Veterinary Epidemiology and Zoonosis

Classification based on epidemiological features (life cycle / maintenance / mode of transmission)

CLASSIFICATION OF ZOONOSES BASED ON EPIDEMIOLOGICAL FEATURES
(LIFE CYCLE / MAINTENANCE / MODE OF TRANSMISSION)

 1. Direct zoonoses

• Zoonotic diseases transmitted directly from the infected to the susceptible host by contact, vehicle or mechanically by a vector, without undergoing any development or propagation in the course of transmission are classified under direct zoonoses.
• A zoonotic pathogen requires only single vertebrate host species for their completion of life cycle.
  • Examples: Bovine and ovine brucellosis by contact, Anthrax by contact and touch, Rabies by bite, Ringworm by touch, Salmonellosis by vehicle

Figure 1: Direction of spread of direct zoonotic diseases

• Direct zoonoses can be any one of the following type
  • Direct-anthropozoonoses
  • Direct-zooanthroponoses
  • Direct-amphixenoses

• (a). Direct-anthropozoonoses
  • The lower vertebrates (domestic and wild animals) are the reservoir hosts for many infectious zoonotic pathogens and these reservoirs transmit infections to the human beings by accidental exposures or direct contact with infected population or contaminated materials.
    • Examples: Rabies, Brucellosis

Figure 2: Direction of spread of direct-anthropozoonotic diseases

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• (b). Direct-zooanthroponoses
  • Zoonotic diseases transmitted from human beings to the lower vertebrate animals by direct contact with infected person or contaminated materials.
    • Examples: Diphtheria, Tuberculosis

Figure 3: Direction of spread of direct-zooanthroponotic diseases

• (c). Direct-amphixenoses
  • Zoonotic diseases maintained in nature either in animal population or in human population are transmitted in both directions by direct contact with infected population or contaminated materials.
    • Examples: Streptococcosis, Staphylococcosis

Figure 4: Direction of spread of direct-amphixenotic diseases

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2. Cyclozoonoses

• The disease requires more than one vertebrate host to complete the life cycle, but invertebrate host is not involved. Depending upon the involvement of human being it may be obligatory cyclozoonoses or non-obligatory cyclozoonoses.
  • Examples: Japanese encephalitis (Flavivirus of Family Flaviviridae), Hydatidosis, Bovine cysticercosis, Trypanosomiosis (tse-tse-borne), Nairobi sheep disease (Bunyaviridae), Leishmaniosis.
• (a). Obligatory cyclozoonoses
  • In obligatory cyclozoonoses, agents require essentially a human being for completion of their life cycle.
    • Example: Taeniosis (Taenia saginata and T.solium) – measly beef and measly pork

Figure 5: Direction of spread of obligatory cyclozoonotic diseases

• (b). Non-obligatory cyclozoonoses
  • In non-obligatory cyclozoonoses, agents require man's involvement as accidental or not essential for completion of their life cycle.
    • Example: Hydatidosis (Echinococcus granulosus).

Figure 6: Direction of spread of non-obligatory cyclozoonotic diseases

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3. Metazoonoses

• Those diseases that are transmitted by the invertebrate hosts (biological vectors) in which the causative agent multiplies (propagative) or develops (developmental) or both (cyclopropagative).
• It requires both vertebrate and invertebrate hosts for their completion of life cycle.
  • Examples: Arborial infections, Schistosomiosis, Fascioliosis (sapro-meta-zoonosis), Plaque, Trypanosomiosis, Filariosis, Trichinellosis (Trichinella spiralis).
• Depending upon the number of hosts required for the completion of life cycle there are four subclasses of metazoonoses.
  • Metazoonoses subtype I
  • Metazoonoses subtype II
  • Metazoonoses subtype III
  • Metazoonoses subtype IV

Table 1: Sub-classes of metazoonoses

Subclass	Number of hosts required		Examples of zoonoses
	Vertebrates	Invertebrates
I	1	1	Japanese encephalitis (Culex tritaeniorhynchus), Yellow fever (Juncle cycle and urban cycle, Aedes aegypti)
II	1	2	Paragonimiosis
III	2	1	Eastern equine encephalitis, Clonorchiosis
IV	1	Transovarian transmission of agent in ticks	Tick-borne encephalitis, Kyasanur forest disease

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• (a). Metazoonoses subtype I
  • It requires one vertebrate and one invertebrate host for the completion of an etiology’s life cycle.
    • Example: Yellow fever, in which juncle cycle or sylvatic cycle and urban cycle requires a monkey or a man and a mosquito Aedes aegypti.

Figure 7: Direction of spread of metazoonotic (subtype I) diseases

• (b). Metazoonoses subtype II
  • It requires one vertebrate and two invertebrate hosts for the completion of an etiology’s life cycle.
    • Example: Paragonimiosis

Figure 8: Direction of spread of metazoonotic (subtype II) diseases

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• (c).Metazoonoses subtype III
  • It requires two vertebrate and one invertebrate hosts for the completion of an etiology’s life cycle.
    • Example: Eastern equine encephalitis

Figure 9: Direction of spread of metazoonotic (subtype III) diseases

• (d). Metazoonoses subtype IV
  • It is the type of transovarian life cycle of zoonotic pathogen in ticks and transmission to human beings.
    • Example: Tick-borne encephalitis

Figure 10: Direction of spread of metazoonotic (subtype IV) diseases

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4. Saprozoonoses

• The diseases which require a non-animal objects like soil, plants, organic matter, water, foods, etc. to serve as a true reservoir of infection or as site for an essential phase of development.
• The etiological agent may develop (Histoplasmosis) or propagate (Ancylostoma brazilliense) or develop and propagate (cyclopropagative - Fascioliasis) in the inanimate (fomites) objects.
• The agent can occur in both saprophytic and parasitic phase in cyclic order.
  • Examples: Visceral larval migrans, cutaneous larval migrans, fascioliosis, mycosis of various forms, botulism, coccidiomycosis, histoplasmosis, etc.
• Based on the direction of spread it may be classified into three sub-classes, such as
  • Sapro-anthropozoonoses
  • Sapro-amphixenoses
  • Sapro-meta-anthrapozoonoses. 

Table 2: Sub-classes of saprozoonoses

Sub-class	Agent in inanimate host	Examples of zoonoses
I	Propagative	Histoplasmosis
II	Development only	Ancylostoma brazilliense
III	Both propagative and development	Fascioliosis (sapro-meta-zoonosis)

• (a). Sapro-anthropozoonoses
  • Zoonotic infections are normally transmitted between lower vertebrates and fomites and involvement of human population is only accidental.
    • Example: Erysipeloid

Figure 11: Direction of spread of sapro-anthropozoonotic diseases

• (b). Sapro-amphixenoses
  • Man and lower vertebrates are equally susceptible hosts and infection can persist in the absence of one or another host.
    • Example: Histoplasmosis

Figure 12: Direction of spread of sapro-amphixenotic diseases

• (c). Sapro-meta-anthrapozoonoses
  • It requires both non-animate, invertebrate host and vertebrate host in its life cycle.
    • Example: Fasciliosis

Figure 13: Direction of spread of sapro-meta-anthrapozoonotic diseases

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