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The Risk of Rabies
What are the chances of letting a rabid dog into New Zealand?
Stuart C MacDiarmid and Kevin C Corrin
The invariably fatal disease rabies has never occurred in New Zealand primarily because of strong import and quarantine policies regulating the importation of dogs, the major carrier likely to bring it in. The Ministry of Agriculture has been assessing which of these policies are needed to ensure that risk levels are kept at a suitably low level.
The virus which causes rabies can infect all mammals, including humans, and is transmitted in the saliva of rabid animals, typically through the bite of a "mad" dog. Until recently, dogs coming into the country had to have lived for at least six months in one of a small group of locales -- the UK, Australia or Hawaii -- considered to be free from the disease. Dogs coming from anywhere else had to be eligible for residency in one of these places and held in quarantine there before they could be on-shipped to New Zealand.
People returning to New Zealand after living overseas have frequently protested at the long, expensive period of separation from family pets required under the restrictive import policy. However recently-gained understanding of the natural history of the disease and the efficacy of newer vaccines has led the Ministry of Agriculture to review import policies.
Wild, Urban Rabies
There are two distinct patterns of disease seen with rabies: the wildlife cycle and the so-called urban cycle. In some parts of the world, rabies is maintained in various species of wildlife, such as bats, raccoons and foxes. In such situations, strains of the virus tend to be restricted to a particular host species in any one region. It is unlikely that a rabies strain adapted to a particular wildlife species would become established should it be introduced into a new species of wildlife in a new region.
In countries in which rabies is not endemic in urban dog populations, infection of domestic animals or humans is a spillover phenomenon and does not usually result in the establishment of a new cycle.
The cycling of the disease in stray dogs, so-called "urban rabies", is more dangerous to humans than rabies in wildlife, and accounts for an estimated 96% of all recorded human cases. While wildlife rabies is more common in the temperate regions of the world, urban rabies predominates in tropical countries. Dogs imported from countries in Africa, Asia and South America present the greatest risk of introducing a rabies strain that could establish a cycle in dogs here in New Zealand.
Immunisation with modern vaccines will protect against infection. Dogs respond to rabies vaccinations by producing neutralising antibodies. The blood concentration, or titre, of these antibodies can be measured, and there is good correlation between antibody titres and protection. The vaccination status of an imported dog can be checked by a simple blood test.
Failure of modern vaccines to protect against rabies is uncommon. For example, in 1988, 33 million dogs and cats were vaccinated in the US and only four cases of vaccination failure (that is, rabies developing in vaccinated animals) were recorded. Other studies have also confirmed the effectiveness of vaccination.
Modelling Disease Risks
The ministry has used a PC-based software package, @RISK, to assess the risk of releasing a rabid dog from quarantine under each of a number of import policies based on quarantine periods of different duration, with or without verified vaccination status. For modelling purposes, in the absence of any safeguards the risk of selecting a rabies-infected animal was considered to be a function of the incidence of rabies amongst domestic dogs in the exporting country and the incubation period of the disease.
The incidence of rabies in pet animals in developed countries has, in general, been decreasing as a result of widespread immunisation. The incidence figures from several countries were used to estimate the risk of randomly selecting a dog infected with rabies. As it is probable that the number of reported cases underestimates the true incidence, a range of incidences was used in the model.
For example, the number of cases of rabies in dogs in the US was 91 in 1988, the year for which figures were available when the rabies risk assessment project was begun. In the risk assessment it was considered that the "most likely" number of cases was actually 50% higher than this, or 137 cases. Had the incidence been grossly under-reported, the actual number of cases could have been five times greater (455 cases).
The incubation period of rabies (the time between infection and the development of signs of disease) can vary from a week to more than a year, but is usually between 3-8 weeks. From published data, estimates were made of the minimum (10 days), most likely (56 days) and maximum (365 days) incubation periods.
When running these figures through the model, it showed that the unrestricted risk of importing an infected pet dog directly, without safeguards, from the US, France, Canada and Germany is small, being fewer than 40 cases per million. Dogs from the US presented the least risk, at 1.6 per million, and dogs from Germany the greatest risk, at 38 per million. The higher incidence in German dogs may be a reflection of the greater exposure dogs there have to fox rabies, more so than American dogs have to raccoon rabies.
Once the magnitude of the unrestricted risk had been estimated for each country, the risk-reducing effects of vaccination and quarantine were assessed, again using a range of values in the simulation model. The effectiveness of vaccination was set for 0.80-0.99, with the "most likely" value being 0.94. The probability of vaccination failing to protect was thus 0.06 (1 minus those values), or six failures in 100 vaccinations.
The effect of each safeguard, or combination of safeguards, is the product of the unrestricted risk and the likelihood of failure of the safeguard. The simulation model was run for 5,000 iterations, examining the risk of introducing rabies following importation of dogs from selected countries after applying different safeguards.
MAF's risk assessment concluded that the risk of introducing rabies through the importation of dogs can be managed by the application of appropriate safeguards. The risk of introducing rabies under a policy of confirmed vaccination is no greater than under a policy of prolonged quarantine alone.
As a result of this risk assessment, the Ministry of Agriculture now permits the importation of dogs from countries in which rabies occurs in wildlife. The risk of a fox- r raccoon-adapted strain of rabies virus establishing here is remote.
Vaccinated dogs may now enter subject to them:
Importation is still not permitted from countries in which rabies is established in the canine population, and New Zealand will only accept dogs from the approved locales. Long separations, however, are now a thing of the past.
Stuart C MacDiarmid and Kevin C Corrin are with the Ministry of Agriculture.
Kevin C Corrin is with the Ministry of Agriculture.
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