Age affected: Piglets pre-weaning, weaners, (all ages).
Causes: Swine pox virus.
Effects: Red spots, brown or black scabs, conjunctivitis, stunting.
Caused by the swine pox virus, a large DNA virus of the swine pox group, and in the past by vaccinia virus. These viruses are extremely resistant to drying and may persist for years in exudates in which they can resist temperatures of 100°C for 10 minutes. Swine pox virus grows in pig kidney cell lines or on the chorioallantoic membranes of eggs to cause recognisable changes (CPE) and intracytoplasmic inclusions.
The virus enters locally via skin abrasions and multiplies there or attacks the skin after viraemia. The incubation period is 4-5 days from infection to the development of pustules and these last 10-14 days. All stages from macule, papule, vesicle, pustule to ulcer may develop. Viraemia (presence of virus in the blood) in sows results in the birth of congenitally-infected piglets. Serum neutralising antibody develops at 7 days, peaks at 20 days and has largely gone by 50 days post infection. Cellular immunity peaks at 11-21 days post infection.
Mode of transmission
Transmission is usually from pig to pig directly, but rubbing or scratching on contaminated pen furniture is an important route of infection, especially if there are protrusions which can break the skin. Bites by insects (lice and flies), cuts and scratches can also allow the entry of infection. Occasionally swine pox may occur in neonates as it can pass the placenta when the sow becomes viraemic. The resistance of the virus to cleaning and disinfection when dried means that it may recur even after extensive cleaning. Both carrier pigs and contaminated items such as implements and clothing can transmit the disease to a clean farm.
The incubation period is 3-6 days but may be up to 14 days and the lesions persist for 1-3 weeks. Slight fever (to 40°C, 104°F) may accompany the appearance of the lesions. Red 1 cm papules (spots) appear on the ventral abdomen and rapidly form circular red brown scabs, which soon blacken. In young piglets the bursting of the vesicular (blister) stage on the face may lead to wetting, scab formation and conjunctivitis.
Slight inappetence may accompany the development of the lesions. Affected pigs appear hairy and growth may be depressed. A large percentage of successive litters of sucking and recently-weaned pigs may be affected but the disease is rare in adults. Mortality is rare although transplacental infection in newborn pigs can cause deaths. Lesions may be seen on the edge of the tongue, lips and all over the body. Badly affected newborn piglets may have to be killed, but all other age groups normally recover.
The size and colour and especially the uniformity and circular shape of the lesions is characteristic. Flank biting, ‘spirochaetal granuloma’ and local infections with Staphylococcus hyicus may also resemble pig pox but the agents of these are usually demonstrable.
Affected pigs rarely die from the disease, but dead pigs may be presented for examination with the lesions described above. Pathological examination confirms a diagnosis of swine pox infection as the skin lesions are those of a typical pox virus. Nuclear vacuolaton and cytoplasmic inclusions may be seen at the papular stage in the cells of the epidermis. A mild vesicular stage occurs but the pustular and crusting stages are most prominent. Secondary bacterial infections may occur. Electron microscopy of scabs, scrapings or culture can confirm the presence of the virus and differentiates if from that of Orf.
Treatment and prevention
Treatment or control are rarely attempted because of the mildness of the disease, but the use of insecticide to eliminate lice and flies which transmit the disease on a farm and the thorough cleaning and disinfection of pens in which outbreaks of the disease have occurred help to reduce the incidence.
Cleaning and disinfection is necessary because the virus may persist in a farm for years between outbreaks in dust and dried secretions in unused pens and fittings. The residual virus infects progeny of newly purchased pigs and weaned pigs from non-immune farms when mixed with carriers or placed in infected pens. Vaccines have been produced experimentally but do not protect completely as fever may still occur on challenge. None are available commercially.