GENERAL INTRODUCTION: PARAMYXOVIRIDAE AND PNEUMOVIRIDAE

Introduction

Nipah virus disease is an acute febrile disease caused by Nipah virus (NiV), which has only occurred in South and South East Asia, where fruit bats of the genus Pteropus are the reservoir host of the virus. It is one of the most important emergent viral zoonotic diseases, with case fatality rates in humans ranging from 38  to 100 per cent. In humans either a respiratory or neurological syndrome presents, while in pigs respiratory signs manifested by paroxysmal coughing and open-mouth breathing are prominent. Adult pigs may die suddenly and sows abort, but many infections of pigs are asymptomatic. In cats the infection is pantropic and results in sudden death. In dogs, infection may be asymptomatic or may result in sudden death. In the Philippines, an outbreak of an uncharacterized henipavirus which is believed to be a variant strain of Nipah virus, resulted in multiple cases of encephalitis in humans and neurological disease in horses, whereas dogs sero-converted without obvious clinical signs.⁸

The geographical range of Nipah virus disease has spread from a relatively small geographic area in Southeast Asia, to a much wider distribution including the West coast of India across to the southern Philippines. All outbreaks have been associated with significant human mortality. In the first recognized outbreak, the virus crossed from a wildlife reservoir to cause an outbreak in pigs, among which it was highly contagious, and from infected pigs, the virus spread to humans. In outbreaks in Bangladesh and India the authors of this chapter have seen direct transmission from the wildlife reservoir to humans, presumably via bat-contaminated raw date palm sap and subsequent person-to-person transmission.

Cases of an unusual encephalitis were identified in pig farm workers in the Malaysian state of Perak in 1998,¹¹ although, retrospectively, earlier human infections were diagnosed. Later in 1998 the number of cases in Perak increased, and a number of humans died. Human cases started to occur in more southerly areas of Peninsular Malaysia and rapidly escalated into a major epidemic in February, March and April 1999. It eventually spread to Singapore, through movement of infected pigs, where cases occurred in abattoir workers processing pigs from Malaysia.^{43, 46, 47}

In March 1999 a paramyxovirus was isolated from human cases and identified as being related to Hendra virus,¹⁰ a paramyxovirus involved in an outbreak of fatal disease of horses and deaths in humans in Australia⁴² (see Hendra virus infection). The virus was shown to be novel, and was called Nipah virus^{9, 10} after the village of Sungai Nipah, where the patients from whom the virus was isolated had lived. In the ensuing investigation the virus isolated from humans was shown to be widely disseminated among pigs on the farms on which the human cases had occurred, and also to have infected dogs, cats and horses.^{1, 2, 4, 9, 12, 35, 43, 45}

The progress of the epidemic was halted by quarantine and culling of pigs on known infected pig farms as well as on those suspected to be infected. Human infection resulted from close contact with infected pigs. Serological surveillance using a newly developed diagnostic capability was designed to detect infected pig farms.^{15, 44, 45}

No new human cases of Nipah virus disease have been diagnosed in Malaysia or Singapore since May 1999, and no newly infected pig farms have been detected since that time.^{43, 47} The last detection of seropositive pigs was in May 2000, and in June 2001 the OIE recognized that Nipah virus infection had successfully been eradicated from the Malaysian pig population.⁴

Bangladesh has experienced Nipah virus outbreaks on an almost annual basis since 2001.³⁴ The virus has also caused outbreaks of human disease in the Indian states of West Bengal and Kerala.³ A variant strain of Nipah caused an outbreak of horse and human disease in the southern Philippine island of Mindanao.⁸

Nipah virus remains a concern to both veterinary and public health agencies in Southern Asia. Bats of the genus Pteropus are distributed in many countries from Madagascar across Southeast Asia to Australia and the Pacific Islands. There is a possibility that further outbreaks of this zoonotic disease may occur as a result of spill-over of the virus from bats.

Aetiology

Nipah virus is a non-segmented negative-stranded RNA virus in the family Paramyxoviridae, genus Henipavirus. This genus includes Nipah virus and Hendra virus, and also the more recently discovered Mojiang paramyxovirus⁶¹ and Cedar virus.³⁶ Cedar virus is not known to cause disease. Mojiang paramyxovirus was implicated in the death of three miners in China in 2012, following potential zoonotic transmission from rats.⁶¹

The NiV genome is one of the largest of all characterized paramyxoviruses with 18246 nucleotides. This increased genome size, compared to other paramyxoviruses, is in part due to the long untranslated regions...