Classification according to the modes of transmission
1. Direct zoonoses: The direct zoonoses are those zoonoses that are transmitted from an infected vertebrate host to a susceptible vertebrate host by direct or indirect contact, by vehicle or by mechanical vector. Only a single vertebrate species is required to perpetuate the cycle and the agent itself undergoes little or no propagative changes during the transmission, e.g., rabies, anthrax, brucellosis, etc.
2. Cyclo-zoonoses: These zoonoses require more than one vertebrate host, but no invertebrate host for completion of the causative agent’s development cycles. Most of the cyclo-zoonoses are cestode infections. The Taenia solium and Taenia saginata infections are obligatory cyclo-zoonoses, that is, man must be one of the vertebrate hosts in the cycle of the infections. Other cyclo-zoonoses, such as hydatid disease, are not obligatory. In these infections, man is sometimes involved but human involvement is an exception rather than the rule, e.g., hydatid disease, taeniasis, etc.
3. Meta-zoonoses: The meta-zoonoses are transmitted biologically by invertebrate vectors. There is always an extrinsic incubation period in the invertebrate host before the transmission of the causative agent to another vertebrate host. In the invertebrate, the causative agent may multiply and serve as a reservoir of infection (propagative transmission) or the agent may merely develop but not act as a reservoir of infection (developmental transmission). Depending upon the hosts required, meta-zoonoses are distinguished into four types.
Subtype I Requires one vertebrate and one invertebrate hosts, viz. sylvatic yellow fever.
Subtype II Requires one vertebrate and two invertebrate hosts, viz. paragonimiasis.
Subtype III Requires two vertebrate and one invertebrate hosts, viz. clonorchiasis. Subtype IV Requires transovarian transmission, viz. tick-borne encephalitis.
4. Sapro-zoonoses: The sapro-zoonoses are those infections which require a non-
animal site to serve either as a true reservoir of infection or as a site for an essential phase of development of their causative agents. The agent may propagate in the non-animal site (food, soil, plant), e.g., histoplasmosis or the agent may undergo essential development without propagation, e.g., Ancyclotoma brasiliense infection.