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Neurological signs and sudden death in horses located in close proximity

Colin Peake, DV Hay, Riverina LHPA and CSU Vet students Suzie Richards and Jo Ward

Posted Flock & Herd August 2015


After cases involving neurological signs and sudden death in horses were reported in south-west New South Wales, the need for further investigation became evident. Given the environmental conditions at the time, arboviruses became an important differential diagnosis given the increased numbers of mosquito vectors present. Kunjin virus is a flavivirus of the family Flaviviridae and is known to cause encephalitis and associated clinical signs in a number of species including horses, birds and humans. Furthermore, one group of affected horses also had access to decaying potatoes as a feed source, botulism could not be ruled out as a differential.

This report details two cases involving neurological signs and sudden death in horses. Two different properties were involved, however both were in relatively close proximity to each other along the Lachlan River.


On Property 1, seven quarter horses of various ages escaped their paddock on 16 January 2011 to be found on a neighbouring property the following day. One 2yo gelding was found dead whilst the others were returned to their paddock. A 2.5yo gelding was affected with progressive clinical signs and was euthanised by the owner on 20 January 2011, before the DV was notified. Two other young horses were also showing mild clinical signs.

Whilst out of their paddock, the horses may have had access to old, mouldy wheat. No animals were kept on the neighbouring property that the horses could have had contact with. The horses were usually kept in a paddock of native pasture, where some fruiting deadly nightshade (Solanum spp.) was present. A post-mortem was performed on the 2.5yo gelding that had been euthanised.

On 18 February 2011, a property down the road (Property 2) reported sudden death in a 15yo riding gelding late the previous day. They had also lost another horse in a similar manner one week previously. Six other horses on the property were all in good condition and did not appear to be showing any abnormal clinical signs. All of these horses had access to decaying potatoes. A post-mortem was performed on the recently deceased 15yo gelding.

On 15 April 2011 a follow up visit was made to both properties to collect bloods for further investigation (plain and lithium heparin). Bloods were taken from four different horses from Property 1, and six horses from Property 2.

Both properties involved with this case were located along the Lachlan River, about 15-20km apart. Increased rainfall had led to an increased population of mosquitoes. Furthermore, a major mice plague was occurring at this time.


Property 1 was first visited on 20 January 2011. A 2yo gelding had been found dead one week previously. At the same time, a 2.5yo gelding was showing signs of lethargy and appeared ataxic, particularly in the hind limbs. The horse was stabled and fed. The following morning, the owner noted that there was no faeces in the stall. At this stage the horse was in lateral recumbency and struggled to rise. Once up, the ataxia appeared worse and the animal had difficulty urinating.

The next day the horse was again found in lateral recumbency and was unable to rise. It was at this point the owner made the decision to euthanize. The owner also reported two other young horses to have shown some signs of lethargy, however they appeared to have recovered.

On 18 February 2011, Property 2, not far from the first, was visited to investigate sudden death in two horses. One had died a week previously, and a 15yo riding gelding had been lost the night before. He was found dead in the morning in lateral recumbency with no signs of a struggle (Figure 1). The owners reported he had not been showing any clinical signs and had appeared perfectly normal the previous day. A small trickle of blood was noted from the anus, no other abnormalities could be found grossly.

Image of dead horse
Figure 1. 15yo gelding died suddenly on Property 2

Post-mortem findings

Post-mortem examination of the 2.5yo gelding on Property 1 was unremarkable. There was no evidence of bowel obstruction or torsion, and there was no inflammation or diarrhoea. A large number of bot larvae were present in the stomach, however this is not thought to have contributed to the clinical signs seen before euthanasia. Some engorgement of subcutaneous blood vessels was noted. Mild consolidation was seen in the ventral lung lobe, speculated to be blood pooling. Samples were taken, however the brain was not removed as the animal had been shot.

Post-mortem of the 15yo riding gelding on Property 2 revealed some congestion of the subcutaneous blood vessels (Figure 2). The gastrointestinal tract showed red/grey discolouration, most likely as a result of post-mortem autolysis. The liver had two 3-5cm diameter sub-capsular areas of fatty discolouration, as well as a khaki colour to part of it however the relevance of this finding was questionable. There was a green, liquid diarrhoea present in the descending colon, but no evidence of bowel obstruction or torsion. The lungs appeared blue/black in colour and were full of clotted blood, however no consolidation was noted. The airways were clear, with no fluid or froth present. Numerous petechial haemorrhages were found along the aorta, dorsal thorax and base of the heart. The heart was otherwise normal. Samples were taken for laboratory analysis.

Image of horse <em>post-mortem</em>
Figure 2. Post-mortem examination of 15yo gelding


Results of histopathology from samples taken from the post-mortem of the 2.5yo gelding showed mild lymphoplasmacytic enteritis and marked hepatic fibrosis with mild biliary proliferation and diffuse vacuolation of hepatocytes.

Results from samples taken from the 15yo gelding were somewhat unclear as some of the tissues had become autolysed. For this reason, some of the reported histological changes may have been agonal/terminal. There was some suggestion of a haemorrhagic problem, as well as evidence of chronic kidney changes. Liver changes were present that may be associated with pyrrolizidine alkaloid toxicity.


Blood samples were taken from four apparently healthy horses on Property 1. Results were as follows:

West Nile Virus IgM ELISA Kunjin Virus Antibody ELISA Murray Valley Encephalitis Virus Antibody ELISA
2yo QH mare Negative Positive Inconclusive
11yo QH mare Negative Negative Negative
3yo QH gelding Negative Inconclusive Negative
2yo QH colt Negative Inconclusive Negative

Blood samples were taken from six apparently healthy horses from Property 2. Results were as follows:

West Nile Virus IgM ELISA Kunjin Virus Antibody ELISA Murray Valley Encephalitis Virus Antibody ELISA
14yo pony mare Negative Inconclusive Negative
12yo pony mare Negative Positive Inconclusive
18yo pony gelding Positive Positive Inconclusive
18yo pony gelding Negative Positive Positive
12yo pony gelding Negative Positive Inconclusive
12yo pony gelding Negative Positive Negative


In this case, three horses experienced sudden death across two properties in relatively close proximity along the Lachlan River. A fourth horse experienced rapid deterioration before it was euthanised by the owner. Post-mortems were carried out on two of these horses, however results were unremarkable and inconclusive.

Differential diagnosis list included:

As the properties were both located along the river, and given the environmental conditions at the time (increased rainfall), arboviruses must become a differential diagnosis given the very large increase in numbers of mosquito vectors present.

Kunjin virus (an Australian subtype of the better known West Nile Virus) and Murray Valley Encephalitis virus are both flaviviruses of the family Flaviviridae and are known to cause encephalitis and associated clinical signs in a number of species including horses, birds and humans. They are both arboviruses, transmitted by mosquitos. The principal vector for Kunjin virus is the fresh-water mosquito (Culex annulirostris) which is found throughout Australia (Roche et al., 2012). Birds generally remain asymptomatic and act simply as a reservoir and amplifying host for virus, however it is also thought that a number of bird and other mammalian species may too play a role in the transmission of Kunjin virus (Gray et al, 2011).

Transmission of the Kunjin virus is believed to be mainly via haematogenous spread following injection of the organism in mosquito saliva. The virus then replicates in local tissue and regional lymph nodes, resulting in primary viraemia and subsequent infection of extraneural tissues. Eventually the central nervous system becomes infected and neurological signs develop, however the mechanisms by which the virus penetrates the blood brain barrier remain largely unknown. The incubation period is thought to vary anywhere from a few days to a few weeks (Kimura et al., 2010).

Initial clinical signs associated with Kunjin virus are generally depression and mild colic. These are typically followed by the onset of neurological signs, which may range from weakness in the hindquarters, hypermetria in the forelimbs and generalized ataxia to facial paralysis and difficulty masticating. Mortality remains fairly low at around 10-15%, however if death occurs it tends to be fairly rapid (Roche et al., 2012).

Diagnosis of Kunjin virus is achieved through serological antibody and antigen testing. Infection is confirmed by a significant rise in antibody titre to the virus in two sequential blood samples, or isolation of the virus antigen (Daffis et al., 2011). In this case, serological testing was completed for West Nile Virus, Kunjin Virus and Murray Valley Encephalitis Virus. It must be remembered that the West Nile Virus IgM ELISA detects IgM against all members of the West Nile Virus group, including Kunjin. A positive IgM result indicates a recent infection.

The results from blood tests from the four horses on Property 1 showed a probable exposure to Kunjin virus. Serology from the six horses on Property 2 was consistent with a recent infection with Kunjin Virus, although infection with another related virus cannot be completely excluded until a second serum test is completed.

The affected horses from the second property had access to decaying potatoes as a feed source, and hence botulism was thought also to be a relevant differential diagnosis. Furthermore, a major mice plague was occurring at the time.

Botulism is a gram positive, spore forming clostridial bacterium. It is an obligate anaerobe that can be transmitted through the soil, rotting carcases, rodents or decaying feedstuffs (eg potatoes). Following exposure to this organism, the botulinum toxin is produced which is the actual cause of disease. The botulinum toxin is a neurotoxin preventing the release of acetylcholine at the neuromuscular junction, resulting in an ascending flaccid paralysis (Kamargo et al 2008).

In horses, clinical signs of botulism generally appear 3-17 days after exposure to the infected material. Severity of disease and rapidity of onset of clinical signs is dependent on the duration and level of exposure. Peracute cases may experience sudden death with no signs of illness. The most common clinical presentation is of a generalised weakness of the limbs and difficulty masticating and swallowing due to paralysis of the jaw and throat muscles (Radostits et al 1994). This clinical picture is similar to that seen with the Kunjin virus.

Definitive diagnosis of botulism is quite difficult. Post-mortem findings are nonspecific, however the toxin may be able to be isolated from the liver, stomach contents or feed source. Perivascular haemorrhages have also been reported in the brain although consistency of this finding is questionable. Therefore diagnosis is generally made by elimination and consistency of history and clinical signs (Radostits at al, 1994).

The most likely hypothesis in this case is an outbreak of the Kunjin virus resulting in neurological signs and relatively sudden deaths of horses in the area. However, as the 15yo gelding from Property 2 died overnight without displaying any clinical signs and with no evidence of any struggle, peracute botulism was thought to be the most likely diagnosis for this animal. After fencing off access to the decaying potatoes, no more deaths were experienced on this farm.


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