Transmissible spongiform encephalopathies (TSE)
Transmissible spongiform encephalopathies
- Geographic distribution
- Cause
- Host species
- Transmission
- Clinical signs
- Diagnosis
- Risk of introduction
- Effects of introduction
- Control
Transmissible spongiform encephalopathies (TSEs) are invariably fatal diseases characterised by lengthy incubation and neurological signs. They are caused by infectious agents of uncertain nature known as prions. The animal diseases in this group are scrapie, bovine spongiform encephalopathy (BSE), transmissible mink encephalopathy (TME) and chronic wasting disease of deer (CWD). TSEs resulting from infection with the BSE agent have been diagnosed in domestic cats and several captive species of felidae and antelopes. Similar diseases (Creutzfelt Jakob disease and kuru) occur in humans.
Also see the TSE Surveillance page for information about Surveillance programs and incentives, Bovine Spongiform Encephalopathy and Ruminant Feed Regulations.
Geographic distribution
Scrapie is a fatal brain disease of sheep and goats. It has been recognised in Europe since the mid-18th century and occurs worldwide. Only a few sheep producing countries (Australia and New Zealand among them) are recognised to be free from the disease. The disease has spread by international movement of breeding sheep from infected countries. It was introduced by that means (sheep from the UK) to New Zealand and Australia in 1952. The disease was eliminated by depopulation of infected and in-contact flocks and four year quarantine of the affected properties.
In recent years a condition known as 'atypical' scrapie or 'Nor 98' has been described. Although having some similarities to scrapie, this condition is clinically, pathologically, biochemically and epidemiologically unrelated to 'classical' scrapie. Atypical scrapie/Nor 98 is probably not contagious and may, in fact, be a spontaneous degenerative condition of older sheep.
BSE is mainly a disease of British cattle, although cases have been reported in all but one of the countries of the European Union (Sweden) as well as several other countries in which the disease has been associated with importations of European cattle or meat and bone meal. The epidemic of BSE in the UK has been waning since 1993 and since 2002 in the rest of Europe.
Feline spongiform encephalopathy (FSE) in domestic cats and zoo cats and TSE of antelopes in zoos have been reported in animals of European origin. These TSEs are caused by infection with the BSE agent.
TME is a rare disease of farmed mink and has been recorded in the USA, Canada, Finland, Germany and Russia. Its most recent occurrence was in 1985, in the USA.
CWD was first observed in the 1960s but until 1997 appeared to be restricted to a rather limited area in the western United States. However, it has subsequently been found in free-ranging and/or farmed cervids in Nebraska, Wisconsin, Montana, South Dakota, Oklahoma, Kansas, New Mexico and the Canadian provinces of Saskatchewan and Alberta. CWD has also been detected in South Korea in an elk imported from Canada.
Cause
TSEs of animals are caused by infectious agents of uncertain nature called prions. Infected neurones contain an accumulation of an abnormally folded form of a normally coded protein known as PrP, which is infectious and is partially resistant to protease. No DNA has been demonstrated though, in many respects, such as strain variation and mutation, prions behave as if a genome were present. The agents causing TSEs are unusually resistant to most forms of disinfection but are not indestructible.
Host species
Sheep and goats are naturally susceptible to scrapie. The definitive host for the BSE agent is cattle, but it has also been demonstrated to be the cause of feline spongiform encephalopathy (FSE) in domestic cats and zoo cats and TSE in antelopes in zoos. Sheep and goats have been experimentally infected with BSE, producing a disease indistinguishable from scrapie but very extensive surveillance has allayed earlier fears that BSE could be present in European sheep and goat flocks. CWD affects several species of deer and elk. TME occurs only in mink.
Transmission
Scrapie is contagious. Lateral transmission occurs naturally, possibly by consumption of or exposure to, infected placentas. Infection may also be transmitted to lambs in the milk of infected ewes.
The origin of BSE is not known. Some experts speculate that it may have resulted from the scrapie agent crossing the species barrier into cattle through the practice of feeding meat and bone meal containing tissue from infected sheep. Subsequent to infection establishing in cattle, it was recycled by bovine material included in meat meal. This might also have had the effect of passaging the agent in cattle and favouring the selection of a cattle adapted strain. Though infectious, BSE does is not contagious. No lateral spread or maternal transmission has been detected by a number of studies.
FSE in domestic cats caused by BSE agent is presumed to have resulted by ingestion of food containing infected cattle offal or meat and bone meal. Similarly TSE in some captive antelopes was probably caused by feeding of meat and bone meal. In the case of disease in felidae in zoos, direct feeding of uncooked cattle carcasses containing central nervous system tissue is implicated. FSE is not contagious.
The origin of TME is not known but it may have originated from feeding scrapie infected carcases to farmed mink. TME is not contagious.
CWD is contagious but the means of transmission is not known.
Clinical signs
Scrapie is insidious in its onset. Exercise tolerance is reduced and the gait unsteady. Affected animals drink small quantities of water frequently and urinate abnormally, passing small quantities of urine. Pruritus (itchiness) is common (though not present in all cases) and animals rub or nibble themselves in an attempt to relieve the irritation, causing loss of wool. Rubbing the back commonly stimulates a nibble reflex. Animals may be nervous or aggressive and may separate from the flock. Hypersensitivity to sound or movement may be seen. Muscular twitches or tremor may occur. Ataxia of the hind limbs is a major feature. Sometimes animals will hop or trot. Some animals, however, are found dead without apparently having shown any of those signs. (This was the case with the animals that died of scrapie during quarantine of sheep imported into New Zealand from the UK in 1972.)
BSE has a peak incidence in cattle 4 to 5 years old. Affected animals may exhibit hypersensitivity to touch or sound, excessive nose licking and teeth grinding, apprehension and frenzy, and abnormalities of posture and gait including low head carriage, arched back, abducted, stiff, straight and straddled hind limbs, hind limb ataxia, swaying, trotting, hypermetria and falling. There is loss of condition and milk yield. The clinical course of the disease usually lasts a few months, but can be as short as 2 weeks or as long as a year or more.
In captive antelopes, SE has a short clinical course. The signs are neurological and mimic those of scrapie and BSE in cattle.
In domestic cats, SE results in ataxia and abnormal behaviour. Altered grooming habits and hyperaesthemia are common. Abnormal head posture, tremors and salivation occur in some cases. There is difficulty in positioning for defaecation or urination. Animals walk with a crouching gait. Unusual aggression or timidity may be exhibited. In captive wild felidae ataxia is a consistent feature.
CWD is characterised by a loss of body condition and changes in behaviour in affected deer or elk. Animals may become anti-social and difficult to handle. They may show repetitive behaviours such as pacing, sleepiness or depression, or may carry their head and ears lowered. In later stages many animals drink and urinate excessively, lose their coordination, drool and slobber profusely and have difficulty swallowing. Death is inevitable.
However, sometimes signs are not so obvious and it is not uncommon to see cases of older animals in excellent condition losing weight quite suddenly, not responding to treatment, and dying from pneumonia caused by inhaling food or cud.
Most cases of CWD are in animals three to seven years old, although the disease has been seen in animals as young as 17 months and as old as 15 years. The length of time the animals are sick may vary from a few days to a year, with most surviving for only a few months. Occasionally sudden death may occur.
In mink affected with TME, hyperaesthesia, hyperexcitability and increased aggression are common signs. Ataxia develops, a creeping gait is adopted, circling and body tremors occur and blindness is common. The course of clinical disease is a few weeks.
Diagnosis
The presenting clinical signs usually suggest the possibility of a TSE. Confirmation can be made by microscopic examination of brain tissue for neuronal vacuolation and other pathological changes. MAF's National Centre for Disease Investigation acquired the new "Prionics" western blot test for TSEs in 2000. This test has the advantage that it can be applied to brain samples which are not fresh enough for microscopic examination.
Risk of introduction
Scrapie would only be introduced by the importation of live sheep from countries where the disease occurs. New Zealand policies permit the importation of sheep embryos and semen, as accumulated scientific research has shown that these commodities are unlikely to transmit scrapie. Nevertheless, importation is via a rigorous quarantine process.
BSE could be only introduced by imported live cattle or meat and bone meal. New Zealand import policies prohibit importation of live cattle and meat and bone meal from countries where BSE occurs.
CWD could be introduced through importation of deer and elk, or their semen and embryos, and for this reason importation of these from North America have been suspended.
Effects of introduction
Introduction of scrapie could have adverse effects on the export of live breeding sheep and embryos. Meat exports would probably not be affected, but the developing biopharmaceutical industries could be severely affected. The cost of any control programme would be considerable.
Introduction of BSE would have a much greater impact. It would require the exclusion of a range of tissues (so-called Specified Risk Materials) from all animal and human food chains, possible exclusion of cattle over 30 months of age from food chains, and possible testing of cattle at slaughter. Exports of meat, biopharmaceuticals and livestock would all be adversely affected.
Control
Any diagnosis of scrapie would almost certainly result in quarantine of the entire sheep or goat flock in which affected animals were found. Further action would depend on the results of epidemiological investigation to determine the extent of the problem and consultation with the sheep industries over their willingness to embark upon a control programme. Destruction of affected flocks and in-contact flocks was the policy followed in the early 1950s when the disease was introduced by sheep imported from UK. Though expensive, it was effective.
BSE would be dealt with by destruction of individual affected animals, and incineration of their carcases. Offspring of clinically infected animals would be traced and probably destroyed.
Page last updated: 28 October 2009