Cowpox

Previous authors: R P KITCHING

Current authors:
S BABIUK - Research Scientific, PhD, National Centre for Foreign Animal Disease, 1015 Arlington Street, Manitoba, Canada, R3E 3MA

GENERAL INTRODUCTION: POXVIRIDAE

Introduction

Cowpox is a rare disease of cattle caused by an orthopox virus and characterized by vesicles, pustules and scabs on the teats and udder. It was the observation by Jenner in 1798 that milkmaids who had been infected with cowpox virus were resistant to smallpox that led to the science of vaccinology.^{22, 45} Cowpox  is a zoonotic disease and humans  can be infected by contact with  infected animals including the teat lesions during milking, cats, pet rats and when hunting rodents.^{10, 44, 46} The lack of orthopox virus immunity in humans since vaccination against smallpox  ceased, has led to more human infections with cowpox virus, from contact with infected animals predominantly cats.

Aetiology

Orthopox viruses have historically been named for the animal where the initial isolates were recovered from, hence the naming of cowpox. However, further research showed that the name is a misrepresentation since the cow is an accidental host.

Cowpox virus  belongs to the genus Orthopoxvirus and the family Poxviridae (see General Introduction: Poxviridae). Orthopox viruses are large, brick-shaped, double-stranded DNA viruses, morphologically indistinguishable from capripox viruses. As with capripox viruses it is probable that under conditions of dual infection with more than one orthopox virus recombination events occur; such is one theory for the origin of vaccinia virus in the smallpox wards of London, as it replaced cowpox virus as the vaccine used against smallpox. Since the animal reservoir for cowpox are rodents, the virus can circulate and possibly recombine with other  poxviruses. For example recombination with ectromelia virus resulted in a novel orthopox virus infection in Tonkean macaques that died in Italy. This orthopox virus is a novel clade lying between cowpox and ectromelia viruses.⁴ Further genetic characterization of this orthopox virus, named abatino, was most closely related to ectromelia with additional genes showing the highest similarity with cowpox indicating that it could be the result of complex evolutionary events.²⁰

Cowpox virus has the largest genome of all the orthopox viruses (more than 220 000 base pairs). It is is also closely related genomically and antigenically to camelpox  and buffalopox viruses.²⁷ Phylogenetic analysis showed that the CPXV strains sequenced clearly cluster into several distinct clades, some of which are closely related to vaccinia viruses while others represent different clades in a CPXV cluster.^{8, 14} Using genomic data available on cowpox, phylogenetic analysis identified five monophyletic clades of cowpox viruses.³¹

Cowpox virus is sensitive to lipid solvents as found in detergents and to most disinfectants such as those containing hypochlorite, quaternary ammonium compounds or phenolics. It is also inactivated by direct sunlight. However, it can persist for many months in the dark of an infected cowshed.

Epidemiology

The natural  and experimental host range for cowpox is very broad. The primary hosts of the virus are small rodents such as wild ground squirrels, gerbils, common voles and wood mice^{2, 12, 24, 34} in Europe and Asia. Rats are also  susceptible to cowpox infection.²⁶ Cowpox has also been reported in wild and domestic felids and in a variety of animal species in zoological collections including the larger members of the feline family, such as the cheetah, panther, lion, puma, jaguar, ocelot and lynx,as well as anteater, elephant, rhinoceros and banded mongoose.^{5, 21, 37} Dogs and related species are more resistant,⁴⁰ but there is one reported case in which a dog spread the virus to a human.^{1, 7} Cowpox virus can also infect a variety of monkey species including squirrel monkeys (leading to death),¹⁹ new world monkeys (including Callithrix jacchus, C. penicillata, C. geoffroyi, Saimiri sciureus, Callimico goeldii), and tamarin species (Saguinus oedipus, S. fuscicollis, S. nigricollis, S. midas, and S. labiatus),³⁰  Barbary macaques, pig- tailed macaques, Japanese macaques, cynomolgus macaques and rhesus macaques.²⁹ Recently cowpox outbreaks occurred in alpaca herds in Germany³⁵ and llamas in Italy.^{3, 38} Cowpox has been reported also in  an aborted foal.¹³

Cowpox is endemic in Europe and Northern and Central Asia.¹¹ It was probably rare, even in the time of Jenner. Now it is very rarely reported as a disease of cattle, and is more commonly found in cats and zoo animals. The virus can spread between cattle by contaminated milking machines, the hands of milkers and udder cloths. Teat injuries, which allow the virus to penetrate the skin, predispose to outbreaks. Biting flies may also mechanically transmit the virus.

Outbreaks of a vaccinia virus (Cantagalo virus) infections in cattle and humans have been reported  in 1999 in Brazil and have been recurring in agricultural areas throughout the country⁹ and the Amazon region of Colombia.⁴³ The reservoir of these outbreaks of vaccinia virus infections was likely rodents. Cats in Brazil have tested positive for vaccinia virus by PCR, but theydid not develop clinical  disease as cats do following infection with cowpox virus.⁶ The virus may be introduced into a cattle herd by rodents, the natural hosts of the cowpox virus, or by...