Vesicular stomatitis and other vesiculovirus infections

C J MARÉ AND D G MEAD

GENERAL INTRODUCTION: RHABDOVIRIDAE

Introduction

Vesicular stomatitis (VS) is a viral disease, most often affecting horses, cattle and pigs, characterized by fever, and by vesicular and erosive lesions on the tongue, gums, lips, teats, prepuce and the coronary bands of the feet.^{18, 19} Infection in dairy herds can result in substantial losses in milk production, and serious weight losses may occur in beef cattle and pigs. In horses, restriction of their use and movement frequently result in disruption of equestrian events. Of particular concern with VS in ruminants and pigs is its close clinical resemblance to foot-and-mouth disease (FMD), one of the world’s most feared livestock diseases. As a result, the diagnosis of VS often results in onerous and economically devastating quarantine measures. Subclinical infection with vesicular stomatitis virus (VSV) is common in livestock,^{36, 51} and probably in many wildlife species.

Human infection with VSV is frequently seen in persons in close contact with sick animals^{21, 54} and in laboratory workers.²⁵ The disease in humans is a self-limiting illness characterized by influenza-like symptoms including fever, nausea, chills, vomiting, headaches and muscular pains. Vesicular lesions are rarely seen.³⁹

The early history of VS has been exhaustively reviewed.^{18, 19} In summary, the first formal report of VSV in the USA was in 1916,⁴⁷ but historical evidence suggests that VSV has been present in that country for well over 100 years.^{8, 20, 52}

The descriptions of ‘sore tongue’ in cattle, horses, and pigs in the eastern USA in 1801/02, and in 1817 accurately fit modern descriptions of the disease. General G.B. McClellan is credited with the first report of what may have been VS during the Civil War in 1862.¹⁸ During that time, over 4 000 horses in the Union Army were incapacitated with what is now believed to have been VS.

Vesicular stomatitis is essentially a disease of the Americas. It has, in the past, been transported to France from the USA via military horses,²⁴ but it did not become established there.

Several South African reports of stomatitis resembling VS^{4, 32, 49, 55} were not confirmed to be VS by virus isolation, nor were they obviously related to animal importation from the western hemisphere. If they were, in fact, VS, there is no indication that the disease became established in South Africa. There is currently no evidence of the existence of VS outside of the western hemisphere.

Aetiology

Vesicular stomatitis is caused by a group of antigenically related but distinct viruses belonging to the genus Vesiculovirus in the family Rhabdoviridae. ³⁷ This very large family of viruses includes viruses of mammals, birds, fish, insects and plants. Several of the vertebrate and plant rhabdoviruses are transmitted by insects. Two serotypes of vesicular stomatitis virus, VSV-Indiana (VSV-I) and VSV-New Jersey (VSV-NJ), have been associated with vesicular disease among cattle, horses, pigs and humans. There are three serologically distinct types of VSV-I:¹⁵ the classical Indiana (type 1), Cocal (type 2), and Alagoas (type 3). Only a single type of VSV-NJ occurs. Vesicular stomatitis virus Indiana was first isolated from a cow in the state of Indiana in 1925,¹² while VSV-NJ was isolated from cattle and horses in the state of New Jersey in 1926.¹³

By far the most frequent cause of VS in the USA has been VSV-NJ, whereas VSV-I, a common cause of VS in Central and South America, was not diagnosed in the USA from 1965 to 1997, when it suddenly reappeared in the southwestern USA. The VS viruses are also known to cause disease in Mexico, Colombia, Venezuela, Panama and Costa Rica.^{28, 43} The Cocal and Alagoas viruses have not been recognized in North America. Cocal virus was isolated in Trinidad and Brazil in 1964 from rodents, mosquitoes and mites,^{26, 27} and was first associated with VS in horses during an outbreak that affected horses on more than 50 premises in Argentina in 1963/64.¹⁵ Alagoas virus, isolated from mules in Brazil in 1964, has been associated with VS in horses, mules and cattle.¹⁵

Epidemiology

The host range of VSV is extremely broad, and the widespread presence of antibodies to the viruses in humans, livestock, pets, and many species of wildlife in endemic areas suggests that most vertebrates may be susceptible to VSV. However, no vertebrate reservoir of the viruses in nature has been identified. Vesicular stomatitis virus has also been shown to replicate in black flies (Simulium spp.),^{31, 33} sand flies (Lutzomyia spp.),^{10, 48} the mosquito Aedes aegypti,⁵ and the leafhopper Peregrinus maidis(Ashri.).²⁹ Thus, it is a virus of both vertebrates and invertebrates.

The aetiological agents of VS, especially VSV-NJ and VSV-I, are among the most thoroughly researched of all animal viruses, and yet, paradoxically, the epidemiology of the disease itself remains an enigma, with major portions of the maintenance and transmission cycles of the viruses remaining unknown. It is clear that contact transmission of VS can occur, but recent evidence suggests that the disease is only mildly contagious. During the 1995 VS epidemics in the southwestern USA stringent quarantine measures were imposed in attempts to limit spread of the disease,⁷ yet the disease continued to move rapidly northwards apparently unimpeded by these restraints. Epidemiological...