Malignant catarrhal fever

Previous authors: H W REID AND M VAN VUUREN

Current authors:
D O’TOOLE - Professor, MVB, PhD, Dip ECVP, FRC Path, Department of Veterinary Sciences, 1174 Snowy Range Road, Laramie, Wyoming, USA, 82070
H LI - Microbiologist, DVM, MS, PhD, Animal Disease Research Unit, Agricultural Research Service, USDA, 3019 ADBF, Washington State University, Pullman, Washington, USA, 99164

GENERAL INTRODUCTION: HERPESVIRIDAE

Introduction

Malignant catarrhal fever (MCF) is a disseminated and generally fatal viral disease of domestic cattle and wildlife. Affected species are primarily ruminants, including multiple species of deer, American and European bison (Bison bison and B. bonatus respectively), water buffalo (Bubalus bubalis), African buffalo (Syncerus caffer) and certain captive and farmed antelope. Some wildlife species are now used as captive wildlife to produce meat on farms and ranches, such as bison in North America and venison from red deer (Cervus elaphus) in the United Kingdom and New Zealand.^{23,99,109,162,169,145} Malignant catarrhal fever also affects domestic pigs.¹ Published reports of the occurrence and impact of MCF in free-ranging wildlife are limited and it is likely the disease is underdiagnosed.^138,193,212 There is, however, no reason to regard it as a population-limiting disease of wild North American ungulates.⁸⁰

The clinical course of MCF in individual animals varies. Major clinical forms are peracute (i.e., few overt signs prior to death), acute, chronic or mild.^68,88,142 The most typical presentation of MCF is acute disease with a fatal outcome. Clinical signs in most affected animals are severe and characteristic. Animals present with fever and severe inflammation of the alimentary and upper respiratory tracts. Bilateral panophthalmitis and keratoconjunctivitis are common and diagnostically helpful, with progressively worsening ocular signs in fatal cases.²²³ This is accompanied by profuse mucoid to mucopurulent nasal discharge and, in some cases, signs of meningoencephalitis.^156,195,196 Generally MCF is a sporadic, low morbidity, high mortality disease.^150,163,167 On occasion, large numbers of animals are affected.  Such outbreaks are rare.  They may involve stressful conditions or unusual management practices, such as penning cattle or bison near or with sheep for extended periods.^{32,109,150,155,157,167,220}

Several excellent reviews of MCF exist, not least two previous iterations of this chapter.^4,9,175,183 Some of the older literature addresses MCF before the 1900s.^42,122 Although published almost 30 years ago, a chapter written by Dr Walter Plowright remains insightful and comprehensive.¹⁵⁹ In a failed effort at brevity, we underscore more recent developments about MCF. These include expansion of the MCF virus (MCFV) group, current diagnostic assays, recognition of additional susceptible species, MCF in pigs, an MCF-like syndrome in sheep, current thoughts about pathogenesis, and prospects for an effective vaccine.

The growing number of gammaherpesviruses in the MCFV group presents a challenge to diagnosticians and researchers. Not all MCFVs have been shown to cause MCF. The disease affects a formidable number of artiodactyl species (>150), many in captive or semi-captive situations.^51,64 Yet it is impossible to predict, and impractical to test experimentally, which of the currently ‘non-pathogenic’ MCFVs are likely to cause disease. We rely on investigation of natural disease episodes, most involving a handful of animals, to help define new susceptible species. It is likely that additional members will be added to the MCFV group over time.⁴⁹ Ten such agents are recognized as of 2018 (Table 1). As a general rule, infection of reservoir host species is a subclinical event, with little or no disease. Six of 10 MCFVs cause mild to severe disease in poorly-adapted host species. Two of the six were responsible for most early reports of MCF.  These two continue to be the most important economically: alcelaphine herpesvirus 1 (AlHV-1) and ovine herpesvirus 2 (OvHV-2).

The natural hosts of AlHV-1 are wildebeest (Connochaetes taurinus and C. gnou). Alcelaphine herpesvirus 1 causes the ‘African’ or wildebeest-associated form of MCF (WA-MCF). Disease can be seen anywhere wildebeest are kept near domestic cattle and other clinically susceptible species.

Domestic sheep (Ovis aries) and wild sheep such as mouflon (Ovis orientalis) are natural hosts of OvHV-2.  Sheep-associated-MCF (SA-MCF) occurs worldwide when sheep are kept near clinically susceptible species. Clinical disease due to AlHV-1 has not been described in the natural alcelaphine reservoir hosts. Sheep infected with OvHV-2 are generally asymptomatic. Under poorly-understood circumstances, domestic and wild sheep develop an MCF-like disease characterized primarily by multisystemic vasculitis (‘polyarteritis nodosa’; PAN).^153,154,199

No commercial vaccines exist to protect livestock from MCF.  Early efforts by Plowright and his predecessors to develop effective vaccines were discouraging.214 Several groups recently renewed efforts to create vaccines to protect susceptible livestock. The most pressing need is one for cattle when they are sympatric with wildebeest in East and southern Africa. It would be helpful to have one for ranched bison that are recurrently exposed to sheep in North America. In the absence of a cost-effective vaccine, control of MCF depends on...