Among experienced poultry farmers, a viral infection like Marek's disease could mean an end of the investment in the affected flock. Marek's disease is a highly infectious lymphoproliferative viral disease mainly affecting chickens. It is associated with tumours in the liver, spleen, kidneys, skin, gonads, iris, and paralysis linked to mononuclear cells infiltration of peripheral nerves. It is becoming a serious concern because of the evolution of its etiologic agent toward higher virulence (Dunn et al 2019).This disease is caused by a cell-associated oncogenic DNA virus that belongs to the genus Mardivirus in the subfamily Alphavirinae of the order Herpesvirales (Davison 2010). This oncogenic serotype 1 Herpes virus has been said to cause high economic losses globally, up to $1-2 billion (Morrow, Fehler 2004). These are losses associated with poor growth, egg production losses, chicken death, etc. (Rozins et al 2019). The Marek disease virus is also classified into four pathotypes based on their pathogenicity in experimental infections. These are the mild (m), virulent (v), very virulent (vv), and very virulent plus (vv+) (Bertzbach et al 2020). This disease occurs more commonly in birds between 10 to 15 weeks of age and more often in females (Abdu, Musa 2019), although it also affects older layers.Transmission of the virus occurs through inhalation of virus-laden dust. The virus usually occurs in feathers, oral, nasal, and tracheal secretions. The incubation period is usually 3 -4 weeks to several months. The mortality ranges from 20% to 60%. This disease occurs in classical form (fowl paralysis), which produces nerve lesions or could be with a high level of visceral tumours (acute Mareks disease) or the cutaneous form (Abdu, Musa 2019). Outbreaks of Marek's disease do occur even after vaccinations of day old chicks in the hatchery. This may be because of errors in vaccination at the hatcheries, loss of potency Abstract Marek's disease (MD) remains a major concern to chicken farmers, especially because of its adverse effect on birds' growth, livability, and egg production. Veterinarians often consider the prognosis poor, especially when tumors are seen in organs like the liver, spleen, and lungs. This is a report of an outbreak of acute MD in commercial chickens. In the present study, carcasses of 16 weeks old Isa brown pullets were presented at the Veterinary Teaching Hospital, the University of Jos for investigation. They had been vaccinated at the hatchery against Marek's disease at day old. The disease was diagnosed as Marek's disease based on mortality pattern, necropsy findings, histological observations, and polymerase chain reaction analysis. Secondary bacterial infection was caused by Staphylococcus aureus, susceptible to gentamicin. The affected flock was revaccinated using Izovac® HVT Marek's vaccine. As a follow-up, a gentamicin-doxycycline-based oral preparation: Doxygen®, was administered for five consecutive days at 100g/200 litres of drinking water. A significant reduction in mortality was obser...
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