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In an exclusive interview, through seven key questions, Dr. David Stallknecht from the University of Georgia gives his opinion on the possible reasons why avian influenza has not caused epizootics as it has in North America.
David E. Stallknecht BS, MSc, PhD is a professor at the Population Health Department of Veterinary Medicine of the University of Georgia in Athens, Georgia (USA).
His research is focused in viral diseases and diseases having natural vectors in wildlife animals. Dr. Stallknecht has worked as a researcher and professor for many years at the University of Georgia’s Southeastern Cooperative Wildlife Disease Study (SCWDS).
Every migratory bird route in North America is connected, at least in part, with Central and South America. Why haven’t North American influenza viruses not caused epizootics in commercial fowls in the Southern part of the Continent?
Even though it is true that migratory routes in the American Continent influence Central and South America, it is important to consider several factors related to avian influenza virus (AIV) ecology in wild birds. For example, the prevalence of these viruses is essential. Wild birds are a natural host for AIV and it is important that viruses circulate in high concentrations for them to actually pose a significant risk.
In other words, the risk is based on numeric questions. The higher the virus concentration, the greater the risk of spreading and infection of avian species which are normally unaffected.
Another important parameter is the geographical distribution of migratory birds and the seasonality of their migrations.
The biological stage of a migratory species is also very important. For example, during the winter months, when birds migrate to the South, the virus infection and dissemination rate is very low, whereas during the spring and summer, when birds are being bred in the North, viral infection and dissemination rate is extremely high. This is partly due to the great concentrations of birds in breeding areas of land, and it is also due to the age range of the involved birds.
Very young birds which are scarcely developing in the North before migrating to the South in winter have not accumulated a totally complete immune response against various AIVs and even if they do not catch the disease or die from the infection, they cannot efficiently control viruses.
Therefore up North, when young birds predominate, the population immunity tends to decrease and the virus dissemination is greater.
By the time birds are ready to migrate toward the south, they will have already developed immunity and they will tend to show a lower prevalence or infection rate, thus representing a lesser risk for commercial poultry breeding in the middle and south of the continent.
Another very important aspect is that, as young fowls age and as they are exposed to several AIV subtypes, they start showing some degree of crossed protection, even among the subtypes.
There are definitely several species of ducks that could be considered as the most important vector for several reasons. The first reason is that ducks tend to exhibit long distance migration patterns, while other avian species cover shorter distances, and therefore the geographic range they cover is smaller.
The same duck species dwelling in the United States easily reach Mexican territory, and some of them even go as far as South America.
In other words, Mexico has approximately the same migratory duck assortment as the United States, and to a certain extent Guatemala and Belize are in the same situation, since they are part of Central America, and because of their proximity to the Caribbean Sea.
The diversity of waterfowl species in North America is progressively diminished toward the south of the continent, with only a few species of ducks and other waterfowls reaching the Southern Cone.
Even though there are several migratory waterfowl species of interest, the two most important species in terms of avian influenza and south- bound migrations are the Blue Wing Teal (Anas sp.) and Mallard ducks. Mallards are probably most important in terms of portability and AIV transmission, whereas Blue Wing Teals are most important with regard to territorial coverage and very long distance migrations. In addition to these two species, various species and subspecies of seagulls and terns, and some coastal birds are also capable of traveling great distances and carry the AIVs toward the south.
It is important to note that ducks tend to migrate within the continent, whereas seagulls and other coastal species; such as the Rudder, migrate along the Pacific and Atlantic shorelines. In fact, coastal birds only represent a hazard in countries where commercial fowls are housed in close proximity to the shore, such as Peru. For the rest of the countries in the continent certain duck species may be more dangerous.
“In the north, when young birds predominate, the immunity of the population tends to be lower, and the propagation of the virus is higher”
Knowing the ecology of influenza viruses and migratory patterns of wild birds in the American Continent, should a greater dissemination of the virus in the environment be expected when birds flock together in the north or when they flock together in the south?
In general, it is much more probable that there is a greater amount of influenza virus in the north than in the south, due to the immaturity of immune response of young birds which have just started to be exposed to influenza virus in the north, in addition to the greater concentration of birds and bird species in the north.
As stated above, the great diversity of species in the north starts thinning out as they migrate toward the south. This does not mean that there does not exist a great diversity of birds in the south. On the contrary, there is a great number of avian species in South America that do not necessarily reside in the north of the continent.
In fact, we should think of AIVs as a group of viruses which tend to represent the birds in North America; and a second, separate group of viruses that are prevalent in the south. That is, somehow the large populations of birds which are influenza virus carriers tend to influence either the north or the south, and the interchange of influenza viruses between the north and the south are actually rare or simply does not easily take place.
At the risk of oversimplifying some concepts, northern migratory birds and their AIVs tend to encompass an area of influence spanning from Canada up to the northern part of South America. One example of avian species which follows this pattern is the Northern Pintail, which covers a great portion of the Canadian territory, United States, and Mexico, plus it has been spotted as far as Asia.
Another reason that could explain the greater prevalence of AIV in North America is that northern regions experience several months of intense cold every year, which helps perpetuate the viability of AIVs, whereas the extreme heat recorded between the tropics of Cancer and Capricorn could reduce AIV viability during most of the year.
What are the requirements for influenza virus circulating among wild (migratory) birds to be able to infect and adapt to avian populations in commercial poultry breeding?
Contrary to the generally accepted opinion, only rarely do influenza viruses from wild birds infect commercial poultry.
As a matter of fact, it is not easy for these wild bird viruses to effectively infect chickens or turkeys, since they are not adapted to these.
Therefore, it is common for influenza virus infections to go unnoticed or cause hardly any mortality and clinical signs. For instance, it is extremely difficult for subtypes H12 to actually infect commercial poultry..
Other subtypes can more readily infect and adapt to chickens. However, once they have been given the chance to replicate in an unrestricted manner into commercial poultry, and once they have adapted to chickens, hens, or turkeys, then clinical signs, or even high mortality, are seen.
It is hard to accept the fact, but most outbreaks of avian influenza are caused by one or a few introductions from wild birds, and it is really the poultry industry personnel, along with biosecurity deficiencies, who are responsible for disseminating such avian influenza outbreaks. Nevertheless, one of the main sources of infection is known to be wild bird droppings, since this is where the virus is found in high concentrations.
In commercial fowls we find high concentrations of the virus in the respiratory tract, whereas the same viruses tend to be present in extremely high concentrations in the intestinal contents of wild birds.
Hence, detection of the virus via molecular methods is more successful from oropharyngeal and tracheal secretions in commercial poultry, compared to samples from the intestinal contents and the intestine itself in wild (water) fowls.
In practice, biosecurity systems must concentrate on preventing mechanical transport of fecal matter into the sheds, which requires taking showers, changing shoes, footbaths, and entry systems emulating the “Danish” system, which implies changing shoes inside the shed, but before entering the zone where fowls are housed, so that the shoes worn to step in pens are never used to walk in high-risk areas.
“Migratory routes from the central region and the Mississippi river and their widely spanning birds would be important candidates to propagate the virus in Latin America”
What are the reasons for a greater influenza virus infection, dissemination, and distribution rate among wildfowl, including migratory birds, in the last few months or years?
It has been speculated that climate changes have favored the permanence of wild and migratory birds in certain areas where they have had to geographically coexist as a species with other species with which they do not normally share their territory as much. This has possibly enabled a greater exchange of virus subtypes (and recombination thereof) among different species of birds and different migratory routes.
It is actually very difficult to be able to objectively answer that question. Maybe there is not a greater amount of influenza viruses in wildfowls, but an excessive concentration of commercial farms with biosecurity deficiencies.
If it were at all possible to speculate about the probable sources of infection in Latin America, what migratory routes within the United States would be the most probable pathways for potential propagation of influenza viruses to Latin America?
Nobody has a definitive answer to this important question, but being aware that some duck species geographically reach much deeper into the south; and that most of such species use the migratory routes in the central region and the Mississippi river, these latter migratory routes and their birds, which cover greater distances, would be significant candidates to disseminate the virus in Latin America.
The truth is that there are always influenza viruses in many species of wild birds, particularly marine and coastal species, as well as in ducks. However, it is important to preclude the chances for direct or indirect contact between these birds and their secretions, and commercial poultry.
The only countries in Latin America with a documented history of avian influenza in commercial fowls have been Mexico, Guatemala, Belize, Chile, Colombia, and the Dominican Republic (through serology but without a clear subtyping).
It is interesting to note that most of these outbreaks were primarily caused by the H5 and H7 viruses. Does this make sense or is it to be expected? Or is it possible that other low pathogenicity subtypes have gone undetected due to the mildness of the respiratory infections they caused?
H5 and H7 are naturally much more closely monitored subtypes because they are the most likely to turn from mild to highly pathogenic, and because in many countries it is mandatory to disclose these viruses, such as under the World Organization for Animal Health (OIE).
H5 and H7 viruses pose a significant problem for International trade, since most of the commercial associates of exporting countries prohibit imports of poultry and poultry products from countries, regions or sections that are affected by these subtypes (or any other highly pathogenic subtype).
Countries with a lower level of sophistication in surveillance, detection and control of avian influenza are probably exclusively concentrating on modest or moderate, active or passive epidemiological surveillance based on HI serology (hemagglutination inhibition test). Although HI tests are excellent to detect seroconversion to field infections or upon vaccination, they are restricted to the detection of H5 or H7 virus antibodies, and they do not detect infections (or seroconversion) by any other subtype but H5 or H7.
Therefore, countries focusing on HI–based surveillance alone are at a great risk of possible infections by other subtypes to go undetected. Performing HI tests is fine, but these should be supplemented by ELISA serology tests, which detect influenza A antibodies, or by molecular methods.
” Performing HI tests is fine, but these should be supplemented by ELISA serology tests”
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