Louping-Ill Virus

Louping-Ill Pathology in Agricultural Animals

Louping-ill has been best characterized in sheep due to its relative rarity in humans. Infected sheep develop fever and weakness, progressing to meningoencephalitis with cerebellar ataxia, generalized tremor, jumping, vigorous kicking, salivation, and clamping of jaws. This may be followed by paralysis, coma, and death. In one study of experimentally infected sheep ( n = 8) and lambs ( n = 4) inoculated intracranially with LIV, all animals developed severe neurological symptoms, progressing from slight ataxia to complete flaccid paralysis 6–18 h later. High levels of viremia were also seen, ending prior to the onset of severe neurological symptoms in all of the sheep, but viruses were still detectable at death in the four lambs. Two of 12 animals inoculated with a 10-fold higher viral dose subcutaneously were killed when moribund and four of the remaining animals developed nonfatal neurological symptoms. Hemagglutination-inhibiting (HI) antibodies were present in the serum of all animals inoculated subcutaneously, but were absent in those inoculated intracranially. Antibodies were detected in sheep significantly earlier than in lambs.

In a separate study by the same research group, 33 sheep were inoculated subcutaneously with LIV. Of these, 12.1% died and an additional 54.5% were killed with severe neurological symptoms. These animals became moribund between days 6 and 11 postinfection. Ataxia rapidly progressed to complete flaccid paralysis within 3–5 h. The surviving sheep did not develop the symptoms of encephalomyelitis, although two of them became chronically debilitated. The maximum levels of viremia were seen on days 3–4, but, as in their previous study of sheep inoculated intracranially, viremia rapidly declined until none of these adult animals remained viremic at the time of death or development of severe nervous symptoms. The extent of viremia reflected the severity of the subsequent pathology. The subsequent decrease in viral titer was linked to the appearance of circulating HI antibody and the rise of neutralizing antibody activity. Antibodies appeared sooner and rose to higher levels in surviving vs susceptible animals and appear to be the major protective factor in survivors, although small amounts of interferon were also present. Unlike the situation in many other viral infections, the immune response protected the animals, rather than leading to pathogenic inflammatory responses.

Neuropathological lesions in sheep infected with LIV either naturally or experimentally are characteristic of nonsuppurative meningoencephalomyelitis with focal microgliosis and neuronal necrosis with neuronophagia. Lesions in lambs are localized and are often distributed along with neuronal processes, suggesting that LIV may be disseminated along axonal pathways. Necrosis is most common in the Purkinje cells of the cerebellum and the medulla and spinal cord. Mononuclear, and some polymorphonuclear, cells may be present in these parts of the central nervous system (CNS). Histiocytes within the areas of gliosis contain LIV antigen and may contribute to viral clearance. The perivascular cuffs contain plasmacytes and sometimes polymorphonuclear cells. Coinfection with LIV and the bacterium Anaplasma phagocytophilum or an acute infection with Toxoplasma gondii , a neurotropic protist, increases louping-ill pathology and is accompanied by a reduced antibody response.

Naturally occurring, fatal cases of louping-ill have been found in cattle in Dartmoor, England, since 2001. When six calves were experimentally inoculated subcutaneously with LIV, all developed short-term low-intensity viremia that subsided when HI serum antibody was detected. One calf developed severe meningoencephalitis and mild neurological changes were seen in one other animal. Due to the low virus levels, calves do not appear to play a role in the transmission of LIV to ticks or result in the infection of other vertebrates. This may or may not be the case for adult cattle.

Naturally occurring LIV-associated neurological disease was found in a Scottish colt, which subsequently recovered within 2 weeks, and in a cluster of seven naturally infected horses in Devon, England. The animals’ ages ranged from 7 months to 20 years and included several breeds. All of the LIV-infected English horses developed some degree of ataxia, with some having a nearly complete inability to stand or move. Other symptoms included muscle tremors of the neck and facial area, anorexia, depression, mania, and avoidance of bright daylight. All of the infected horses lived on the edge of a moorland region that contained LIV-infected sheep. Upon testing 68 other, apparently healthy, horses living within a two-mile radius, an additional five seropositive animals were found, two of which had previously demonstrated neurological symptoms. Four LIV-infected free-range horses in Ireland also developed encephalitis, resulting in the death of two of the animals. LIV was isolated from both the brain and spinal cord of one horse. Experimental infection of horses results in fever, moderate viremia for 2–3 days (one-thousandth of the level present in sheep), and antibody production. Age was not a factor in LIV infection of horses. Based on these findings, horses may have the potential to serve as amplifying hosts for LIV.

Louping-ill has also been reported in an alpaca on the edge of Dartmoor. It presented with posterior weakness, progressing to recumbence when under stress, and tremors of the head and neck. While the farm did not have a recorded history of tick-associated disease, it is surrounded by common moorland grazing areas and neighboring farms had reported louping-ill in both cattle and sheep within the previous 2 years. Louping-ill in this alpaca is similar to that previously reported in a llama on the Island of Lewis.

Louping-Ill Pathology in Birds

Experimental infection in the tarsal pad of red grouse results in a high rate of mortality and sufficient viremia to infect tick vectors. Symptoms in infected grouse include depression, anorexia, muscular weakness, and regurgitation of crop contents on handling. In this study, within 2 weeks, 78.4% of the birds died ( n = 37). Louping-ill additionally impairs breeding success in red grouse, according to a 1978 study that found that the number of chicks per brood was only 0.6 in areas with high tick abundance compared to 5.5 per brood in low-tick areas. Similarly, in high-tick regions, 84% of adult grouse had anti-LIV antibody ( n = 61), while only 1 of 10 grouse was seropositive in low-tick regions. Infected chicks (15.4% of 162 tested) also weighed significantly less than similarly aged uninfected birds. Viremia in adult grouse peaks at day 4 of infection and its decline is associated with the presence of high titers of HI antibody. The magnitude and the duration of viremia in those birds that die are greater than those seen in surviving grouse and also than those found in sheep. Grouse are the only wild animal species that regularly develop sufficient levels of looping-ill viremia after experimental infection to be able to infect the tick vectors.

The presentation of encephalitis in experimentally infected grouse differs from that in sheep, being more rostral, with less prominent motor neuron necrosis. It should be noted, however, that the routes of inoculation differ among studies in grouse and other animals. In grouse, mild foci of nonsuppurative meningitis are seen in the more anterior regions of the brain, over the cerebrum and optic lobes, on days 5–7 postinfection, while nonsuppurative encephalitis is seen on day 8. During the next week, the disease increases in severity and becomes more generalized. The early inflammatory response consists of pericapillary and perivenular cuffs of two types of unclassified, dividing mononuclear cells in the meninges and underlying brain tissue. Lymphocytic infiltrates were only seen in 1 of 32 infected birds. Degenerate neurons were rare and primarily localized in the anterior brainstem and basal ganglia. A similar disease is seen following experimental infection of the moorland willow grouse and the tundra ptarmigan. Subclinical infection with low levels of viremia occurs upon infection of several species of woodland birds, including black grouse, capercaillie (a species of woodland grouse), and the ring-necked pheasant. It appears, therefore, that LIV first became established in the forests and woodlands, where the avian species developed immunity. LIV appears to have much more recently moved into the upland grazing areas due to the introduction of sheep into these regions. The red and willow grouse, therefore, have yet to develop an effective resistance to LIV infection.

Louping-Ill Pathology in Rodents

Intraperitoneal infection of mice with LIV leads to 100% mortality, but only when using high levels of virus. Antiserum against LIV prevents clinical manifestation, infection, and mortality in mice inoculated with either LIV or TBEV. Inoculation of mice with cerebrospinal fluid (CSF) from a louping-ill patient causes disease in a small number of animals, some of which subsequently leap wildly about their cages. After several passages in mice, rodents that were inoculated intracranially develop symptoms and lesions typical of those seen in murine louping-ill. The virus initially circulates in the blood, but not the CNS. Later, the virus is absent from the blood as a HI antibody response develops and, although the animals appear to be well, the virus is present in the brain and CSF. These mice then develop encephalitis since insufficient levels of circulating antibody pass through the blood–brain barrier.

Although experimental infection of field voles, bank voles, and wood mice induces anti-LIV antibody production, it fails to produce a sufficient level of viremia to support transmission to ticks. In Scotland, field-caught small mammals carry few ticks and these are uninfected larvae. Additionally, none of these mammals are LIV-infected or have seroconverted. Very low levels of viremia are also found in brown rats. It is thus unlikely that rodents play a significant role in LIV maintenance or transmission in nature.

Louping-Ill Pathology in Nonhuman Primates

Rhesus macaques, patas monkeys, and vervet monkeys are resistant to LIV inoculated subcutaneously with LIV. They do not develop clinical signs and, only rarely, brain lesions. These monkeys do, however, become viremic and some develop antibodies. Following intranasal or intracranial inoculation, however, all monkeys develop lesions characteristic of acute encephalitis, whether or not they are symptomatic. Small levels of LIV are present after day 9 postinfection, followed by very high levels between days 14 and 16 that decrease slowly. Lesions in the CNS consist of very large meningeal cellular exudates, multilayered perivascular cuffs containing lymphocyte-like cells, and diffuse and focal microglial infiltration accompanied by neuronal degeneration. Eosinophilic necrosis, disappearance of neurons, and spongy degeneration occur in Purkinje cells and large neurons in the dentate nucleus of the cerebellum. In severe cases, microglial infiltration occurs together with very high degrees of proliferation and hypertrophy of astrocytes. The anterior spinal cord columns, the reticular formations and spinal nuclei of the medulla, the substantia nigra of the midbrain, and the medial nucleus of the thalamus are also damaged.