Experimental encephalitozoonosis in the blue fox. Neonatal exposure to the parasite.

Newborn and young pups up to the age of 15 days were exposed to E. cuniculi, either by keeping the pups in cages together with orally inoculated foster-mothers and their offspring, or by oral inoculation with E. cuniculi spores. A majority of pups appeared sero-positive to E. cuniculi with the india-ink immuno-reaction from 35 to 87 days post exposure; spores of E. cuniculi were detected in organs of some of the animals. The non-inoculated pups kept together with the orally inoculated pups became seropositive from 49 to 129 days after the oral inoculations. However, the exposure of newborn and young pups failed to induce clinical encephalitozoo-nosis, and when killed at the time of pelting the body weights and fur quality appeared to be within the normal range in all exposed foxes. No macroscopic lesions were detected in the various organs. Histologically focal interstitial nephritis occurred in the great majority of the seropositive animals. Meningoencephalitis was seen in some of the foxes, whereas slightly thickened walls of some arteries, mainly in the myocardium, were found in a few animals. The lesions of the brain and kidneys seem to be very similar to those seen in chronic cases of rabbit encephalitozoonosis. Polyarteritis nodosa and severe encephalitis and interstitial nephritis with extensive proliferations of plasma cells, which are almost constant findings in cases of clinically diseased foxes, were not detected in any of the subclinically infected animals. Various factors that might be of significance in the pathogenesis of the disease are discussed, and it is concluded that intrauterine infection of the pups via the transplacental route appears to be an essential supposition for the establishment of clinical fox encephalitozoonosis.