**4.2 Clinical signs**

*Viruses and Viral Infections in Developing Countries*

**3.6 Factors supporting occurrence of outbreaks**

poultry farms [44].

typical live bird scenario.

**3.7 Control measures**

possible source of introduction and reintroduction of HPAI viruses into Nigeria's poultry. On the other hand, rural poultry mainly kept in free-range, multispecies, multiage holdings with low biosecurity levels are exposed to many at-risk contacts and therefore act as the epidemiologic link between the wild reservoir of AI viruses and industrial poultry [22, 35]. The predominant species in the rural poultry sector of Africa is the scavenging indigenous domestic fowl (*Gallus gallus domesticus*) [43], which in most African countries has no regular health control program, may or may not have shelter, and scavenges for most of their nutritional needs [43]. In fact, village chickens have been reported to act as potential reservoirs and carriers of infections to themselves and to the more susceptible exotic breeds in commercial

Following initial introduction of the virus from a possible migratory wild bird [22] or water fowl [20] through direct means into commercial poultry or by resident water fowls and ducks that form bridges of virus transmission from migratory wild birds [35], the virus is further spread by inadequate farm biosecurity, trade in poultry, human and vehicular exchanges between farms, and uncontrolled farm visits by poultry and poultry product vendors. Live bird markets have also been shown to harbor [39] and transmit the virus among birds and finally to household poultry upon introduction of poultry bought from live bird markets [20]. Mixed-species poultry farming has been shown to have higher odds of HPAI virus infection [39] and could harbor apparently healthy but infected duck species which may transmit the virus to susceptible chickens and turkeys. In a study conducted during the first introduction of HPAI into Nigeria, it was shown that mortality rate was much higher in mixed species flocks (*P* < 0.0001) and ranged from 4.92 to 73.15% with the chicken-duck-turkey mixed flock farms having the highest rate (73.15%) [39]. Also, it was established that higher risk of HPAI disease occurred in multiple, mixed species poultry than in single species poultry production [39]. **Figure 3** depicts a

Enforcement of movement control, surveillance (active and passive), and prompt payment of the revised compensation to the affected farmers, including reorganization of the LBMs which led to the successes recorded during the

*A typical live bird market where poultry from different origins are sold. ©Prof. T. U Obi, University of Ilorin.*

**66**

**Figure 3.**

In gallinaceous domestic poultry, infection with HPAI viruses produces severe depression, severe decrease in feed and water consumption, high morbidity and mortality rates, sudden death, and occasionally nervous signs if they survive the peracute syndrome. However, the frequency of clinical signs and gross lesions varies with virus and species of bird and is not consistent in all birds [3] (**Figure 5**).
