Main viral diseases in broilers in the USA
Some of the viral diseases with greatest economic impact in US broiler production will be discussed in this article. Many of them are immunosuppressive, which means that they impair the functioning of the immune system and make animals prone to suffer other infections...
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Some of the viral diseases in broilers, with greatest economic impact in US broiler production, will be discussed in this article. Many of them are immunosuppressive, which means that they impair the functioning of the immune system and make animals prone to suffer other infections, making diseases much more severe and difficult to overcome.
More specifically, avian infectious bronchitis and Marek’s, Newcastle and Gumboro diseases will be described. First, their main clinical signs and then, the lesions observed in the post-mortem evaluations that can help in their diagnose.
Finally, we will describe several natural mechanisms available in the country to prevent and minimize their impact in the farms.
MAIN VIRAL DISEASES IN BROILERS
Avian infectious bronchitis
The severity of this disease is greatly influenced by the age and immune status of the flock, the environmental conditions and the presence of concomitant diseases .
Morbidity and mortality also depend on the virus serotype or variant that currently affects the farm. The most frequently observed clinical signs are respiratory, such as eye and nasal discharge, panting and coughing. Despite this, there are viral variants with trophism towards the respiratory and renal systems, and the signs caused by them are the presence of urates in the dejections and a decrease in egg production and shell alterations (in breeders) , .
Serous, catarrhal or caseous exudate in trachea and foam in the air sacs can be observed in the post-mortem evaluations. If kidneys are affected, nephritis and ureter distention than can evolve to renal and visceral gout (presence of urates in the organs in the coelomic cavity) are a usual finding. In females during the latter stages in their productive cycle, salpingitis in the right oviduct (unevolved) and fibrinous material with alteration of the tissue consistence in the left oviduct  are the lesions observed.
Vaccination is the commonly used tool to prevent infectious bronchitis but, due to it is a highly mutagenic virus with many variants and serotypes, the protection obtained with vaccination may not be totally effective . Very often, viral variants in the USA have their origin in the virus used in vaccinations, such as Mass, Conn and Ark . Another issue to consider is that, when animals are infected, this disease has an immunosuppressive effect that reduces the efficacy of further vaccinations.
Marek’s disease is neoplastic and caused by a lymphotropic virus that causes the formation of T-cell lymphomas . In affected birds, tumors in the organs and skin and alteration of the nerves, particularly the peripheric ones, iris and skin caused by the infiltration of lymphocytes can be observed.
The clinical picture can be nervous, with unilateral flaccid paralysis of the legs and wings, ataxia, and alterations of the sciatic verve; visceral, with tumors in different organs, especially the liver and spleen, which lead to diarrhea and low but constant mortalities; or with flaccid neck, with birds affected by flaccid paralysis of this region associated to highly virulent viral strains. Apart from the lesions already described, atrophy of the lymphoid organs (thymus and bursa of Fabricius) are the other common findings.
Vaccination is the most used prevention tool, as this disease has severe consequences in the affected farms. Vaccines need to consider the pathotypes and serotypes present in the farm in order to choose the right vaccine. Besides, despite vaccination prevents animals from getting sick, it does not protect birds against the infection, so that animals remain as a source of the virus . This is why vaccine strains can easily recombine with wild variants .
Newcastle disease is a viral infection that usually causes acute respiratory signs, whose severity and mortality depends on the variants present in the farm (lentogenic, mesogenic, velogenic) . Mortality can vary between 10% and 80%.
Respiratory signs include coughing, panting and nasal discharge. The other frequent signs are depression, watery diarrhea and neurological alterations , derived from the damaged central nervous system, that go from trembling and spasms to flaccid paralysis of the extremities, torticollis (wry neck), and neck and head swelling.
In the necropsies, there is also congestion and exudates in the mucosae of the respiratory tract, as well as petechiae and hemorrhages in the serous membranes of the digestive system and Peyer patches, splenomegaly, and spleen lesions, in addition to oedema in the thymus and bursa of Fabricius.
Outbreaks caused by highly pathogenic strains of this disease in the USA are infrequent and usually related with the importation of birds from Mexico. For the prevention, the USDA recommends the application of severe biosecurity measures that include the implementation of vaccination plans. Despite this, keep in mind that this virus has a strong immunosuppressor activity.
Also called infectious bursal disease or infectious bursitis, this disease usually affects 3- to 6-week-old-birds. It causes immunosuppression in birds because it targets the mature B lymphocytes in the bursa of Fabricius.
In its acute or classic form, birds suffer from depression, whitish diarrhea, anorexia, ruffled feathers, and lethargy. Some of them show sudden death. Birds infected before week 3 of life suffer the subclinical disease with growth retardation associated to other diseases.
The most depicting lesion is an enlarged bursa of Fabricius, which is also hyperemic and has longitudinal marks in its tissue. It turns from white to creamy white. After several days, the bursa becomes atrophic, its size decreases and it turns edematous and greyish. Other lesions that can also be observed are muscle hemorrhages (legs, pectorals), presence of mucous in the gut and nephritis or nephrosis
Most vaccination plans for broilers in the USA include vaccinations against Marek’s disease. They are important to prevent the disease, as well as to consider the predominant variants in the area that cause a rapid atrophy of the bursa of Fabricius, which are different from the classic variants and may need specific vaccine to be kept under control.
NATURAL STRATEGIES FOR THEIR PREVENTION
The strategies to prevent these diseases should focus on improving vaccines’ efficacy and minimizing the immunosuppressor effect of these viruses. In this sense, immunostimulant pronutrients can be useful.
Pronutrients are botanical molecules that stimulate cell’s physiological activity. They act like a stimulus that activates and boosts the activity of certain genes in the target cells related to specific functions. Immunobooster pronutrients activate immune cells in birds in order to maintain their physiological activity and, therefore, keep the animal’s defense system in an optimal status.
Its administration in feed or drinking water at small doses can activate, up to physiological levels, the adaptative and innate immune systems in a way that birds are ready to fight any infectious agent that comes across. Besides, these pronutrients improve the response to vaccination in order to obtain the best protection against specific diseases such as Gumboro or Newcastle diseases or avian infectious bronchitis.
The chart below shows results obtained in broiler chickens vaccinated against Newcastle disease on days 10 and 23 (LaSota). Birds in the control group (131,480) did not receive any immunobooster, while birds in the pronutrients group (56,200) received immunobooster pronutrients at 1 ml/l from day 1 to 10 and at 0.5 ml/l from day 11 to 20.
Antibody titters for Newcastle disease on day 38 got increased by 36.2% in the group with pronutrients, which means that the response to vaccination was greater and, consequently, birds were better protected against this disease. The administration of pronutrients also had a positive effect of reducing mortality, which decreased by 15%.
Infectious bronchitis and Marek’s, Newcastle and Gumboro diseases are the most important viral diseases in broilers in US broiler farms. Their economic impact is derived from the consequences they have on performance, morbidity, and mortality, as well as to their immunosuppressor effect, which makes animals prone to suffer other infections.
The most used prevention mechanism is vaccination. Despite this, the broad variety of virus causing these diseases and the technical difficulties to manufacture and administer these vaccines, cause vaccination to lose efficacy.
Therefore, it is highly recommended to use immunobooster pronutrients together with vaccination, since these natural molecules help to improve antibody production to achieve greater protection levels against specific diseases. Their administration helps to prevent the immunosuppression caused by these diseases and reduce their consequences.
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MV. Júlia Pié Orpí
Veterinary Technical support to the area of Latin America at Biovet S.A. Laboratories Official Veterinary Services (SVO) in poultry slaughterhouse