Morbus Aujeszky; mad itch
Aujeszky's disease is primarily a viral disease of pigs. However, dogs, cats and virtually all other domestic mammals can also contract it. For all susceptible species except pigs, the disease is fatal. The symptoms are similar to those of rabies, which is why the disease is also called pseudo-rabies. According to current knowledge, humans are not susceptible to the infection.
Pigs (domestic and wild) are the natural reservoir. Dogs, cats, other carnivores (minks, ferrets) and ruminants (cattle, sheep, goats), as well as equids, on the other hand, are false hosts that hardly play a role in the spread of the disease. The disease is fatal in all abortive hosts, only pigs survive the disease - also depending on their age.
Transmission occurs from pig to pig via the respiratory or digestive tract, in wild boar also via mating. Other animals become infected through direct contact with pigs. For carnivores, the main source of infection is ingestion of meat and offal of infected (wild) pigs. In the current epidemiological situation in Austria, hunting dogs are particularly at risk.
In pigs, symptoms range from subclinical disease (often in feral pigs) to abortions, respiratory disease, neurological symptoms and death in very young piglets. False hosts always react to the infection with a fatal disease of the central nervous system, often accompanied by restlessness, itching, self-mutilation and seizures.
In Austria, the disease does not occur in domestic pigs. Aujeszky's disease is notifiable in domestic pig herds. Vaccination is prohibited. In order to maintain this status, serological surveillance programmes are carried out every year. In addition, abortions of domestic pigs sent to AGES are also examined for Aujeszky's disease. The examination of wild boar is carried out exclusively by private order or within the framework of research projects.
Situation in Austria
Due to the permanent surveillance programme, Austria has been officially free of Aujeszky's disease in domestic pigs since 1997. In 2020, 16,135 pigs from 2,042 holdings were examined as part of the surveillance. Within the framework of abortion examinations, 137 pig foetuses were tested. A total of 18,749 pigs were serologically tested for antibodies (AK) against Aujeszky's disease (also as part of other investigations, e.g. insemination centres). All examinations yielded negative results.
The causative agent of Aujeszky's disease is suid herpesvirus 1 (SuHV-1), syn. pseudorabies virus from the subfamily Alphaherpesvirinae, genus Varicellovirus. The virus strains vary in virulence but behave in a serologically uniform manner. Weakly virulent virus strains are strictly neurotropic and, in contrast to the strongly virulent strains, do not cause further organ damage. Strongly virulent strains are detectable in the lungs (infestation of alveolar macrophages) and in the genital tract, as well as in the semen of infected boars.
The virus replicates primarily in the epithelia of the nasal and pharyngeal mucosa and the tonsils, or in the genital mucosa, and subsequently spreads to the nervous system. From the primary site, the virus travels to the CNS via afferent nerve pathways. Nervous disease symptoms develop when damage to neurons has occurred. If the primary infection is survived (only in pigs), the animals remain latently infected. At this stage they are not infectious, however, due to stressors (transport, crowding, mating season, parturition) the virus may reactivate and as a result spread.
The virus can survive in the environment at 25 °C for up to 40 days. Inactivation of the virus is achieved by heating above 55 °C or by chlorine-, ammonium- or formalin-based disinfectants. However, alcohol and phenols are ineffective.
In domestic pig herds, the pathogen is usually transmitted to healthy pigs by direct contact with infected pigs. In heavily infected herds, transmission may also occur during animal care via hand contact, through feed, inanimate objects, and/or even via air movement ("aerogenic") in close proximity. In areas with dense pig populations, infection spreads rapidly.
Other sources of infection are meat, organs, milk and semen. Pregnant sows spread the virus through aborted fetuses, placenta and vaginal discharge.
Not only susceptible but also vaccinated pigs can become virus carriers. Vaccination is therefore prohibited in Austria. After a disease has been overcome, the virus withdraws into the trigeminal ganglia and possibly the tonsils, or into the sacral ganglia (especially in wild boars), depending on the point of entry (latent infection). Stress factors such as transport etc. can lead to reactivation and excretion of the virus. The transmission of the viruses is not bound to a season. For carnivores, the most important source of infection is the ingestion of meat and offal of infected (wild) pigs.
Piglets: Initially fever, vomiting, movement disorders, circular movements, paralysis of swallowing, strong salivation; thereafter central nervous disorders: Muscle tremors, convulsions, paddling of limbs and partial paralysis; in piglets up to 2 weeks old, mortality is 100%; in piglets 3-4 weeks old, still 50%. Young animals 1-3 months of age show low appetite, rhinitis (nasal discharge), mild fever, and respiratory distress. Death usually occurs only in central nervous disorders.
Runners/fattening pigs: respiratory tract disorders, high fever, dejection, poor weight gain, rarely central nervous disorders. The incubation period is 3-5 days with a disease rate of 100% and a mortality rate of 5%.
Sows/boars: fertility problems, including abortions.
Wild boars: Usually show symptoms not as pronounced as domestic pigs - often without signs of disease.
Dogs/cats/cattle/small ruminants: inflammation of the brain and spinal cord with central nervous symptoms, salivation and severe itching. The disease is always fatal in these animals, usually after 1-3 days.
In contrast to rabies, diseased abortive hosts are thirsty, carnivores do not show symptoms of aggressiveness and ruminants are not water-shy and show pronounced symptoms of the respiratory tract, e.g. increased panting or shortness of breath.
In Austria the use of protective vaccination is prohibited. The live vaccines developed for pigs are pathogenic for cattle, dogs and cats, inactivated vaccines are not effective enough. Due to the occurrence of Aujeszky's disease in wild boars, it is recommended to review the appropriate biosecurity measures on one's own farm and to ensure appropriate prevention of contact between domestic and wild boars (e.g. by double fencing dense with wild boars).
Symptoms in dogs and cats
Diagnosis is based on clinical signs and rapid progressive course. Conspicuous symptoms are reluctance to eat, pathological sensitivity, high degree of itching followed by self-mutilation (automutilation), extreme sensitivity to touch, deafness, salivation, reddened eyelids and oral mucosa, increased respiratory rate (60/min.) as well as a frequent pulse (160/min.) In contrast to rabies, the diseased animals show thirst but not aggressiveness. The disease progresses so rapidly that no antibodies have been formed by the time of death. A reliable diagnosis is only made after death by appropriate laboratory testing.
Information for hunters
An infection of hunting dogs in contact with infected blood cannot be excluded. However, infection can be avoided if blood tested for freedom from the virus or tested organ material from a dead wild boar is used for training. Infection by inhaling the scent of wild boars is unlikely. However, since no studies on this or on the survival of the AD virus in urine are known, a definite statement cannot be made.
Direct contact between wild boar and hunting dog should be restricted as far as possible during hunting operations. In any case, "comradeship" with organs of wild boar, intensive contact with entry and exit wounds, cutting of wild boar, contact with openings or wild boar tracks should be avoided. If the protective measures are observed, the infection of hunting dogs with Aujeszky's disease can be reduced with a high degree of certainty.
Laboratory diagnostics in (wild) pigs
- Examination for antibodies from blood (serum) by ELISA or serum neutralisation test: The result allows a statement as to whether the animal has had contact with the virus; however, the result of the test does not indicate when, where and to what extent the virus is acutely present or whether it is excreted. The detection of antibodies also does not imply immunity. In international trade, the differentiation of vaccinated animals may be important, which is possible with the help of special ELISA tests due to the use of so-called marker vaccines.
- Examination by PCR from organs (tonsils, brain, spinal cord, lungs, spleen, kidney, liver, lymph nodes), including aborted material, as well as from naso- and oropharyngeal swabs: PCR directly detects the virus or virus components. Even latent infection can be detected by PCR if the corresponding target tissues (trigeminal or sacral ganglia) are sampled.
- Virus isolation: in contrast to PCR, this allows a statement to be made as to whether the virus is capable of infection, as well as further typing. Virus isolation is rarely used in routine diagnostics.
Laboratory diagnostics in case of false hosts
- As no antibodies are formed due to the rapid course of the disease, only direct pathogen detection by PCR (if necessary also by virus isolation) is useful. In this case, CNS (brain and/or spinal cord, in the latter case especially the region of the spinal cord which was responsible for the supply of the skin sites where the itching was observed) should be brought for examination.
In all cases, the shipment of samples to the laboratory should ideally be carried out with the addition of refrigerants and consideration of the appropriate transport regulations (UN3373) by an authorized logistics company.