EPIDEMIOLOGY

GEOGRAPHIC DISTRIBUTION AND HOSTS

Several huge epizootics have affected domestic chickens and turkeys since 1927. The geographic distribution of ND in free-living birds is markedly affected by three virus sources: i) free- living birds, which acquire NDV as an overspill from infected poultry; ii) the pigeon variant of APMV1 (also named pigeon PMV1, PPMV1), which was introduced to Europe from Middle Eastern countries in 1981—1982 and has since occurred not only in free-living feral pigeons (Columba livia) but also in other columbi- formes such as the Eurasian collared dove (Streptopelia decaocto), wood pigeon (Columba palumbus) and other species that live in urban environments⁽⁵⁾ (after the intro­duction of the pigeon variant of APMV1 to European countries, large proportions of wild birds succumbed during the following years⁽²⁾); iii) migratory birds, which are a less important source, that fly from Africa to Europe within the incubation or latent period or are traded within European countries.

Exact data on the geographic distribution of APMV1 for all European countries are not available. In Germany, examination of3,672 cloacal samples from 78 wild migrat­ing and resident bird species collected during 2003 to 2005 resulted in detection of 52 APMV1-positive samples⁽³⁾. APMV1 were obtained from mallards (Anas platyrhynchos), Eurasian coot (Fulica atra), a black-headed gull (Larus ridibundus) and a mute swan (Cygnus olor). APMV4 was obtained from mallards, teal (Anas crecca), European wigeon (Anas penelope), a black-headed gull and a magpie (Picapica). APMV6 was detected in mallards, a mute swan and a greylag goose (Anser anser). The results indicated that predominantly apathogenic APMV1 circul­ates among waterfowl and waders (Anseriformes and Charadriiformes) that migrate from their Nordic breeding grounds to central Europe⁽³⁾.

In a separate study, in Germany, a mallard yielded an APMV9⁽⁶⁾. APMV1 was not isolated from 658 organ samples of 110 greylag geese. However, haemagglutination inhibition (HI) antibodies against APMV4 were detected in one of 13 serum samples, against APMV6 in 10 of 13, and against APMV8 in two of 13 serum samples from this species⁽⁶⁾.

ND has been described in 241 avian species from 27 of 50 orders⁽⁷⁾ (Table 3.1). It appears reasonable to conclude that all species of European birds are susceptible to infect­ion by NDV. However, the signs of disease and its outcome vary considerably with the affected host species. Many reports describe virus detection in birds that were found dead and none describe major die-offs among free-living birds in Europe. Young birds that acquire infection by virulent NDV before the development of immunocompe­tence seem to be more susceptible to new infection and disease than mature, immunocompetent birds. Convales­cent birds with serum antibodies are resistant to disease but not to infection. The sex of birds does not affect infect­ion and outcome of disease.

Virulent APMV1 as defined below in the diagnosis section, are of major importance as notifiable epizootics in

TABLE 3.1 Host range of NDV in free-living European birds (abridged; for details see Kaleta and Baldauf, 1988(⁷)).

CNS — nervous signs including torticollis, opisthotonus None — either not reported or not observed

chickens and turkeys. Such strains have been frequently isolated from free-living gallinaceous, passerine and other terrestrial birds.

Mesogenic APMV1 have been detected so far only in domestic chickens. Lentogenic strains are frequently iso­lated from aquatic birds such as ducks, geese, swans and many shore birds. Avirulent enteric APMV1 has not been associated with any disease expression. Numerous viruses of this pathotype have been, in most cases, isolated from subclinically affected aquatic birds. As compared to APMV1, APMVs of the types 2 to 9 are of minor import­ance as causes of disease and losses. Such viruses are occa­sionally isolated from a variety of birds, mainly passerine and captive psittacine birds⁽²⁾. Only strains of the APMV3 cause respiratory signs and pancreatitis in turkeys and, in addition, disorders of the central nervous system in some captive psittacine birds, namely Australian parrots of the genus Neophemd-^sK

ENVIRONMENTAL FACTORS

Climate per se has no reported effect on the prevalence of disease. However, cold winters with frozen surface waters force aquatic birds to aggregate in large numbers on a few remaining open waters, which facilitates frequent and effective virus transmission. As NDVs are readily transmit­ted by the faecal- oral route, virus transmission is more likely. However, NDV in excreted faeces or saliva is readily inactivated by ultraviolet light from sunshine.

Moist soil and decomposing vegetation preserve the infectivity of the virus, whereas dry materials and higher temperatures inactivate virus rapidly. Several studies provide evidence for survival of NDV for days to months, but the data cannot be applied to the highly variable envi­ronmental conditions in nature, such as decomposition of organic matter, pH, organic acids found in the environ­ment, microbial colonization (bacteria, yeasts, moulds), survival in arthropods and following ingestion by earth­worms. Viral infectivity in carcasses, for example of hunted birds, can be retained for several months at freezing tem­peratures. Animate and inanimate vectors are not neces­sary for virus spread, and arthropod vector transmission does not occur.

TRANSMISSION

Saliva and faeces of ND-affected domestic poultry contain high amounts of NDV. Birds with access to poultry farms (house sparrows, swifts, swallows, crows) acquire infection by direct contact with virus-containing faeces or consumption of contaminated feed and water for poultry. Thus, horizontal transmission is the most important route. Also, live- bird markets displaying domestic, hobby, pet and captured wild birds facilitate spread among birds. Local spread among waterfowl is likely to be due to con­taminated water. International and intercontinental trans­mission may occur via commercial trade in live poultry and poultry products or offal. Virus transmission may occur from migrating wild birds returning from Africa to Europe. Free-ranging chickens in many African countries suffer from endemic ND, which facilitates transmission to migrating birds.

An epidemiological link between ND outbreaks in poultry and virus isolations in birds of prey has been observed, with a causal relationship between ND-infected chickens and domestic pigeons, as the prey of Falconi- formes and Strigiformes, in which birds virus is sub­sequently detected(⁴).