Bats
NiV
antibodies have been found in five bat species, and NiV has been isolated
from the urine and frass (chewed fruit pulp) of one species, P. hypomelanus
(Johara et al. 2001, Chua et al. 2002). Diagram: H. Field & C. Smith.
Johara et al. (2001) shot and mist netted 324 bats of 14 species from peninsular Malaysia from April 1 to May 7 (see figure). Statistically relevant samples were only obtained for at most four species from the wild: Cynopterus brachyotis, Eonycteris spelaea, Pteropus hypomelanus, and Scotophilus kuhlii (an insectivore). P. vampyrus and P. hypomelanus had 17% and 31% seroprevalence, respectively, and C. brachyotis, E. spelaea, and S. kuhlii had 3-5% seroprevalence. PCR and viral isolation attempts on heart, liver, kidney, spleen, and fetal tissue (fetal tissue was submitted for S. kuhlii, E. spelaea, C. brachyotis, Taphozous melanopogon but was not collected for P. vampyrus or P. hypomelanus) were negative for NiV. Titers ranged from 1:5 to 1:40 (median 1:10) and were slightly higher in P. hypomelanus and P. vampyrus. Inadequate samples, all seronegative, were obtained for C. horsfieldi (n = 24), Ballionycterus maculata (n = 4), Macroglossus sobrinus (n = 4), Megaerops ecaudatus (n = 1), Rhinolophus affinis (n = 6), T. melanopogon (n = 4), T. saccolaimus (n = 1), Hipperosiderus bicolor (n = 1), and Rhinolophus refulgens (n = 1).
Mohd Nor et al. (2000b) presented seroprevalence (by serum neutralization test) results from apparently another, unpublished study in bats. Antibodies were found in 8.7% (n = 57) of P. vampyrus, 26.8% (n = 41) of P. hypomelanus, 2.7% (n = 74) of C. brachyotis, 4.5% (n = 44) of E. spelaea, and 2.0% of S. kuhlii. Details were not available.
NiV antibodies were found in P. hypomelanus on Tioman Island, far from the outbreak sites, and Lam and Chua isolated virus from the same population. 50 pooled urine samples from 17 August 1999 and 275 samples from 23 August 1999 were taken from the roost site, but NiV was not found in any. (Tioman virus was found in the 17 Aug 1999 sample.) 263 pooled urine samples and 27 swabs of partially eaten Eugenia aquea fruits were collected from 11-13 June 2000, and two NiV isolates were obtained from urine samples (each sample contained 4-5 drops of urine) and one isolate was obtained from a fruit swab. Tioman and an unidentified virus were also obtained from the June samples (Chua et al. 2002).
Johara
et al. (2001) plan to determine seroprevalence for the five
antibody-positive species at additional locations in peninsular Malaysia and
in Sabah and Sarawak, Borneo.
Pigs
Upon development of ELISA in April 1999, the Malaysian government arrested the culling program and initiated a surveillance program. Between 21 April and 20 July 1999, 889 farms were tested. Each farm was tested twice, and samples were obtained at least three weeks apart from a minimum of 15 sows or six sows/barn. Fifty of the 889 farms tested positive and their pig populations (totally 172,750 pigs) were destroyed in July. On average, 5.6% of all pig farms in peninsular Malaysia were seropositive (Mod Nohr et al. 2000).
In one previously affected farm, more than 95% of sows had NiV antibodies, and more than 90% of piglets had antibodies, probably maternal (Mod Nohr 1999). Ali et al. (2001) report that the prevalence of sick pigs on farms in cull areas was 68.6% in Negri Sembilan and 72.9% in Selangor.
A survey of 100 “finisher” herds in Queensland in May 1999 yielded no evidence of NiV infection (Black et al. 2001).
Dogs
J Mills, Asiah, M Bunning, and T Ksiazek screened dogs and peridomestic animals for IgG reaction to HeV and reported 15% seroprevalence in the 465 dogs tested (unpublished results Field et al. 2001). In March and April they opportunistically sampled domestic dogs near infected pig farms around Bukit Pelandok and found a prevalence of 46% (n = 92). Mohd Nor et al. (2000b) report 55% (n = 66) seroprevalence by ELISA in March 1999 and 23% (n = 26) prevalence in April 1999 but give no details. A transect study in May found four antibody positive dogs out of 249 tested, and all were within 5 km of the disease endemic area. 114 dogs sampled from veterinary clinics and pounds in the Kuala Lumpur area were negative (Field et al. 2001).
Other animals
Blood samples from 18 wild boars, 16 hunting dogs, and 25 rodents in Perak were obtained by nonrandom sampling between April 1 and May 7 (Johara et al. 2001). None of the sera were positive by indirect ELISA using NiV antigen. Mills et al. screened peridomestic animals, including rats (Rattus rattus), chickens, jungle fowl (Gallus gallus), house shrews (Suncus murinus), domestic ducks, and pigeons (Columba livia). No antibody to NiV was found in any of the 110 birds, 316 rodents, or 37 insectivores (Field et al. 2001). NiV antibodies were found in two of 47 polo horses from Ipoh, one of 23 cats (Mod Nohr 1999), goats (Lam and Chua 2002), and sheep (Uppal 2000). Mod Nohr et al. (2000b) report 1.5% (n = 65) seroprevalence by ELISA in goats, 6.4% (n = 24) in cats, 6.4% in birds (n = 109), and 0.4% in rodents (n = 278), again without supplying details.
Humans
Parashar et al. (2000) found anti-HeV [sic] antibodies in 6% (n = 166) of persons from farms without reported encephalitis patients and 11% (n = 178) of persons from farms with encephalitis patients; all individuals had been asymptomatic during the outbreak.
Between bats
No transmission experiments in bats have been conducted. Experiments on HeV indicate viral shedding may be related to parturition. P. hypomelanus on Tioman Island shed virus in urine in June but not in August (Chua et al. 2002); in the Philippines, this species gives birth in April and May. P. vampyrus gives birth in March and April (Mickleburgh et al. 1992). It would be extremely useful to determine the date of acute infection of the six seropositive Ipoh patients in 1997.
Bats to pigs
Transmission may occur if infected bats’ urine contaminates pigs’ water supply, if pigs consume partially eaten fruit discarded by infected bats, or if they consume aborted fetuses or afterbirth of infected bats.
Between pigs
Transmission between pigs occurs through aerosol, even when the “barking cough” is not produced, and probably through direct contact with nasal secretions (Middleton et al. 2000).
Pigs to humans and other animals
Three studies have attempted to gage the risk factors for human infection in Malaysia:
1) Of the 65 serologically confirmed NiV infections in Negri Sembilan, 56 (86%) of case-patients reported touching or handling pigs before the onset of illness. Of those 56, 36 (64%) reported contact with pigs that appeared sick (MMWR 30 Apr 1999).
2) In another study, encephalitic Nipah case patients were significantly more likely to report increased numbers of sick and dying pigs on the farm (59% v. 24%) than persons from nearby farms without encephalitis patients. Nipah patients were also significantly more likely than affected farm control patients to perform activities requiring direct contact with pigs (86% v. 50%). Forty-one percent (n = 80) of case patients reported no sickness in pigs on their farm. An additional eight percent of case patients reported no direct contact with pigs at all (Parashar et al. 2000).
3) In Kuala Lumpur, 41% (n = 87) of Nipah patients who had regular contact with pigs recalled that these pigs were sick. Two other patients recalled direct contact with sick dogs. 56% of patients had infected family members (Goh et al. 2000).
Comparing figures from the three studies, 41-49% of encephalitic patients recalled contact only with asymptomatic pigs, and 5-14% of encephalitic patients had no direct exposure to pigs at all. Thus, 50-59% of case patients during the outbreak did not come into contact with obviously ill pigs.
In the Singapore abattoir, all infected individuals but only 63% (n = 41) of control subjects reported contact with live pigs. Infected individuals also had significantly more exposure to pigs’ urine and feces (Chew et al. 2000). Ali et al. (2001) found that of 1412 military personnel who assisted in the cull, only six (0.4%) had detectable HeV IgM antibody, and one had IgG antibody. Though the cull involved 1638 military personnel, only two became sick with Nipah encephalitis. Sixty-three percent (n = 1412) reported physical contact with live pigs and 30.9% with dead pigs.
Human to human
Human-to-human transmission has not been documented (MMWR 30 Apr 1999) but may be physically possible (Chua et al. 2001).
Author: S. Cobey.