954 
ANIMAL DISEASES 
Figure 5. — Portion of a nearly healed granuloma of 
canine histoplasmosis with central caseation and min- 
eralization. HE X 32. 
RESPIRATORY DISEASES OF CALVES 
Calf Pneumonia 
The most important subclinical disease of 
calves is pneumonia,^^ Data on the incidence of 
calf pneumonia are lacking, although the im- 
pression gained from observations in slaughter 
houses and autopsy laboratories is that subclini- 
cal, chronic pneumonia occurs in the majority 
of calves from one week to six months of age. 
The lesions occur in the dependent portions 
of the apical, cardiac, intermediate, and ante- 
rior diaphragmatic lobes, affecting from a few 
lobules up to one-fourth of the lung (Figure 7) . 
The affected areas are dark red and sharply de- 
marcated from the adjacent normal-appearing 
lung. Microscopically the lesions are character- 
ized by peribronchial and perivascular accumu- 
lations of lymphoid cells and atelectasis (Figure 
8) with varying admixtures of purulent bron- 
cho-pneumonia. 
The causes of calf pneumonia and whether it 
represents more than one disease are still un- 
certain. The lesions are thought to be caused by 
a virus and secondary bacteria. Parainfluenza 
or adenoviruses are the most frequently incrim- 
inated viruses although they are infrequently 
isolated from the lesions of chronically diseased 
calves. Pasteurella multocida is most often iso- 
lated and Corny ebacterium pyogenes is not un- 
common. The role of chlamydial organisms and 
mycoplasma as etiologic agents is still uncer- 
tain. 
Tt is not clear whether bacterial pneumonias 
occur independently in calves or whether they 
represent complications of viral pneumonia. Oc- 
casionally severe suppurative bronchopneu- 
monia occurs with a paracardial distribution in 
the dependent lobes similar to that of viral 
pneumonia. The lesions in calves with subclini- 
cal disease are somtimes accompanied by evi- 
FiGURE 6. — Canine dirofilariasis. Branches of the pul- 
monary artery containing dead nematodes exhibit in- 
timal hyperplasia and medial hypertrophy. HE x 32. 
