Rabies is one of the better known encephalitides, caused by the rabies virus of the genus Lyssavirus in the family of Rhabdoviridae (George M B, 2007). About 55000 human rabies cases are reported each year worldwide with ≥ 90% caused by dog (Canis lupus familiaris) bites (Zhang Y Z, 2006). In countries where dog rabies has been controlled due to active vaccination campaigns and surveillance, dog rabies is no longer prevalent, and human cases in these countries or regions are usually caused by wildlife reservoirs (Rupprecht C E, 2006). Foxes, raccoons, skunks, jackals, mongooses, and bats have been successively reported to be responsible for human deaths in these regions (Hanlon C A, 2007). Nevertheless, in China, dog rabies is still considered to serve as the main source of infection for human rabies (Hu R L, 2008) and in the total number of 2000-3000 human rabies cases reported annually at the national level, ≥ 95% were dog-associated rabies, according to retrospective epidemiology investigation. Cat, mongoose and other wildlife-associated human rabies in China have been generally considered to be infrequent and isolated incidents result from spillover from rabid dogs without epidemiological significance.
In an independent survey investigating Chinese Ferret Badger-associated human rabies, we found that ferret badger rabies cases in some counties in Zhejiang and Jiangxi provinces were more prevalent than dog-associated in the same regions (Wang Z, 2006; Zhang S F, 2009). For example, from 2002-2004 in Hangzhou district, Zhejiang province, 29 out of 42 human rabies cases were badger-associated. After the successive outbreaks of badger-associated human rabies, the local center for disease prevention and control (CDC) advised residents to be careful to avoid contact with these kinds of animals, and once bitten, a rabies Post-Exposure Prophylaxis (PEP) regimen should be started immediately. However, these recommendations have not been made in other regions such as in Jiangxi province. With the increase of badger-associated human rabies, we hypothesize that the rabies transmission in ferret badgers has formed an independent cycle. However, evidence about whether an independent transmission cycle exists in badger populations or the badger rabies was just a spillover from dogs is lacking. To investigate this hypothesis, from 2007-2010, we performed an extensive epidemiology and molecular epidemiology study in Wuyuan, Leping and Jingdezhen counties, Jiangxi province. 15 rabies viruses from ferret badgers in these counties in Jiangxi province were isolated as part of this surveillance program, combined with other publicly available isolates and investigated from an epidemiological standpoint.
From 2007-2009, 6 people (3 in 2007, 2 in 2008 and 1 in 2009) died of rabies in Wuyuan county, Jiangxi province, while there were no dog-associated human rabies cases anywhere in this county. Of these six cases, 3 were hunters and 3 were farmers. The contact between the hunters and badgers were frequent and direct as they manage the animals for sale almost every day, and they were not generally concerned with injuries when bitten. This is highlighted by three cases. The first case was a female farm worker who found a sick badger when walking along the road, when she tried to catch the badger on its tail, the badger bit the hand of the woman; in the second case a man found 3 badgers playing with each other in the field, and was bitten when he tried to separate them in order to catch one of them. The third one was bitten in his house when he got up at midnight; a sick badger had entered his house and bit him on the foot when he stepped on it. None of the cases sought post-exposure prophylaxis (PEP). All 6 people died, 5 died 2-3 months after the bite, and 1 died 1 year after the bite.
From the beginning of 2007 to the end of 2009, a total of 178 ferret badger specimens were collected from Jiangxi and Zhejiang province. After DFA assay, 15 (8.9%) of them were found positive. Virus isolates were obtained from all the positive samples (Table 1).
Table 1. Background information for isolates collected from ferret badger brain specimens as part of the rabies wildlife surveillance program
Both the rabies virus positive ferret badger brains and the infected mouse brains were used for gene amplification and sequencing. The 15 sequenced isolates obtained in this study in Jiangxi and Zhejiang provinces were aligned, compared and a phylogenetic tree was constructed (Fig. 1). The tree contained three major clades (Clades 1 to 3) with strong bootstrap support. 14 of the 15 isolates formed a single sub lineage within Clade 2. They shared 94.7%-100% nucleoprotein homology with one another, but only 87.8%-95.7% homology with other dog origin isolates in Clade 2, 88.0%-90.2% homology with isolates in Clade 1, and 85%-87.9% homology with the dog isolates comprising Clade 3. There one ferret badger exception, JX09-17, collected in this study which didn't place in the main ferret badger clade was isolated from Fuzhou, a county of Jiangxi province and far from where the 14 isolates were isolated (Zhang S F, 2010). The nucleotide sequence of both nucleoprotein and glycoprotein shared higher homology with the dog isolates, but showed some differences in amino acid sequence for the glycoprotein and was considered a variant of dog rabies virus origin. Up to now, no dog rabies virus isolate has been reported to be closely associated with these isolates (Fig. 1).
Figure 1. Phylogenetic tree of nucleoprotein of ferret badgers rabies virus isolates. 14 of all the 15 ferret badgers rabies virus isolates (marked in red) are clustered into a single lineage, The homologies are 85.0% to 95.7% between ferret badger and dog isolates compared to 94.7% to 100% amongst ferret badger isolates.
It can also be inferred from Fig. 1 that the rabies viruses maintained in ferret badger populations may share some common ancestor with dog isolates in Clade 2, but have experienced long term of adaption in ferret badgers (based on the branch lengths and observed nucleotide mutations).
Comparison of the amino acids among the 14 isolates and with other dog isolates showed that the amino acid sequences of these 14 isolates from ferret badgers are highly conserved, ranging from 99%-100%. However, they shared 96.7%-98.8% homology with the isolates in Clade 2; 96.4%-98.3% with Clade 1, which is the emerging lineage in the current rabies epidemic in China (Yu J N, 2012); 94.5%-96.7% with the isolates in Clade 3. The high homology between the isolates from ferret badgers and the other isolates in Clade 2 also indicates a shared common ancestor at some earlier time. There are characteristic amino acid mutations in the ferret badgers amino acid sequences at sites 181 and 374 in the nucleoprotein, at sites 201, 408 and 467 in the glycoprotein, and at site 70 in the matrix protein as shown in Table 2. These results also reflect the long term adaptation in ferret badgers. Whether these typical amino acids represent interspecies transmission requires further investigation. It should be noted that all the amino acids at glycosylation sites and the most recognized major antigenic sites keep unchanged.
Table 2. The characteristic amino acid signatures identified in the major structural proteins of rabies virus isolates from ferret badgers