fatty acids, pigments and rate of secretion (smyth JD and GAD Haslewood, 1963). If the hydrated tapeworm (E. granulosus) enters and unfavourable host, the host bile can rapidly destroy the invader by lysing its body cuticle or in some way providing toxic to the parasite, or interfering with its metabolism, while when enters in a favorable host, the host bile can provide the stimulus for further growth.

Physical Factors

Along with biochemical factors, some physical actor like pH, temperature, and salt concentration may play an important in determining the host specificity.

Immunologic factor Host specificity may be based on some short of immunologic tolerance of the host for certain parasites and not for others. The basis must be immuno chemical. According to this view, the parasites have developed in the course of evolution which have antigens resembling those of their host (molecular mimicry). The host is thus unable to produce antibodies effective against its parasites till the parasite population reaches a critical level of abundance. Now, the host reacts immunologically against the parasite and eliminate most or all off its population.