Epidemiology of Aujeszky's disease in swine and cattle

Viggo Bitsch; DVM, DVSc
viggo.bitsch@gmail.com
ISBN 978-87-994685-1-5

Genital/venereal infection was earlier common in small swine herds in many European countries, and there is evidence that infection of cattle resulting in pruritus on the hindquarters had regularly been sexually transmitted from swine by man.

Abstract Aujeszky's disease – infection withSuid herpesvirus 1 (SuHV1) – is an infection of pigs, which under certain conditions can be spread to several other animal species, usually with a fatal outcome. It has been found world-wide, but has now been eradicated in a number of countries. Denmark is the country, where Aujeszky's disease has been studied most intensively, and the results from investigations of cattle have been of greatest importance for the understanding of the epidemiology of the infection in swine. Denmark was the first country to initiate eradication and was also the first to complete eradication of the indigenous infection. In this review, important features are recapitulated at the end of each section or subsection.

In cattle, the infection will most often give rise to an intense pruritus of a skin area. In iatrogenic cases the site of virus introduction will determine the site of pruritus, but in natural cases pruritus will appear (1) in the head region or on the chest, which is associated with respiratory infection, or (2) on the hindquarters of females, which is indicative of vaginal infection. In cases of respiratory infection, virus can be found in the mucous membranes of the nasal or pharyngeal cavity of animals with head pruritus, while in cases with chest pruritus virus can be found in lung tissue. In cases with pruritus on the hindquarters, virus can most often be found in the vagina. Pruritus must be considered a phantom sensation due to stimulation or damage to the central nervous system including sensory ganglia. Practically speaking, respiratory infection of cattle is a dead-end infection, as spread among cattle has never been demonstrated. Relatively high virus titres found in mucous membranes in a few cases, however, may be taken to indicate that such transmission cannot be totally excluded.

The source of respiratory infection in cattle has always been pigs infected by the respiratory route. Cattle at risk are not necessarily those placed near the infected pigs, as virus in an animal house has been seen transmitted by air currents determined by ventilators over distances of 10-20 meters, in some cases even from pigs in a neighbouring room.

In swine, the infection was considered to be exclusively respiratory for a very long time, and naturally occurring genital infection was not demonstrated until 1981. In herds with outbreaks in cattle characterized by pruritus on the hindquarters (vaginal infection), respiratory infection of pigs was never observed, but a sow would characteristically have been served by a foreign boar from 5-14 days earlier. In some such cases investigated, genital infection was confirmed in sows on the same farm, which clearly linked cases in cattle with a posterior localization of pruritus to a concurrent genital infection of swine. Naturally occurring genital infection has later been found also in wild pigs in the USA.

Respiratory infection of pigs was demonstrated late in the history of the disease. The first outbreak in Denmark was in 1964, and in the preceding 33 years there had been only 3 outbreaks in cattle showing an anterior localization of pruritus indicative of respiratory infection, but close to 60 outbreaks, where the affected animals showed a posterior localization of pruritus indicative of genital infection.

The development of the disease situation in pigs in Denmark has clearly illustrated that SuHV1 has the ability to change in the degree of pathogenicity over time. Changes occurred in two steps. In the early 1960's respiratory strains developed, which were spread rapidly between herds due to animal contacts, mainly by trade, and later in the 1970's strains developed, which had an even higher degree of pathogenicity for both cattle and swine. These new strains were found to be syncytial in contrast to earlier isolates from traditional outbreaks. And that the new respiratory or syncytial strains had not been introduced from abroad was confirmed with absolute certainty by restriction fragment pattern analyses of virus DNA.

That syncytial virus strains were more virulent than non-syncytial strains was later substantiated by results from examination of isolates from England and Northern Ireland.

As concluded from the studies of respiratory SuHV1 infection in cattle, the decisive spread of a respiratory infection among swine in a ventilated animal house will be by air currents over many meters determined by the ventilation system and not by close animal-to-animal contacts. This feature also illustrates that during an acute outbreak in a swine herd considerable amounts of virus will be blown out into the surroundings by ventilators, exposing neighbouring herds to a risk of infection. Early observations in Denmark allowed the conclusion that airborne spread between swine herds may occur over several kilometres, but after eradication of the indigenous infection, new virus strains were introduced from abroad, which demonstrated that airborne spread of virus between herds took place over even 10 to 30 kilometres. Syncytial strains of SuHV1 have shown to possess a pronounced tendency to airborne transmission among swine herds, which implies that if a syncytial strain is first introduced into an area, it is likely to be the prevailing type in that area within a few years.

The special manifestation of Aujeszky's disease in cattle showing pruritus on the hindquarters was regularly associated with use of a boar from a boar centre for natural service of a sow shortly before appearance of the clinical disease. In most cases examined, virus was found in the vagina of the affected bovine animals, although in low titres, and in three outbreaks – the only cases investigated early enough to be successful - virus was demonstrated in the vagina of a sow. It was found that animal sodomy (zoophilia/bestiality) seemed to play a role in the transmission of the genital infection from swine to cattle, and this conclusion is further substantiated by comprehensive supplementary information given in this review article. From the fact that genital infection in cattle is closely correlated with contemporary genital infection in swine on the same premises and from the many reports over the years on infection in cattle showing pruritus on the hindquarters, it can additionally be concluded that the SuHV1 infection in a great part of the 20th century was maintained as a porcine genital infection in many European countries.