Small but DEADLY

W hat to know about com­mon poul­try dis­eases

The Poultry Bulletin - - SMALL BUT DEADLY -

In­fec­tions and poul­try dis­eases are caused by sev­eral micro­organ­isms, namely bac­te­ria, viruses, pro­to­zoa and fungi. If the bird has a good im­mune sys­tem, it can re­sist low grade in­va­sions of these or­gan­isms. If the bird comes un­der stress which of­ten low­ers im­mu­nity, the bird be­comes sus­cep­ti­ble. These or­gan­isms then mul­ti­ply rapidly in the bird and sick­ness fol­lows.


Dis­ease-caus­ing bac­te­ria are found in large num­bers in the poul­try house. Once they invade the bird, they mul­ti­ply rapidly through cell di­vi­sion. One cell di­vides into 2, 2 cells di­vide into 4, 4 di­vide into 8 and so on. Bac­te­ria can sur­vive and mul­ti­ply out­side of the bird pro­vid­ing there is food and the con­di­tions are right. They mul­ti­ply un­der lab­o­ra­tory con­di­tions form­ing colonies. It’s the pat­terns of the colonies

they de­velop which helps iden­tify the bac­te­ria.

Some bac­te­ria pro­duce the con­di­tion of the dis­ease in the bird. Other bac­te­ria just lower the bird’s re­sis­tance, so al­low­ing other bac­te­ria to invade the bird and pro­duce dis­ease. Bac­te­ria pro­duce tox­ins which are an­tag­o­nis­tic to the bird.

The greater the num­ber of bac­te­ria, the larger the amount of toxin pro­duced un­til the bird dies. These tox­ins are of var­i­ous sorts and af­fect the bird in dif­fer­ent ways. The vir­u­lence (pathogenic­ity) of these or­gan­isms is a mea­sure of how fast it pro­duces a dis­ease. The more vir­u­lent, the faster the dis­ease de­vel­ops. This vir­u­lence may vary within a species of bac­te­ria. The bac­te­ria within the bird may not mul­ti­ply very rapidly un­til con­di­tions are right and then it ex­plodes, mul­ti­ply­ing rapidly so pro­duc­ing large amounts of toxin. This is es­pe­cially so on multi-aged farms where the vir­u­lence will in­crease as the dis­ease goes through each batch of birds.

The bird must be in a sus­cep­ti­ble state for the dis­ease to be­come es­tab­lished and the bird has var­i­ous mech­a­nisms to help pre­vent in­va­sion. The most im­por­tant of these is keep­ing the bird stress free. Good man­age­ment in en­sur­ing good tem­per­a­ture con­trol, good feed, good hous­ing and cor­rect stock­ing den­si­ties, etc. This en­sures that the im­mune sys­tem does not weaken so mak­ing bac­te­rial out­breaks less likely. A healthy skin free of cuts and scratches will keep bac­te­ria out. The cilia and the mu­cous on the mem­branes in the res­pi­ra­tory tract will also rinse bac­te­ria away. That’s why it’s es­sen­tial to keep am­mo­nia lev­els down as am­mo­nia will de­stroy the cilia.

Cli­mate and sea­sons also play a role in that some dis­eases af­fect birds more in cold weather and some more dur­ing hot weather. Some species of birds are more re­sis­tant to cer­tain bac­te­ria than oth­ers. The bac­te­ria will invade a type of bird and grow and mul­ti­ply with­out pro­duc­ing any ev­i­dence of the dis­ease but in an­other type of bird be dev­as­tat­ing to its health. The age of the bird can in­flu­ence its sus­cep­ti­bil­ity to cer­tain bac­te­ria. With this in mind, good man­age­ment is the most im­por­tant fac­tor in pre­vent­ing dis­ease out­breaks.


Viruses are the small­est of the micro­organ­isms that cause dis­ease and can only be seen with an elec­tron mi­cro­scope. Most can only sur­vive for a few hours out­side

of the body. They are killed by di­rect sun­light, heat and some dis­in­fec­tants. They can­not mul­ti­ply out­side of liv­ing tis­sue and can only mul­ti­ply within a host cell. They can­not be treated with an­tibi­otics. Vac­ci­na­tion is the only way to pre­vent vi­ral dis­eases. Each vac­cine is spe­cific for each virus i.e. vac­ci­na­tion for New­cas­tle dis­ease will only be ef­fec­tive against New­cas­tle dis­ease.


These are sin­gle celled an­i­mals which are par­a­sitic in poul­try by liv­ing on the con­tents of the cell. They have a com­plex life cy­cle. An ex­am­ple of this is Coc­cid­io­sis.


Fungi are a group of or­gan­isms which in­clude moulds and yeasts. They grow out­side of the bird and can be in­gested or in­haled by the bird. They pro­duce tox­ins in the bird.

Vi­ral Dis­eases New­cas­tle Dis­ease (NCD)

This is caused by a virus of which there are four forms: Len­to­genic (mild pathogenic­ity), Me­so­genic (in­ter­me­di­ate pathogenic­ity), Vel­o­genic or Neu­ro­topic (high pathogenic­ity), Vis­cero­topic Vel­o­genic (very high pathogenic­ity). The mild form is used as a vac­cine, the mod­er­ate is of­ten seen in birds that have not been prop­erly vac­ci­nated, and the very vir­u­lent form can cause ex­tremely high mor­tal­i­ties in un­pro­tected flocks.

The virus can be trans­mit­ted through the air over rea­son­ably short dis­tances. Cough­ing dis­lodges the virus from the res­pi­ra­tory tract and this is then car­ried in the air from bird to bird. The virus can also be car­ried by me­chan­i­cal means, such as on cloth­ing, equip­ment, trucks, etc .This is prob­a­bly the most com­mon form of trans­mis­sion and can be pre­vented by strict biose­cu­rity. Short rest pe­ri­ods and multi-age sites of­ten re­sult in the older birds re-in­fect­ing the younger birds. Viruses can→

also be spread by wild birds, neigh­bour­ing poul­try, or even ver­min like rats and mice.

The in­cu­ba­tion pe­riod is 4 – 6 days and the spread of the dis­ease is very rapid. Symp­toms are res­pi­ra­tory dif­fi­culty, twisted necks and di­ar­rhea. The tail feath­ers by the Cloaca are of­ten stained green from Bile as the birds stop eat­ing. Egg pro­duc­tion drops and eggshell qual­ity is re­duced. Upon post­mortem, hem­or­rhag­ing of the proven­tricu­lus is seen and le­sions are some­times found in the res­pi­ra­tory tract. In the pic­ture the hem­or­rhag­ing of the proven­tricu­lus can be clearly seen.

There is no treat­ment for this dis­ease. Birds can only be pro­tected by vac­ci­na­tion. It is best to con­sult with a vet­eri­nar­ian to de­sign a vac­ci­na­tion pro­gramme for your area and cir­cum­stances. Vac­ci­na­tion tech­niques need to be good to achieve titer lev­els that will re­sist a field chal­lenge. Titer is a mea­sure of an­ti­body pro­duc­tion or im­mu­nity.

Gum­boro Dis­ease/in­fec­tious Bur­sal Dis­ease (IBD)

This is a vi­ral dis­ease which at­tacks the im­mune sys­tem of the chicken. In young birds the Bursa of Fabri­cius is mainly re­spon­si­ble for pro­duc­ing an­ti­bod­ies which kill off dis­ease caus­ing pathogens. This or­gan can be found next to the Cloaca and when cut open has folds in it. This dis­ease at­tacks this or­gan thus se­verely af­fect­ing the bird’s im­mune sys­tem. In­cu­ba­tion pe­riod is about 2 days. In a very vir­u­lent form, it can cause high mor­tal­ity but nor­mally Coli sep­ti­caemia fol­lows on from the dis­ease. There is no cure, pre­ven­tion is by vac­ci­na­tion, but an­tibi­otics are of­ten used to pre­vent se­condary bac­te­rial in­fec­tions.

The dis­ease is con­fined to younger birds. Birds be­come mor­bid, stop eat­ing, have ruf­fled feath­ers and can have whitish di­ar­rhoea. Birds can of­ten be seen pick­ing at their vents.

On post mortem when the skin is peeled back some haem­or­rhag­ing can be seen on the thigh mus­cles and some­times the breast. The Bursa is swollen and when cut open haem­or­rhag­ing can be seen. It is best to send birds to the lab­o­ra­tory for a pos­i­tive di­ag­no­sis. Note the swollen Bursa of Fabri­cius.

Con­trol should rather be by pre­ven­tion so biose­cu­rity is very im­por­tant. The virus is ca­pa­ble of re­main­ing vi­able out­side of the chicken for sev­eral months so proper wash­ing and dis­in­fec­tion of houses be­comes very im­por­tant. It is mainly spread by peo­ple and on equip­ment.

There is no treat­ment for this dis­ease as it is a vi­ral dis­ease, but an­tibi­otics are of­ten given for se­condary in­fec­tions. Par­ents are→

Gum­boro Dis­ease

New­cas­tle Dis­ease

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