diffuse interstitial pneumonia. The alveolar interstices are infiltrated with histiocytes, eosinophils, and lymphocytes that form occasional eosinophilic granulomas (Figure 4).2

Comparison With Human Disease

Lymphatic dwelling filariae induce a range of manifestations that include: 1) asymptomatic microfilaremia, 2) filarial fevers with transient edema and recurrent adenolymphangitis, 3) persistent lymphedema and elephantiasis,

and 4) the tropical eosinophilia syndrome. It is considered that the disease manifestations are immunologically mediated and that asymptomatic, microfilaremic infections are associated with a suppressed or restricted immune reactivity.6 Ferrets infected with B. malayi have a similar spectrum of clinical features. Asymptomatic microfilaremia is observed but most ferrets develop manifestations associated with high immune responsiveness. The inflammatory reactions in the lymphatics and lymph nodes parallel the lesions reported in humans.7 The acute episodes of adenolymphangitis with fever, common in humans, are not obvious in the ferret but episodic lymphedema is present. Chronic lymphedema, a major medical problem in lymphatic filariasis,8 is induced by multiple infections in the ferret. Histopathologic changes are consistent with filarial lymphedema, although the extreme dermal hyperplasia of elephantiasis has not been noted.

Tropical filarial eosinophilia (TE) is considered to be a hyperresponsive syndrome associated with immune clearance of circulating microfilaria in the lung and other organs. The hallmarks are amicrofilaremia, elevated IgE levels, and high peripheral eosinophilia. The syndrome is often expressed by respiratory signs resembling asthma (tropical pulmonary eosinophilia).9 Early microscopic changes of human pulmonary disease include histiocyte infiltration of the interstitium that progresses to diffuse eosinophilic pneumonitis. Occasional eosinophilic abscesses are centered around microfilariae or their remnants.9 Less attention has been paid to lesions in other organs. In some patients, however, adenopathy and eosinophilia may be the main presenting features. Hepatomegaly and lymphadenopathy are especially common in children and these organs usually contain microfilariae, in the form of MK bodies, trapped within grossly visible eosino
Figure 2. Subcutaneous lymphatic vessel

with fibrotic walls and chronic inflammation (X100).

Figure 1. Persistent edema of tbe right hind paw in a ferret af ter multiple infections with B. malayi third-stage larvae.