Implementation of an extended ZINB model in the study of low levels of natural gastrointestinal nematode infections in adult sheep.
BACKGROUND
In this study, two traits related with resistance to gastrointestinal nematodes (GIN) were measured in 529 adult sheep: faecal egg count (FEC) and activity of immunoglobulin A in plasma (IgA). In dry years, FEC can be very low in semi-extensive systems, such as the one studied here, which makes identifying animals that are resistant or susceptible to infection a difficult task. A zero inflated negative binomial model (ZINB) model was used to calculate the extent of zero inflation for FEC; the model was extended to include information from the IgA responses.


RESULTS
In this dataset, 64% of animals had zero FEC while the ZINB model suggested that 38% of sheep had not been recently infected with GIN. Therefore 26% of sheep were predicted to be infected animals with egg counts that were zero or below the detection limit and likely to be relatively resistant to nematode infection. IgA activities of all animals were then used to decide which of the sheep with zero egg counts had been exposed and which sheep had not been recently exposed. Animals with zero FEC and high IgA activity were considered resistant while animals with zero FEC and low IgA activity were considered as not recently infected. For the animals considered as exposed to the infection, the correlations among the studied traits were estimated, and the influence of these traits on the discrimination between unexposed and infected animals was assessed.


CONCLUSIONS
The model presented here improved the detection of infected animals with zero FEC. The correlations calculated here will be useful in the development of a reliable index of GIN resistance that could be of assistance for the study of host resistance in studies based on natural infection, especially in adult sheep, and also the design of breeding programs aimed at increasing resistance to parasites.
Departamento de Producción Animal, Universidad de León, Campus de Vegazana s/n, 24071, León, Spain.
M
M
Atlija
Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow, Bearsden Road, Glasgow, G61 1QH, UK. joaquin.prada@princeton.edu.|Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, 08540, USA. joaquin.prada@princeton.edu.
J M
JM
Prada
Departamento de Producción Animal, Universidad de León, Campus de Vegazana s/n, 24071, León, Spain.|Instituto de Ganadería de Montaña, CSIC-ULE, 24346, Grulleros, León, Spain.
B
B
Gutiérrez-Gil
Instituto de Ganadería de Montaña, CSIC-ULE, 24346, Grulleros, León, Spain.|Departamento de Sanidad Animal, Universidad de León, Campus de Vegazana s/n, 24071, León, Spain.
F A
FA
Rojo-Vázquez
Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow, Bearsden Road, Glasgow, G61 1QH, UK.
M J
MJ
Stear
Departamento de Producción Animal, Universidad de León, Campus de Vegazana s/n, 24071, León, Spain.
J J
JJ
Arranz
Instituto de Ganadería de Montaña, CSIC-ULE, 24346, Grulleros, León, Spain.
M
M
Martínez-Valladares
D000818:Animals; D005260:Female; D007070:Immunoglobulin A / Q000097:blood; D007411:Intestinal Diseases, Parasitic / Q000662:veterinary*; D015233:Models, Statistical; D009349:Nematode Infections / Q000662:veterinary*; D010270:Parasite Egg Count / Q000662:veterinary; D010641:Phenotype; D012756:Sheep; D012757:Sheep Diseases / Q000469:parasitology*
D007070:Immunoglobulin A
Egg count; Gastrointestinal nematodes; IgA; Prevalence; Sheep; ZINB