Published Date: 2012-08-05 18:27:41
Subject: PRO/AH> Rabies - Peru (05): comment
Archive Number: 20120805.1229534
RABIES - PERU (05): COMMENT
A ProMED-mail post
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International Society for Infectious Diseases
Date: Sun 5 Aug 2012
From: Patricia Burke [edited]
Re: Rainforest Peruvians with antibodies to rabies virus [ProMED-mail archive nos. 20120802.1226037 & 20120805.1228965:
I am not sure why this should come as such a surprise. Once the rabies virus makes it to the CNS, and clinical disease has developed, rabies is almost always fatal in humans, with a few spontaneous recoveries documented, and a larger, but still tiny percentage of dogs and other species (the SOP [standard operating procedure] is to euthanize when any clinical signs [appear], minimizing [the chance of] discovery of this phenomenon).
The way the virus is transmitted; the extremely long and variable incubation period (1 week [after] experimental inoculation to over a decade in humans) and local immunity; inapparent, abortive infections; partial protection by maternal antibodies (dog and cats are not vaccinated until 3-4 months of age to minimize blocking passive immunity), allowing the individual to be 'immunized'. This happens all the time in viral infections and offers reasonable explanations for why exposure does not equal infection, species are more or less susceptible to various strains, and terrestrial epizootics in wildlife when strains are unnaturally introduced into a new area.
From Hemachudha T, et all. Human rabies: a disease of complex neuropathogenetic mechanisms and diagnostic challenges. Lancet Neurol. 2002 Jun;1(2):101-9. "Once rabies virus is successfully inoculated into wounds, the initial infectious event may depend on the lyssavirus genotype. In the case of canine rabies virus, the viral glycoprotein may bind to nicotinic acetylcholine receptor on the muscle whereas in the case of some bat lyssaviruses, the virus may bind to unknown receptors in the epidermis or dermis. Another characteristic is the long incubation period or eclipse phase, which may be explained partly by localization of virus within the muscle or other cells at the neuromuscular junction.
Studies in skunks showed that rabies-virus antigen and genome could be demonstrated as long as 2 months after inoculation into muscle. In certain circumstances, this persistence may provide an opportunity for host immune clearance and for post-exposure treatment."
This article also reviewed 3 humans who recovered from rabies - all had atypical presentations.
George Baer's Natural History of Rabies 1991, states "Animals, especially dogs with specific rabies-neutralizing antibody in enzootic areas provide indirect evidence for inapparent transmission of rabies, aborted rabies infection and immune response to sublethal amounts of virus."
[In the published report] Rabies in a captive colony of big brown bats (Eptesicus fuscus), J Wildl Dis. 2004, 40(3):403-13., Shankar et al. showed neutralizing antibodies in some of the bats, who never succumbed to the disease.
In an area with a large number of vampire bats, it makes sense some individuals would be exposed and develop immunity. Screening unvaccinated cattle in the area for protective antibodies to the vampire bat strain would be interesting.
Patricia Burke, DVM
[ProMED-mail thanks Dr. Burke for her comments, providing additional information about rabies antibodies in a variety of unvaccinated domestic and wild mammals. This thread is now cut. - Mod. TY]