Cowpox

Cowpox

R P KITCHING

Introduction

Cowpox is a rare disease of cattle caused by an orthopoxvirus and characterized by vesicles, pustules and scabs on the teats and udder. However, the cow is an accidental host, the primary hosts of the virus being small rodents such as wild ground squirrels, gerbils, voles and woodmice in Europe and Asia. It 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, anteater, elephant and rhinoceros;2, 4 dogs and related species are more resistant,12 but there is one reported case in which a dog spread the virus to a human, who is also an occasional host.1, 3, 11 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.5, 14

Aetiology

Orthopoxviruses are large, brick-shaped, double-stranded DNA viruses, morphologically indistinguishable from capripoxviruses. As with capripoxviruses it is probable that under conditions of dual infection with more than one orthopoxvirus 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. Cowpox virus is also closely related genomically and antigenically to camelpox virus and buffalopox virus.8

The 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

Cowpox 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.

The virus may be introduced into a cattle herd by rodents, the natural hosts of the cowpox virus, or by infected bought-in cattle. However, the virus does not cause a persistent infection, and recovery results in solid immunity. Following the introduction of an infected cow into a susceptible dairy herd, depending on the level of hygiene and preexistent teat injuries, the virus will spread through the herd and become established on the farm. Subsequently only the first-calf heifers will develop disease. The presence of cowpox can reduce milk yields by making milking difficult, and predispose to secondary mastitis. The milk from affected cows is safe to drink following pasteurization.

Clinical signs and pathology

After an incubation period of three to six days, a small red macule is seen in the skin, usually of a teat, at the site of viral entry. This develops into a solid, light-coloured papule surrounded by a red, hyperaemic zone, and then into a yellow vesicle with a depressed centre which develops into a pustule, and eventually a thick, red scab, approximately 10 to 20mmin diameter, forms. In milking cows, the vesicle quickly ruptures, leaving a deep ulcer. Secondary lesions may develop on the same and other teats and in the skin at the base of the udder. Occasionally it spreads to the inside of the thighs and perineum, and onto the muzzles of calves suckling infected cows. In the absence of secondary bacterial infections, lesions heal in two weeks.

Cowpox virus infection in wild and domestic cats usually presents as an infected bite wound. However, a secondary nodular body rash appears ten days later and the nodules may ulcerate. These are replaced by scabs, which resolve in six to eight weeks. Secondary pneumonia may complicate the clinical picture,6 particularly in the larger wild feline species. A fatal form of cat pox has been reported in a domestic cat simultaneously infected with feline immunodeficiency virus, but this may have been coincidental, as many nonfatal dual infections have been encountered.

Human infection with cowpox virus is rare and usually localized in the skin of the hands and forearms at the site of introduction. In some cases, however, it may spread to the mucous membranes of the eyes and cause mild systemic symptoms including lymphadenopathy. Those suffering from infection with the human immunodeficiency virus may be at greater risk.1, 3, 11

Diagnosis

A rapid diagnosis can be achieved by identifying the orthopox virion by electron microscopy. When vaccinia virus was still being used to protect humans against smallpox it was not unusual to find that it had spread from a milker to the udder of a cow. Vaccinia virus is morphologically indistinguishable from cowpox virus. Parapoxvirus is distinct from orthopoxvirus. In the absence of an electron microscope, cowpox virus can be grown in tissue cultures of rabbit kidney cells and Vero cells, in the chorioallantoic membrane of embryonated hens’ eggs and in most laboratory rodents and rabbits. Unlike vaccinia it will not grow on chicken-feather follicle cell cultures. In stained histological sections of skin lesions and in tissue culture cells, type B and later in infection, type A inclusion...

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