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Sat, Aug 31, 2013 - Page 8　News List

Mice and dogs are equally worthy

By Chang Chao-chin 張照勤

All life is worthy of respect. The question of whether animal experiments need to be carried out in relation to rabies should therefore be answered solely according to whether they would contribute to the overall task of disease prevention, and whether it is worth sacrificing the animals involved for the sake of ensuring a healthy life for many more animals.

The statement that “rabies can infect all warm-blooded animals” is true, but it is not the whole truth. To get a more accurate idea of the science involved, a more detailed explanation is needed. That is, “different viral strains will exhibit different pathogenicity and transmissibility in any one animal species, while the pathogenicity and transmissibility of any viral strain will vary in different species.”

The viral strains found among ferret-badgers in Taiwan up to now belong to three evolved genotypes or strains. From a virological point of view, if all that is needed is to know whether there are any differences between these strains as regards their antigenicity and pathogenicity, this could indeed be deduced from experiments using mice or rats.

However, results obtained from tests on mice and rats could not be used to deduce anything about what would happen to infected dogs.

Taking oral rabies vaccines used in other countries as an example, some of them are attenuated viral strains that are not pathogenic for foxes or transmissible among them, and can therefore be used as oral vaccines for wild animals, but these viral strains are virulent for rodents.

Another example comes from Europe, where foxes, dogs and cattle were inoculated with a fox rabies strain. When the results were compared, foxes were quite highly susceptible to it, but dogs were the least susceptible. That is because different animal species will show different susceptibility to the same rabies strain. It would therefore be problematic if anyone used the results of experiments done on mice to predict whether particular virus strains would be pathogenic in dogs.

If the purpose of doing animal studies is to find out whether the viral strains affecting ferret-badgers in Taiwan are pathogenic for dogs, then the results of such experiments would not influence the main strategy for preventing the disease, which is currently to reach or exceed a 70 percent vaccination rate among dogs and cats.

However, if more in-depth studies were done to find out how contagious the viruses are after infecting dogs, and how susceptible they are to the ferret-badger rabies virus, then such experiments would make a practical contribution to disease prevention.

The transmissibility of rabies viruses is what determines whether those viruses continue to circulate. Experiments in which foxes were infected with a fox rabies strain showed that 99 percent of the foxes shed viruses in their saliva three days before symptoms appeared, but only 20 to 30 percent of dogs shed viruses after infection.

As for foxes that were inoculated with a rabies virus that originated from bats and is pathogenic in humans, the foxes got sick, but no viruses were detected in their brains or saliva.

These experimental results show that the same strain of rabies virus will exhibit varying degrees of transmissibility after infecting different species of animal.

If animal experiments can be used to find out how contagious a virus is after infecting dogs, especially whether the virus is shed in the dogs’ saliva, and, if so, in what quantities, that could also serve as a reference for determining what percentage of stray animals producing protective antibodies may be sufficient to stop the spread of the disease once it occurs.

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