Cowpox

Also known as

Vaccina

Description

Cowpox is a benign, contagious skin infection characterized by pox lesions on the teats and udder of cows. The virus (an orthovirus) is zoonotic, meaning that it is transferable between species, such as from animal to human.

Transmission of the disease from cows to humans was first observed in dairymaids who milked infected cows and developed the same kind of pustules on their hands.

Today cowpox is fairly rare, and more commonly found in animals other than cattle, such as rodents. Cowpox is similar to, but much milder than, the highly contagious and often deadly smallpox. Its close resemblance to the mild form of smallpox and the observation that dairy farmers were immune to smallpox inspired creation of smallpox vaccine by the English physician Edward Jenner, using the cowpox virus.

The word “vaccination,” was coined by Jenner in 1796—a term derived from the Latin adjective vaccinus, meaning of or from the cow. After vaccination, humans develop antibodies that make them immune to cowpox, but also immune to smallpox (Variola virus). The cowpox vaccinations and later vaccines were so successful that in 1980, the World Health Organization announced that smallpox was the first disease to be eradicated by vaccination efforts worldwide.

Other orthopox viruses remain prevalent in certain regions and continue to infect humans, such as the cowpox virus in Europe, vaccinia in Brazil, and monkeypox virus in Central and West Africa. Though once common, cowpox is now rare and is generally reported only in western Europe.

The virus of cowpox is closely related to vaccinia and smallpox viruses. The cowpox virus and vaccinia were originally considered to be the same virus, and can only be differentiated by sophisticated laboratory techniques.

Before vaccination of the general population against smallpox was discontinued, some outbreaks of cowpox in cows in North America and Europe were due to infection with vaccinia from recently vaccinated persons. Vaccinia-related viruses continue to cause occasional outbreaks of teat infections in dairy cattle in South America and buffalo in India. These viruses often spread to the people in contact with cattle.

Cowpox causes inconvenience when milking cows because the teats are sore. Milk from affected cows is safe to drink unless the cow develops mastitis. Some affected cows do develop mastitis if the lesions involve the teat openings.

The virus is spread from cow to cow by milker’s hands or the teat cups of a milking machine.  Spread from one herd to another is probably due to introduction of new (infected) animals into the herd, or by human touch, or possibly carried by biting insects.  The horsepox virus can be carried to cows and causes typical cowpox lesions.

The virus probably gains access to the tissues through injuries in the skin; any nicks or abrasions on the teats makes the cow susceptible; small breaks in the skin can even occur when walking through tall grass that may have stiff or abrasive stems or seed-heads.

The virus can spread rapidly through a herd and affect cows for several weeks or months, but recovered cows have immunity that can last several years.

Infection with cowpox virus goes through 5 stages.  After an incubation period of 3 to 6 days, there is reddening of the skin, followed by firm, raised papules that are light in color with a surrounding warm zone around the base due to more blood in that area. Then a yellow blister forms, with a pitted center. The pustular stage soon follows, and then the development of a thick red scab.

These lesions are usually only seen on the teats. They become sore and the cow may resent being milked. Individual lesions usually heal within 2 weeks but in some cows there are fresh crops of new lesions that keep appearing for a month or more. In severe cases the lesions may spread to the insides of the thighs and more rarely to the vulva. Calves that nurse affected cows may develop lesions around their mouths, and bulls may develop lesions on the scrotum.

Signs

• Pustules or blisters on the teats, commonly ruptured during milking, followed by thick scabs (yellow-brown to red in color) about ¼ to ¾ inch in diameter.

Causes

The causative virus (orthopoxvirus) of true cowpox is closely related to several other viruses including horsepox and smallpox. These three may actually be variants of one virus that has simply adapted to different host species. They are also closely related to buffalopox virus.

Cowpox spreads by contact during milking. After an incubation period of 3–7 days, during which cows may have a mild fever, papules appear on the teats and udder. Fluid-filled blisters may not be evident, or may rupture readily, leaving raw, ulcerated areas that form scabs. These lesions heal within about a month. Most of the cows in a milking herd may become affected. The people milking the cows may develop fever and have lesions on the hands, arms, or face.

Most outbreaks of teat lesions today are actually due to pseudocowpox or ulcerative mammillitis. Pseudocowbox lesions are larger and more numerous but are less painful.  This condition may be more prolonged than cowpox, with a longer course of disease. Bovine ulcerative mammillitis is a more severe disease, with plaques and ulcerations on the teats. Occasionally all three diseases will be present in the same herd.

In an infected herd, most cows show signs unless adequate preventative measures are taken, or the cows have already have developed immunity from previous attacks.

Prevention

Measures to prevent spread of cowpox within a herd are based on segregation—keeping infected cattle away from the rest of the herd--and hygiene. Prevention is difficult since the virus is readily transmitted through direct or indirect contact with an infected animal. Hands, milking machines and any materials that touch or are used to clean the udder should be disinfected after contact with an infected cow. Dipping the teats in a suitable disinfectant can usually prevent immediate spread.

Treatment

There is no treatment for the virus; the only helpful treatment is to make the cow more comfortable, using a soft cream before milking and an astringent lotion after milking, to help the cow recover faster.