The Extinction of Rabies?

June 9, 2009 at 11:25 pm 2 comments

Although canine rabies has been successfully eliminated from Western Europe and North America, someone in an undeveloped country dies from this horrible disease every ten minutes.  Because domestic dogs are the primary vector for the disease, understanding how rabies is transmitted among them in areas where the disease is still a problem is vitally important.

Fortunately, as reported in PLoS Biology, the unique pathology of the disease allowed a group of scientists to trace case-to-case transmission of the disease during an outbreak in Tanzania. They used this data analyze how the disease was transmitted between dogs and how it was transmitted from dogs to humans.

Rabies is a terrible disease. Not only do its victim suffer horribly, it also has the highest human fatality rate of all known infectious diseases. Every year approximately 55,000 people die of it and nearly all of these deaths occur in developing countries.  The disease is rarely seen in humans in Western Europe and North America where vaccination has successfully eliminated the disease from domestic dog populations.

Rabies is an acute viral encephalitis that is spread through the saliva of infected hosts. Clinical manifestations vary, but the neurological phase often includes increased aggression and the tendency to bite and thereby transmit infection; rapid progression to death is inevitable. These distinctive signs make transmission of rabies easier to track than that of most other diseases and provide an unusual opportunity to explore epidemiological patterns at the scale of the individual.

Transmission is the key factor in understanding the spread of infection.  Unfortunately, in the case of rabies, it has also been poorly understood.  At least until now.

The research team monitored the spread of infection by tracing contacts. This is where the odd symptoms of rabies proved to a benefit as the events that lead to transmission (typically a bite from an infected animal) were found to be both discrete and memorable. The team was able to record more than 3,000 potential transmission events between 2002 and 2006. Domestic dogs comprised more than 90% of their observations of rabid animals and were therefore considered to be the sole vector of rabies transmission to humans.

A bite from an infected animal is, of course, the primary way that rabies is transmitted. The biting behavior of dogs that were suspected to have rabies varied a lot with the mean number of bites per rabid dog being slightly more than two.  The probability that an unvaccinated dog would develop rabies after being bitten by an infected animal was approximately 50% (if it wasn’t killed or vaccinated immediately after being bitten).

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While canine rabies vaccinate rates are far lower in Tanzania than they are in Western Europe or North America, several domestic dog vaccination campaigns were carried out in the area during the study period. The researchers found that no rabies outbreaks occurred when domestic dog vaccinate rates exceeded 70 percent. The most severe outbreaks occurred in villages where less than 20 percent of the dogs were vaccinated.

More than 300 vaccinated dogs were identified by contact tracing as having been bitten by rabid animals. Only ten of these animals showed any signs indicative of rabies, although in the absence of vaccination approximately 50% of these would have been expected to succumb to the disease. Individual actions by dog owners such as tying or killing exposed or infectious animals also had an impact.

Why weren’t vaccinations completely effective in preventing transmission of the disease?  Tanzanian dogs don’t receive the kind of care that our American dogs do. Because they suffer from poorer nutrition and more health problems they may not develop the same degree of immunity from vaccination that our dogs do. In undeveloped country other factors like failure to maintain vaccinations at proper temperatures, improper vaccination procedures and lack of tracking / registration information can also adversely affect the effectiveness of vaccines.

With the goal of determining what percentage of the domestic dog population needed to be vaccinated to prevent rabies outbreaks, the team set out to collect data. Their goals were: to understand the transmission dynamics of rabies in domestic dog populations; to understand the demographics of domestic dog populations in Tanzania; and to collect information about the practicality and effectiveness of vaccination.

Their data showed that demographics of the local dog population appear to be a controlling factor on the percentage of animals that need to be vaccinated to control rabies transmission.

When vaccinations are carried out in pulses, births and deaths within the host population will continuously reduce the level of herd immunity attained during campaigns. Turnover of domestic dogs in rural Tanzania is very high; therefore, annual campaigns should aim to vaccinate 60% of the dog population to maintain vaccination coverage above [critical levels] for the duration of the interval between campaigns. When successive campaigns have achieved this, rabies incidence has declined dramatically despite high endemic levels in adjacent areas. Domestic dog population turnover therefore appears to have had a marked influence on rabies dynamics that explains the variable success of vaccination efforts.

They calculated that if 70 percent of the domestic dogs were vaccinated, rabies could effectively be eliminated from human and domestic dog populations.  Vaccination of all dogs is not a feasible goal in undeveloped areas like rural Tanzania but 70 percent coverage may be an achievable goal, even in areas where canine birth and death rates are high.

The problem of canine rabies has often been considered intractable in rural Africa, because of poor infrastructure, limited capacity, and the misperception that large populations of wild carnivores are responsible for disease persistence. Our analyses show that global control of canine rabies is entirely feasible and that successful elimination of canine rabies in many parts of the world has likely been achieved precisely because R0 is so low and institutional commitment to maintain high levels of vaccination coverage has been sustained. Achieving vaccination coverage of 60% or more in dog populations in Africa is both logistically and economically feasible through annual vaccination campaigns. The resultant reduction in costs of human post-exposure prophylaxis suggests that vaccination interventions targeted at domestic dog populations could translate into appreciable savings for the public health sector.

It would be wonderful if vaccination programs could make rabies as rare as polio or smallpox. The beauty of a program like the one proposed here is that it doesn’t rely on new, or even advanced, technologies. And the cost (in dollars, and more importantly – in lives) of an appropriately designed, sustained domestic dog vaccination campaign, will absolutely be less than the costs we’ll pay if we don’t do it.

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2 Comments Add your own

  • 1. SmartDogs  |  June 9, 2009 at 11:55 pm

    Two things I need to add here:

    First; given the relative simplicity of this study – I wonder why no one has studied the efficacy of canine vaccination protocols to control rabies transmission before?

    Second; I really wish that the wingnuts at WVU would read this. The Tanzanian group made assumptions – but they identified their assumptions, gave valid reasons why they thought the assumptions were valid and noted in appropriate places where the assumptions may have led to erroneous conclusions. They collected data from a large, representative population. They compared the study data to data collected in other parts of the world. They looked at temporal issues. And – they at least appeared to conduct their work with an open mind (i.e. they didn’t specifically set out to prove that vaccination was the answer.

  • 2. Jessicaw  |  June 10, 2009 at 2:53 pm

    Fascinating! I’m a little surprised that the annual estimated cases is 55,000, as I got different numbers first-hand in Ethiopia last year when I was bitten by a dog there. At the vetrinary hospital, where I had to report for treatment, I asked what the instnaces of infection are, and the lead vet told me that they have 20,000 human cases of Rabies per year in Ethiopia. (OMG!) What he told me is that most frequently the infections come from dogs and cats, but also the donkeys, because people don’t think they can get sick from donkeys.

    Thanks for posting this–it’s good to know that efforts are being made to eradicate rabies in developing nations. Treatment in ET is slow and painful and not wildly successful (even as a preventative treatment) I ended up bypassing their treatment option because I was so close to my return date to the states, but they would have made the vaccine for me (none was readily available) and they were concerned about the strength and safety of the human vaccine.

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