Elements of epidemiology concerning the influenza

This article on the elements of epidemiology concerning the influenza is one of the sub-chapters of the article Avian flu . Without having vocation to replace a Veterinary treaty , it treats aspects diagnoses and Prophylaxie

The virus of the Avian flu transfers easily and frequently. Several of its forms can infect at the same time the Man and Animal.
Ceci particularly returns the éco-epidemiology of this complex virus , the more so as the Mondialisation of the breeding and the trade, as well as the acceleration of transport, are factors suitable for have considerably transformed the causes, the width and the incidence of the Zoonose S and in this case of the human epidemics.

The epidemiology and the éco-epidemiology have as a finality the action against the disease. In the case of the avian flu, the contamination of the breedings by the Avian flu (and particularly by the virus H5N1 worries the human Médecine and Vétérinaire, that UNO invites to work in concert.

From 2003 to 2005, the hearths were primarily detected in breedings. They were initially allotted to a contagion starting from wild birds (breedings located on the way of migratory birds e.g.), but a highly pathogenic Virus can emerge by change or recombination within the breedings themselves. The contamination can then diffuse via exchanges of birds, chicks and besides eggs or circulation of machines or materials contaminated…
Des given recent coming from the genetic analysis of the Asian viruses let think that the H5N1 HP found in the wild birds appeared in a breeding.

- a hearth produced by a not very pathogenic and not very contagious stock remains generally localized (sporadic); it dies out itself, without much economic impact. Transport of poultries, eggs or matters at the risk can however contribute to extend this type of epidemic.

- conversely, a highly pathogenic stock can in a few weeks or month to spread itself in an area (cf trades of birds, eggs, by-products, materials or people soiled…) by causing heavy losses. And in a few years it can colonize a continent or a great part of the world.

Comme shows it the Histoire of epizooties of avian flu, of the enzootic forms can discuss locally, and give place to important regional hearths. Exceptionally, a virus known as “animal” (is this denomination justified?) can pass to the Man for, even more exceptionally to be at the origin of a Pandémie, either directly, or while recombining with another virus grippal.

Origin, ancestor of the virus

Who brought the first virus to the Man, the pig and chicken?
One is unaware of it, but one found in the birds and only in the birds all the known Sérotype S of the virus of the grippe.
So and more recently by deduction starting from the phylogenic tree (filiations by branches or Clade S) of the grippaux viruses one deduced that the bird was probably the original host of the virus, and even as it was originating probable in Asia in the case of the H5N1.

But there was very little research of the prevalence of the virus grippal at other species that the Man, the pig, the horse and the birds. However for a few decades that one starts to seek some, of the grippaux viruses have been detected at a good number of other species of animals with blood chaud.

- It is known that grippaux viruses affecting of the species as different as the man or the pig and the bird can recombine between them.

One can imagine that the virus grippal coévolue with the birds (and mammals?) for a very long time, even that there was an ancestor of the influenza at the Dinosaure S, remote ancestors of the birds.

the question could be posed differently :

- does the bird lodge all the types of germ because it is the original host and that it lodged the ancestor of the contemporary viruses?
- or because thanks to the flight and with its extraordinary capacities of migration, it is the only animal to attend the near total of the biosphere (terrestrial environments and sailors)?

the virologists try to answer this question by reconstituting the phylogenetic tree of the known viruses, with the assistance of the epidemiology and the éco-epidemiology.

Reserve of the virus

- grounds or cold water of the palearctic zone (of Canada in Siberia while passing by Norway, Sweden.) could play a passive part of winter tank for the virus.
- the grippaux viruses of the birds constitute in any case one of the mobile layers of viral genes . The mankind, the Pig S, hens, Duck S, Goose S, Ruail S, horses, Camel X and others Autruche S constitutes others of them (tanks);
- These domesticated species , by their promiscuity, their genetic homogeneity, and the fact that they have had escaped for a few centuries or decades with the natural processes of selection undoubtedly constitute also a stock of viral genes and targets of choice for the influenza…
- It may be that links ecoepidemiologic of the virus are still ignored ; As example, some Mucus (ex: snails) animal are rich in acid sialic (biochemical target of the virus in the lungs) and one can imagine that the virus could starting from the droppings the oisaux one resist it dehydration, UV and heat while being transported and likely to be introduced by birds or mammals or other species.

Lifespan of the virus

the virus of the Bird influenza is famous to better survive in fabrics where it reproduced, in the excrements which are the first source of contamination for the bird, and in water (except if it is acid or hot).

- It resists 7 days 20°C, 35 days with 4°C, according to the course of Jean-Pierre GANIERE of ENVN of House-Alfort (given the semione).
- It resists 3 months in a water slightly Basique and at temperatures moderated according to the AFSSA (2005).
- But of other sources increase this duration to 105 days, even several months when the temperature is lower than 0°C.
- It resists 105 days in the manure and several hours in the Mucus dried, with can be differences according to the variable ones or of the conditions badly included/understood (content of water oxygen, exposure to UV solar, nature of the Biofilm, etc).
See on this subject

Incubation

incubation is considered to last 24/48 hours to 7 days depending on the Isolat, the amount of Inoculum, the species, the age and the state of the infected bird and perhaps according to other conditions badly included/understood (interaction with other pathogenic? season.).
Un vaccinated bird or which survived a first contact with the virus, or variable near is immunized but it can develop a discrete or asymptomatic form of the disease, i.e. excrêter some time a certain quantity of virus.

- In chickens, incubation is sometimes very short (24/48 H). And a breeding can know a death rate close to 100% in 48 hours.
- In 2006, the duration of incubation remains badly observed in nature and the phenomenon is still badly included/understood in the asymptomatic carrying birds, the pig, the cat-like ones (of which the cat or in other mammals.

Contagion

It would be done primarily by direct transmission (contact) but so indirect (food contaminated by droppings of wild birds, soiled packing…). The infected egg (believed or badly cooked) is contaminant.
- the ways of penetration are digestive and/or respiratory.
- the anatidés migrating ones often lodge in an unapparent way 38 of the pathogenic stocks for chickens…. (source: Court veterinary surgeon of diseases considered contagious of Houses-Alfort, updated semi-2005)
- the cured turkeys and poultries still carry the virus influenza during months, still constituting a source of contamination for other poultries or wild birds (pigs?).
- vaccinated poultries still excrete of the viruses at least during a week.
- the ground soiled or with many soiled objects can also play a part of “inanimate” vector.

the virulence of the viruses influenza depends on the nature of the site of cleavage of their hémagglutinine. All the stocks of the type H5 or H7 are potentially pathogenic for the poultry. The sequencing of the site of cleavage of gene ha makes it possible to identify the pathogenic stocks.

Localization of the virus in the organization

the virus of the traditional seasonal human influenza infects usually initially the lungs then sometimes the digestive tract. Receivers cellular of this virus are rather grouped in part high of respiratory system (bronchi, pharynx, trachea and nasal mucous membrane), which explains the strong contagiousness of this influenza, and on the other hand, the weak current transmission interhumaine of the H5N1 whose targets are located in the content of the lungs. One however always fears a change allowing the H5N1 to infect the upper part of the respiratory system.

the virologists had already noted in 2005, qu ' contrary to the seasonal flu, the avian flu of the type has (H5N1) could at the Man produce viral titles (rate of virus in the Mucus) higher in the throat than in the nose (cleanings would have of which rather to be made in bottom of throat. Samples of the lower respiratory tracts could offer more significant means of diagnosis, and that tests made on samples collected in the nose, but there remains interesting to sample the nose to detect a possible change making the virus able to infect the high part of the lungs, of which the sphere ORL).
a study concluded at the beginning of 2006 that the human receivers of the H5N1 HP (for the variable ones identified of 2003 to 2005) are rather distributed in the air cells, i.e. in the major part of the lungs , whereas the banal seasonal flu infects especially the top of the lungs.

Certain grippaux viruses, highly pathogenic (of which the H5N1) have an unusual capacity to infect other bodies quickly that the Poumon S, at the Man or other mammals.

- WHO and FAO had already confirmed in December 2005 that certain viruses H5N1 HP (highly pathogenic) were propagated with almost all the parts of an infected bird, including their flesh and the brain. It is said that the attack is Systémique .
- The same applies in the cat, the mouse (in laboratory), and probably at the Human one. This is an éco-epidemiologic problem, because the urine, sperm, the vaginal Mucus or the excrements could then become additional vectors of the virus (to be checked).

  • Four remarks/comments in connection with the studies published at the beginning of 2006.

- Science/March 23rd, 2006 (www.sciencexpress.org) Page 1/10.1126/science.1125548
- Nature 2006; 440: 435-36

  1. According to the authors, the sub-type H5N1 which emerged in 1997 with Hongkong was ready to infect the human higher respiratory tracts than those which circulate since. The fact then of having cut down all chickens of the territory (1,3 million heads) thus perhaps avoided a pandemia. The H5N1, in another form then spent six years to remake surface, elsewhere in Asia. This variable of the H5N1, like the precedent causes viral pneumonias and nondue to secondary infections, which was also the case of Spanish influenza H1N1 of 1918.

  2. For the moment, the work carried out by Yoshihiro Kawaoka (University of the Wisconsin) is based on animal models infected by the H5N1 for work of Japanese, and on human fabrics studied by Dutch who also worked on the cat and the pipe cleaner. But the assumption remains to be checked at the Man by a more systematic study of the viral titles in the various compartments of the human lung and in their secretions, study made difficult by the critical condition of the patients.
  3. the team of Dutch Thijs Kuiken (Erasmus Medical center, Amsterdam, Université of Rotterdam) note that the type of fixing of the virus in the human respiratory tracts is rather similar to that found in the pipe cleaner or the cat whose receptivity with the H5N1 and the influenza is studied by this team since 2004. Cat and pipe cleaner would be thus of good experimental models, from this point of view.
  4. Thijs Kuiken, announces at the beginning of 2006 that its team will seek with to model which changes would allow the virus avian flu to be fixed in the higher tracts respiratory of the man, so in particular helping WHO to know which changes to supervise.

Tentative conclusion

In 2006, in spite of significant progresses, one still knows little thing about the éco-epidemiology of the avian flu and the prevalence of the virus in nature. With development of studies on follow-up of migratory birds, and classification of virus grippaux highly pathogenic in Notifiable disease (i.e. indicated by the Administration veterinary surgeon like virus “whose detection or suspicion must be carried immediately to the knowledge of the Veterinarian authority, in accordance with the national regulations, according to the GOOSE), knowledge should progress. Certain experts fear that by too targeting research on the bird, one omits part of the cycle of the virus which could possibly be related to other species. It should in particular be included/understood how this famous virus being destroyed by heat survives so well in countries such as Indonesia.

To return in the principal page

See too

Internal bonds

Zh-min-nan: Khîm-liû-kám

Random links:Ségos | Furnaces (Nievre) | Brahe | Aryabhata (crater) | Élections_de_sénat_d'Etats-Unis,_1984