CHICKEN COCCIDIOSIS: A FRESH LOOK AT LESIONS
R. B. Williams
Coxitec Consulting, Hertfordshire, UK
E-mail address: ray.coxitec@tesco.net
Macroscopic coccidial lesions in
chickens are usually assessed by the subjective scoring systems developed by
Johnson & Reid (1970). Johnson & Reid’s system was developed using naïve
birds; the grades correlate negatively with weight gain or FCR. In immune birds
(drug-treated or vaccinated) lesion scoring can be misleading because mild
lesions may sometimes occur in birds with normal weight gain or FCR. Furthermore,
any lesions in vaccinated birds that are clinically protected frequently contain
few or no observable parasites, whereas those in naïve birds are packed with
parasites.
Association of specific pathogens with a parasitic lesion is not, by definition,
a necessity, but may help with lesion identification. The similarities in gross
appearance of lesions and the differences between associated parasite burdens in
naïve and immune birds suggest that coccidial lesions are due more to host
responses than to the parasite itself. The lesions sometimes present in immune
birds in the field are presumably the result of successful repulsion by the host
of a parasite challenge. What, then, accounts for the gross appearance of
lesions; and are there fundamental differences between those occurring in naïve
and immune birds?
Perhaps in naïve birds, white plaques caused by Eimeria acervulina or E.
necatrix are due to endogenous parasites (schizonts, gametocytes or oocysts),
fibrin and/or invading leucocytes. Red lesions (E. brunetti, E. maxima, E.
necatrix, E. tenella) may be due to haemorrhage resulting from rupture of
capillaries or diapedesis. But in immune birds with lesions superficially
similar to those in naïve birds, immunological responses alone may be
responsible for the white lesions, and inflammation (vasodilatation, hyperaemia,
congestion) may account for the red lesions. But why do E. mitis and E. praecox
not cause gross lesions? Explanations might be provided by histological and
immunological techniques.
A few commercially-vaccinated birds may develop lesions directly resulting from
vaccination, but attenuated vaccines do not harm performance. Lesions may also
result later from natural field challenge in some vaccinated birds, again
without affecting their performance. Clearly, lesion scoring alone under-rates
vaccine efficacy, and should not be used to judge the immune status of birds,
but do lesions in vaccinated birds damage the gut epithelium? Direct
microscopical examination under physiological saline of the fresh intestinal
mucosa of affected birds facilitates immediate clinical assessment of their gut
integrity, and might provide the answer.