Animal models of disease shed light on Nipah virus pathogenesis and transmission

Abstract

Nipah virus is an emerging virus infection that causes yearly disease outbreaks with high case fatality rates in Bangladesh. Nipah virus causes encephalitis and systemic vasculitis, sometimes in combination with respiratory disease. Pteropus species fruit bats are the natural reservoir of Nipah virus and zoonotic transmission can occur directly or via an intermediate host; human-to-human transmission occurs regularly. In this review we discuss the current state of knowledge on the pathogenesis and transmission of Nipah virus, focusing on dissemination of the virus through its host, known determinants of pathogenicity and routes of zoonotic and human-to-human transmission. Since data from human cases are sparse, this knowledge is largely based on the results of studies performed in animal models that recapitulate Nipah virus disease in humans.

INTRODUCTION

In September 1998, an infectious disease outbreak presenting as a febrile illness with encephalitis was detected in Malaysia [1]. Concurrently, pigs in the outbreak area were suffering from respiratory and neurological disease, albeit with low mortality [2]; since human cases occurred in people who had contact with pigs it was concluded that people were acquiring the infection from pigs. The outbreak spread further through Malaysia and even Singapore by March 1999 through the transport of infected pigs [1-3]. By late May 1999, the outbreak was under control, mainly through the culling of close to 1,000,000 pigs [2], with a total of 276 human cases, including 106 fatalities [4].

The disease observed in this outbreak was shown to be caused by a previously unknown Paramyxovirus, subsequently named Nipah virus, classified in the Henipavirus genus together with Hendra virus [4]. Thus, Nipah virus is an enveloped, non-segmented, negative strand RNA virus. Six structural proteins are encoded by the Nipah virus genome: the nucleoprotein N, phosphoprotein P, matrix protein M, the two envelope glycoproteins F (fusion) and G (receptor binding) and the RNA-dependent RNA polymerase L. Three additional proteins are encoded by the P gene: an alternative start codon results in protein C; the insertion of one or two nontemplated G residues results in proteins V and W, respectively [5].

Like Hendra virus, the natural reservoir of Nipah was shown to be Pteropid fruit bats, also known as flying foxes [6-12]. Serological and virus detection studies have found evidence for Nipah virus infection in multiple flying fox species, such as the island flying fox (Pteropus hypomelanus), the Malayan flying fox (Pteropus vampyrus), the Indian flying fox (Pteropus giganteus) and Lyle's flying fox (Pteropus lylei) [6-8,10-12]. The circulation of Nipah virus within these natural reservoirs has been detected over a wide geographical range in south East Asia including Malaysia, Bangladesh, India, Papua New Guinea, Cambodia, Indonesia, East Timor, Vietnam, and Thailand [7-19].

Despite the presence of Nipah virus in flying foxes in Malaysia, no human cases of Nipah virus have been detected there since 1999. However, outbreaks of Nipah virus have occurred almost every year since then. In 2001, an outbreak of 66 cases of encephalitis occurred in Siliguri, India with a ~74% case-fatality ratio. Around 75% of cases were the result of nosocomial infections acquired by patients, medical staff and family contacts in four hospitals in Siliguri [20]. Nipah virus was again detected in India in 2007, when there was a small cluster of five human cases of Nipah virus infections, all of which were fatal. Four of these cases were likely the result of human-to-human transmission from the index case [21].

Human cases of Nipah virus infection in Bangladesh were first recognized in 2001, when 13 people acquired Nipah virus encephalitis, nine of whom died [22,23]. Nipah virus disease outbreaks and/or sporadic cases of Nipah virus infection have been detected in Bangladesh almost every year since then. Transmission in Malaysia and Singapore occurred through pigs as intermediate, and amplifying host and nosocomial transmission was the main source of Nipah virus infections in India. However, Nipah virus infections in Bangladesh are either the result of foodborne transmission, most likely through the consumption of raw date palm sap contaminated with Nipah virus by fruit bats during collection [24,25], or of human-to-human transmission, which has been shown to account for 50% of cases during some outbreaks [26] (Fig. 1).

Figure 1.

Zoonotic transmission cycles of Nipah virus in Malaysia and Bangladesh. Pteropus species fruit bats are the natural reservoir of Nipah virus. In Malaysia (left side of panel), Nipah virus was transmitted from bats roosting in fruit trees on pig farms to pigs. Subsequently, pigs transmitted Nipah virus to people in close contact with the pigs. In Bangladesh (right side of panel), Nipah virus is thought to be transmitted via the consumption of raw date palm sap. While date palm sap is collected, bats drink from the sap stream or collection pots and contaminate the sap with Nipah virus through their saliva or urine. People become infected with Nipah virus after drinking this contaminated date palm sap. Subsequently, these infected people can transmit Nipah virus to others via close contact.

In this review we discuss our current understanding of the pathogenesis and transmission of Nipah virus. Since data from human cases are sparse, this understanding is largely based on the results of studies performed in animal models.

NIPAH VIRUS DISEASE IN HUMANS

Clinical disease

Nipah virus infected patients generally present with fever and altered mental status; neurological signs of disease progress over time, leading to coma and ultimately death [3,27-30]. Although neurological disease is the main complication with Nipah virus infection, respiratory disease also occurs to varying degrees: during the 1998-1999 outbreak in Malaysia and Singapore ~25% of cases presented with respiratory disease, vs. 69% of cases in Bangladesh [31]. In some of the patients from Bangladesh radiographic findings were consistent with acute respiratory distress [30]. The case-fatality ratio in Nipah virus infected people is high: ~40% during the outbreak in Malaysia and Singapore; in Bangladesh and India the overall case-fatality ratio is ~70%, but may be 100% in individual outbreaks. In Nipah virus disease survivors, long-term neurological deficits occur frequently [27,32]. Moreover, late-onset or relapse encephalitis has been observed; in one case this late-onset encephalitis occurred 11 years after the initial Nipah virus infection [32-35].

Histopathological findings

All available data on histopathological changes in humans infected with Nipah virus were obtained in a single study performed during the Nipah virus outbreak in Malaysia and Singapore [36]. The main site of infection in Nipah virus patients was the endothelium. Systemic vasculitis was observed in almost all patients, mainly involving blood vessels in the CNS, but blood vessels in lungs, heart and kidneys were also affected [36]. Perivascular cuffing, thrombosis, necrosis and endothelial cell syncytia were also observed. In the CNS, neurons were clearly infected with Nipah virus as indicated by immunohistochemistry. In the lungs, fibrinoid necrosis was noted in alveoli adjacent to small vessels with vasculitis, and alveolar hemorrhage and pulmonary oedema were observed frequently. In the spleens of many cases, acute necrotizing lesions in the periarteriolar sheaths and lymphoid depletion were observed. In the kidneys of one third of patients focal glomerular fibrinoid necrosis was seen [36].

ANIMAL MODELS OF NIPAH VIRUS DISEASE

Nipah virus host species: flying foxes and pigs

Experimental Nipah virus inoculations have been performed in some of its natural reservoir species, grey-headed fruit bats (P. poliocephalus) and large flying foxes (P. vampyrus), and in the intermediate host, domestic pigs.

Pteropid bats inoculated with Nipah virus did not develop clinical signs of disease [37,38]. Virus shedding upon inoculation of large flying foxes was minimal, with viral RNA detected in only two swabs from one out of eight infected bats (one throat and one rectal swab) [37]. Nipah virus RNA was detected in the urine of one of 17 inoculated grey-headed flying foxes, but not in any of the conjunctival, tonsillar, nasal or rectal swabs collected from these animals [38]. Nipah virus isolation from bat tissues was rare and was only successful on day 7 after inoculation. Although some histopathological lesions were observed in inoculated bats, there was no detection of Nipah virus antigen associated with these lesions, so whether these lesions were caused by Nipah virus replication remains inconclusive. All inoculated bats seroconverted, although virus neutralizing titres were generally low [37,38].

Experimental Nipah virus inoculations have also been performed in its intermediate, amplifying host, pigs. Oral inoculation of pigs did not result in clinical disease, but animals did develop a neutralizing antibody response [39]. Subcutaneous as well as a combination of intranasal, ocular and oral inoculation resulted in neurological and respiratory disease, albeit not in all inoculated animals [39,40]. The most common histopathological findings in pigs were meningitis and encephalitis; in pigs with severe clinical disease bronchointerstitial pneumonia, systemic vasculitis and focal necrosis of spleen and lymph nodes were also observed. Viral antigen was detected in endothelial and smooth muscle cells of the brain, lungs and lymphoid system. Neurons, glial cells, epithelial cells of the upper as well as lower respiratory tract were also positive by immunohistochemistry [39,40].

Syrian hamsters

Upon inoculation with Nipah virus, Syrian hamsters developed respiratory and neurological disease; higher inoculum doses resulted in rapid development of lethal respiratory disease and lower doses resulted in the development of neurological signs [41,42]. Several studies have shown that the route of inoculation has an effect on the development of disease, with disease progressing more rapidly and time to death being shorter upon intraperitoneal inoculation as compared to intranasal inoculation [43,44]. Like human patients, the main histopathological finding in Syrian hamsters infected with Nipah virus was systemic vasculitis. Moreover, animals developed encephalitis and neurons in the CNS were clearly infected; viral antigen was detected by immunohistochemistry and inclusion bodies were observed that were shown to be nucleoprotein aggregates by electron microscopy (Fig. 2) [42,44,45]. In hamsters that developed severe respiratory disease, usually upon intranasal inoculation with a high inoculum dose, rhinitis was observed, with virus replicating in the respiratory as well as olfactory epithelium [41,42,46]; bronchointerstitial pneumonia developed in the lungs of these animals [41,42,46]. An overview of histopathological changes in Nipah virus infected Syrian hamsters, also observed in human cases, is displayed in Figure 3A.

Figure 2.

Ultrastructural changes in the CNS of Syrian hamsters infected with Nipah virus. Samples of the brain of Nipah virus infected hamsters were collected at 12 days post inoculation. Examination of the ventral cortex revealed (A) perivascular inflammation with extravasation of plasma cells and monocytes; (B) degeneration of neurons (n) with mitochondrial swelling (asterisk), dilatation of rough endoplasmic reticulum (arrow) in an adjacent plasma cell and axonal myelin degradation (arrowheads); (C) nucleoprotein aggregates were observed in axons showing myelin degradation. Scale bars in (A) and (B) represent 2μm, bar in (C) represents 0.5μm. Adapted from [45].

Figure 3.

Histopathological changes induced by Nipah virus infection. (A) Syrian hamsters were inoculated with Nipah virus. Lung and brain tissue were collected and stained using haematoxylin-eosin (HE) and with a monoclonal antibody against Nipah virus nucleoprotein by immunohistochemistry (IHC). The panels represent typical Nipah virus lesions observed across animal models of Nipah virus disease and in fatal human cases. The left panels show multifocal vasculitis in the lungs with recruitment of inflammatory cells with effacement of the tunica intima and tunica media; rare Nipah virus antigen can be detected in these lesions. The middle panels show acute bronchointerstitial pneumonia centered on terminal bronchioles, with influx of inflammatory cells, thickening of the alveolar septa, deposition of fibrin and haemorrhage; Nipah virus antigen is abundantly present in this lesion. The right panels indicate encephalitis with perivascular cuffing and gliosis; Nipah virus antigen can be detected in neurons and microglia. (B) Lungs of a Nipah virus infected Syrian hamster were stained with DAPI to detect cell nuclei, CD31 (green) to detect endothelial cells and with a monoclonal antibody against Nipah virus nucleoprotein (red). Nipah virus antigen is present in CD31-positive cells, indicating that Nipah virus replicates in endothelial cells. Original magnification: top panels ×100, middle and bottom panels ×400, panel B ×1000

Ferrets

Ferrets inoculated oronasally with Nipah virus developed respiratory as well as neurological signs of disease. Systemic vasculitis was present in infected ferrets. Histologically, the upper respiratory tract, with rhinitis, tonsillitis and nasopharyngitis, as well as the lower respiratory tract, with necrotizing bronchointerstitial pneumonia, were affected; focal necrosis was detected in the spleen [47,48]. Encephalitis was not detected, but nonsuppurative meningitis occurred in a subset of animals and viral antigen was detected in brain endothelial cells and meninges, and occasionally in neurons and glial cells close to infected endothelium.

African green monkeys

African green monkeys inoculated intratracheally, or orally and intratracheally, developed a lethal acute respiratory distress syndrome. Histologically, systemic vasculitis was observed, including in the lungs and brain; Nipah virus antigen was abundantly present in the lungs of all animals and in neurons of most animals [49].

Cats

Cats inoculated with Nipah virus subcutaneously or intranasally and orally developed signs of respiratory disease. Histologically, cats developed bronchointerstitial pneumonia and, in some animals, meningitis [39,50].

Mice

Standard laboratory mouse strains such as Balb/c and C57BL/6 do not develop disease upon intraperitoneal or intranasal inoculation with Nipah virus [51,52]. However, intracerebral inoculation of these animals resulted in a lethal infection [51]. In aged mice, intranasal Nipah virus inoculation also did not result in clinical disease, but most animals seroconverted and vRNA and infectious virus could be detected between 2 and 15 days after inoculation in the lungs of most animals [52]. In IFNAR-KO mice lacking the type I interferon receptor, intraperitoneal inoculation resulted in neurological disease. Histologically, these animals developed vasculitis, perivascular cuffing and meningeal inflammation; the lungs exhibited bronchointerstitial pneumonia [51].

NIPAH VIRUS PATHOGENESIS IN ANIMAL MODELS

Dissemination of Nipah virus through its host

Nipah virus has a very broad cell tropism: it can infect endothelial cells (Fig. 3B), smooth muscle cells, neurons, glial cells, macrophages, epithelial cells of the upper respiratory tract and alveolar pneumocytes (reviewed in [53]). Nipah virus cell tropism seems largely determined by the distribution of its cellular receptors, ephrinB2 and ephrinB3 [54,55]. This is nicely exemplified by the fact that, in Syrian hamsters, Nipah virus antigen could be detected in small and medium size arteries but not in veins [46], in line with the expression of ephrin B2 in arterial but not venous endothelium [56]. However, although Nipah virus receptors may be widely available in its hosts, dissemination of the virus through the host is an essential step in reaching and infecting these cell types.

One obvious mechanism for dissemination of Nipah virus through its host is via the blood. Replication in endothelial cells resulting in viraemia offers the opportunity to travel through the infected host and is likely the cause of the systemic vasculitis observed in patients and animal models. However, Nipah virus uses several other mechanisms to spread through its host.

Nipah virus antigen can be detected in neurons in most animal models and nucleoprotein aggregates have been observed in (degenerating) axons (Fig. 2C); this seems to be an efficient mode of transport for spread throughout the CNS after entry. There is evidence for multiple routes of entry of Nipah virus into the CNS: after disruption of the blood-brain barrier as a result of replication in endothelial cells [40,42]; by attaching to leukocytes without infecting them with subsequent extravasation into the brain parenchyma (or other tissues) [57]; and via transport along olfactory neurons from the nasal cavity through the cribriform plate and into the olfactory bulb and beyond (Fig. 4) [40,45]. Evidence for transport along olfactory neurons was mostly derived from experiments in which pigs, Syrian hamsters and ferrets were inoculated intranasally [40,45,47], potentially skewing towards infection of olfactory neurons and subsequent transport of Nipah virus along these neurons. Nipah virus antigen was not detected in nine olfactory bulbs from infected patients [36]; however, Syrian hamsters infected with Nipah virus through drinking of artificial palm sap spiked with Nipah virus also showed evidence of entry of Nipah virus into the CNS via olfactory neurons [58].

Figure 4.

Nipah virus transport into the CNS via olfactory neurons. Nipah virus-infected neurons (stained red-brown) can be seen extending from the nasal cavity, through the cribriform plate and into the olfactory bulb of Syrian hamsters infected with Nipah virus. Tissues were collected on day 4 after inoculation and stained with a monoclonal antibody against Nipah virus nucleoprotein. Asterisks indicate positive neurons within the olfactory nerve fibre (ONF); OE = olfactory epithelium; OB = olfactory bulb; C = cribriform plate. Taken from [45].

Nipah virus disease in Malaysia vs Bangladesh

Several differences in the epidemiological and clinical characteristics between the Nipah virus outbreak in Malaysia and Singapore and the outbreaks in Bangladesh have been pointed out. Firstly, in Malaysia, zoonotic transmission occurred through an intermediate, amplifying host [1,59-61], whereas zoonotic transmission in Bangladesh seems to occur directly via the consumption of date palm sap contaminated with Nipah virus by fruit bats drinking from the sap stream during collection [24,25,62] (Fig. 1). Secondly, human-to-human transmission played an important role during Nipah virus outbreaks in Bangladesh [26,63,64]. Thirdly, respiratory signs were more common in patients in Bangladesh than in Malaysia [31]. Finally, case fatality rates were generally much higher during outbreaks in Bangladesh (70-100% in individual outbreaks) than during the outbreak in Malaysia and Singapore (~40%) [31].

This has raised the question whether the Nipah virus isolates from Malaysia and virus isolates from Bangladesh, recently proposed to be two different genotypes [65], are intrinsically different, thereby explaining the observed differences, or whether other factors affected the epidemiology and clinical course of Nipah virus disease outbreaks. Unfortunately, autopsies have only been performed on deceased patients from the outbreak in Malaysia and Singapore and no histopathological comparison of the two proposed genotypes is available in humans. Thus, this question was addressed in the Syrian hamster model and the ferret model. A Nipah virus strain from Malaysia and one from Bangladesh were also used to infect aged BALB/c mice, but since these animals do not develop disease upon inoculation with Nipah virus this experiment did not shed light on the differences between these two proposed genotypes [52]. In Syrian hamsters inoculated intraperitoneally, the LD50 of the isolate from Bangladesh was approximately 10-fold higher and disease progression was slower than with the isolate from Malaysia.

Histopathologically, both viruses caused marked, multifocal to coalescing, subacute bronchointerstitial pneumonia with vasculitis, necrosis, oedema and fibrin deposits; however, the progression of lesions lagged 2 days behind in the animals infected with the isolate from Bangladesh [43]. Upon intranasal inoculation of Syrian hamsters with the same two isolates, no difference in severity of rhinitis and bronchointerstitial pneumonia could be detected 4 days after inoculation, although these lesions seemed slightly more severe in animals infected with the isolate from Bangladesh on day 2 after inoculation [46].

In oronasally inoculated ferrets, no clinical differences were observed with either strain. Higher levels of viral RNA were detected in oral swabs obtained from ferrets infected with the isolate from Bangladesh than Malaysia. Regardless of the isolate used, ferrets developed rhinitis, bronchoalveolitis and some vasculitis; viral antigen was detected in the CNS of animals in both groups. No differences in lesion severity or antigen distribution were observed [48].

Thus, the studies in the Syrian hamster and ferret models indicate that intrinsic differences between the viruses did not underlie epidemiological and clinical differences between the two proposed genotypes. Rather, inoculation dose and route (transmission from pigs versus foodborne transmission), health care practices or cultural differences could be the cause of the observed epidemiological and clinical differences. However, it should be mentioned that the described comparative studies have two drawbacks. First, the model systems used were largely skewed towards development of severe respiratory disease through the inoculation dose and route. And second, each study used only one, identical, isolate from Malaysia and Bangladesh, despite the genetic variation that exists, especially among isolates from Bangladesh.

Determinants of pathogenicity

The determinants of Nipah virus pathogenicity are poorly understood. Again, the lack of patient materials has hampered our understanding of the disease process and underlying mechanisms. It was shown that Nipah virus infection resulted in overexpression of CXCL10 (IP-10) in endothelial cells and perivascular infiltrating cells in the brain of patients [66], as well as Syrian hamsters [42,66]. Although this expression of CXCL10 likely plays a role in the accumulation of inflammatory cells in infected areas of the CNS, it is unlikely that CXCL10 is solely responsible for this process; currently it merely is the only chemokine for which expression in the CNS of human cases of Nipah virus disease was analyzed. Increased expression of CXCL10 was also detected in Nipah virus infected primary human respiratory epithelial cells and in infected human lung tissue in a human lung xenograft mouse model [67,68], confirming the role of CXCL10 in Nipah virus pathogenesis.

Reverse genetics techniques were used to study the role of the nonstructural proteins C, V and W in pathogenesis. Recombinant viruses lacking one of these nonstructural proteins were generated; although all three proteins have been determined to play a role in antagonizing the interferon signaling response [69], the three viruses lacking one of the proteins inhibited this signaling to a similar extent to wild type virus in vitro [70]. However, in vivo only the virus lacking W was as virulent as the wild type virus, whereas the viruses lacking C or V were attenuated in Syrian hamsters, with 100% survival of inoculated animals and a complete absence of disease signs. In animals inoculated with the virus lacking C there was evidence of virus replication, albeit to much lower levels than in animals infected with wild type virus, but in animals inoculated with the virus lacking V the only evidence of infection was seroconversion [70]. Thus, the proteins C and V play an important role in Nipah virus pathogenesis. A follow-up study with the virus lacking C showed that more inflammatory cells were recruited to the lungs, though histopathological lesions were less extensive, and that less prominent vasculitis occurred in other tissues of Syrian hamsters inoculated with the virus lacking C as compared to animals inoculated with the wild type virus [71]. Combined with findings in endothelial cell cultures that showed that cells infected with the virus lacking C expressed a higher level of proinflammatory cytokines and chemokines than cells infected with wild type virus, these experiments led to the conclusion that protein C inhibits the early proinflammatory response at sites of infection, thereby preventing control of the infection by the immune system [71].

The role of innate immune signaling in Nipah virus pathogenesis is further exemplified by the fact that Nipah virus causes disease in IFNAR-KO mice lacking the type I interferon receptor that are thus deficient in interferon signaling, but not in wild type mice [51].

NIPAH VIRUS TRANSMISSION IN ANIMAL MODELS

Nipah virus transmission in its natural and intermediate hosts

How Nipah virus is transmitted between bats is unknown, and this question has thus far not been addressed experimentally. In fact, data on naturally as well as experimentally infected bats are sparse. Nipah virus is shed primarily via urine and also via saliva. This detection of Nipah virus in the urine and saliva of fruit bats and from partially eaten fruits together with large colony sizes and abundant social interactions suggest that direct contact transmission as well as fomite transmission are important routes of transmission within the natural host species.

It is obvious from the epidemiology of the Nipah virus outbreak in Malaysia and Singapore, with the virus spreading quickly across farms and states with the movement of infected pigs, that pigs are very susceptible to and efficient spreaders of Nipah virus [2]. Transmission of Nipah virus in the intermediate host was not studied in detail experimentally; however, inoculated pigs transmitted Nipah virus to two naïve cage mates. One of the two contact pigs developed mild, transient signs of disease; both animals shed infectious virus and seroconverted [39].

Zoonotic transmission

As indicated above, zoonotic transmission of Nipah virus occurred through an intermediate host in Malaysia and Singapore. Experimental inoculation of pigs resulted in virus shedding from the nose, throat and eyes; viral RNA as well as infectious virus could be detected [39,40]. In contrast, detection of virus shedding was sporadic in experimentally inoculated flying foxes and isolation of virus from swabs or urine of naturally infected bats is rare [72,73]. Thus, the role of pigs as an amplifying intermediate host being in relatively close contact with humans during the outbreak in Malaysia and Singapore is clear.

There is epidemiological evidence that zoonotic transmission of Nipah virus during outbreaks in Bangladesh occurs mainly through the consumption of raw date palm sap [24,25]. This date palm sap is harvested at night in Bangladesh during the winter months and bats have been observed drinking from the sap stream or the collection pot [62]. It is thought that bats contaminate the date palm sap with their saliva, urine or faeces containing Nipah virus while drinking from it and that people who subsequently drink the sap can acquire Nipah virus infection that way. This zoonotic transmission route was investigated experimentally in the Syrian hamster model. Five out of eight Syrian hamsters exposed to a high dose of Nipah virus via drinking of artificial palm sap containing the virus indeed developed neurological signs requiring euthanasia. Virus replication was detected in the respiratory tract and in the CNS, but not in the intestinal tract, thus resembling the disease in humans in Bangladesh and providing experimental support for the foodborne zoonotic transmission of Nipah virus there [58].

Human-to-human transmission

Nipah virus can be transmitted from infected patients to naïve individuals. During the outbreak in India in 2001, ~75% of cases acquired the infection through nosocomial transmission [20]. Human to-human transmission seems to occur regularly during Nipah virus disease outbreaks in Bangladesh [63,64,74-76]; for some of the outbreaks it was estimated that ~50% of cases were the result of human-to-human transmission [26].

Human-to-human transmission was modeled in the Syrian hamster to determine the route through which Nipah virus is transmitted. Upon intranasal inoculation of Syrian hamsters with Nipah virus, transmission to naïve hamsters was observed only in hamsters that were in direct contact, but not via fomites or aerosols [41,58]. The animals that were infected through transmission did not develop signs of disease, but shedding of viral RNA was detected and animals developed a neutralizing antibody response against Nipah virus. Surprisingly, a Nipah virus strain isolated from a fatal case in Malaysia was transmitted as efficiently as a strain isolated from a fatal case in Bangladesh, indicating that the apparent absence of human-to-human transmission during the outbreak in Malaysia was most likely not due to a difference in the ability of the virus to transmit, but most likely due to different health care practices and cultural differences resulting in more contact between sick individuals and their family members [77].

CONCLUDING REMARKS

Although animal experiments have shed some light on viral and host factors that contribute to Nipah virus pathogenicity, Nipah virus pathogenesis is currently poorly understood. Despite this lack of understanding of how Nipah virus causes disease, antiviral treatments are being developed with promising results. The monoclonal antibody m102.4 protected African green monkeys from succumbing to an otherwise lethal Nipah virus challenge, even when administered after the onset of clinical disease in these animals [78]. However, it is not evident that medical treatments can be implemented easily in the poor, rural areas where Nipah virus outbreaks generally occur. There, the currently ongoing efforts to implement the use of bamboo skirts to prevent bats from accessing date palm sap collection pots are targeted at preventing zoonotic transmission and eliminating the need to treat Nipah virus disease [79-81].