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Hugh W. Reid

and Mark P. Dagleish

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date: 24 January 2022

The poxviruses are a large family of complex viruses infecting many species of vertebrates as well as arthropods, and members of the three genera Orthopoxvirus, Yatapoxvirus and Parapoxvirus are the cause of sporadic zoonotic infections originating from both wildlife and domestic livestock. Infections of humans are generally associated with localized lesions, regarded as inconvenient rather than life-threatening, although severe illnesses have occurred, particularly in immunologically compromised individuals.

The most celebrated of the orthopoxvirus infections is cowpox — a zoonotic infection which has been exploited to the enormous benefit of mankind as it had a pivotal role in the initiation of vaccination strategies that eventually led to the eradication of smallpox. Cowpox occurs only in Eurasia and in recent years it has become evident that infection of cattle is fortuitous and the reservoir of infection is in wild rodents. Monkeypox is another orthopoxvirus causing zoonotic infections in central and west Africa resembling smallpox and is the most serious disease in this category. While monkeypox does not readily spread between people, the potential of the virus to adapt to man is of concern and necessitates sustained surveillance in enzootic areas.

The third orthopoxvirus zoonoses of importance is buffalopox in the Indian subcontinent, which is probably a strain of vaccinia that has been maintained in buffalo for at least 30 years following the cessation of vaccination of the human population. Likewise in Brazil, in recent years widespread outbreaks of vaccinia have occurred in milkers and their cattle.

Orf virus, the most common of the parapoxviruses to cause zoonotic infection, is largely restricted to those in direct contact with domestic sheep and goats. Generally, infection is associated with a single localized macule affecting the hand which resolves without complications. Infection would appear to be prevalent in all sheep and goat populations and human orf is a relatively common occupational hazard. Sporadic parapoxvirus infections of man also occur following contact with cattle infected with pseudocowpoxvirus, and wildlife, in particular seals.

A final serious consideration with the poxvirus zoonoses is the clinical similarity of such infections with smallpox. In view of the potential for smallpox virus to be employed by bio-terrorists there can be an urgency for laboratory confirmation of unexplained zoonotic poxvirus infections. Thus there is a requirement to maintain the capacity for rapid confirmation of poxvirus infections by molecular technique. As representatives of the known poxviruses have all been sequenced, generic and virus specific Polymerase Chain Reactions (PCR) can readily be performed to ensure rapid confirmation of any suspect infection.

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