It has been widely hypothesised that the acute death syndrome in D. immitis infected cats is due to a fatal anaphylactic reaction resulting from antigen (Ag) release during filarial death. However, there is virtually no objective research support for such a hypothesis. The aim of this thesis was therefore to document the physiological, haematological, pathological and biochemical changes that occur in D. immitis-sensitised cats when they are challenged intravenously with D. immitis Ag. Clinical comparisons were then made of the character and severity of the reaction when challenge was induced by D. immitis Ag, presented in various physical forms. In the second part of the experimental protocol, the reaction was further modified by pharmaceutical agents or mediator blockers, in an attempt to attenuate the reaction and describe its pathogenesis in more detail.
Young healthy cats were sensitised to D. immitis Ag over a six-week period by weekly subcutaneous injections of AlOH-adjuvanted D. immitis Ag. The course of vaccination produced significant increases in both D. immitis-specific serum IgG and IgEconcentrations. Five to eight weeks after the sensitisation period, general anaesthesia was induced and baseline measurements were made. Respiratory rate, blood O2 saturation, expired CO2, heart rate and blood pressure were measured and dyspnoea was scored subjectively. Blood was collected for serotonin, histamine and mast cell tryptase assays and complete blood count. Ag challenge was then achieved by the intravenous (IV) insertion of D. immitis, either in a whole filarial form or as soluble Ag in normal saline. A wide range of progressive measurements was then taken until apnoea or the experiment was terminated at
140 minutes after challenge. D. immitis-sensitised cats, sham-challenged with IV normal saline, were the source of control data.
The lungs were removed at necropsy and assessed for gross pathological changes. A bronchoalveolar lavage was then performed and fluid was submitted for cytological examination. Any pulmonary histopathological changes were then compared to those in the lungs of D. immitis-infected cats that had died naturally.
Ag challenge resulted in a combination of hypotension, dyspnoea, hypoxaemia and haemoconcentration. The clinical reaction was much less severe when a whole live filaria or an unbroken dead filaria was inserted intravenously for induction, than when a dead transected filaria was inserted or when soluble D. immitis Ag was administered.
The amount of gross pulmonary haemorrhage present at necropsy was highly variable and could
not be consistently correlated to the effects of Ag challenge. The main pulmonary histopathological findings included oedema, congestion, haemorrhage, bronchial and bronchiolar constriction and alveolar flooding. Alveolar rupture and bronchial and bronchiolar constriction were more prevalent where there was an acute, severe clinical reaction to Ag challenge, whereas pulmonary congestion and alveolar flooding were more prevalent in less severely affected groups. Severely affected cats had extremely high BAL fluid erythrocyte counts.
The severity of the Ag-induced shock reaction was not related to the concentration of D. immitis-specific serum IgG or IgE present when challenge took place. The reaction was characterised by the systemic release of serotonin but not histamine or mast cell tryptase, both proposed markers of mast cell degranulation in cats. In separate groups of cats, intramuscular adrenaline, IV prednisolone sodium succinate, IV
APC-1390 (a mast cell tryptase blocking drug) and pre-treatment IV with anti-Interleukin-5 (IL-5) antibody was used to modify the reaction to challenge by IV D. immitis Ag in saline. Mortality rate was significantly lower in the Control, Prednisolone and Anti-IL-5 groups than in the 20ng/mL Ag group.
While it appears that an acute and often fatal shock reaction did take place when the D. Immitis-sensitised cats underwent IV Ag challenge, there is no evidence to support the involvement of mast cells or their degranulation products. These findings in cats are similar to recent reports of rodent models of Ag-induced anaphylaxis, where acute reactions were inhibited by serotonin inhibition but mast cell degranulation and serum IgE were shown not to be involved.
In this feline D. immitis Ag model, serotonin is likely to be the primary vasoactive mediator driving the physiological,
haematological and pathological effects of the reaction. The most probable source of serotonin is the degranulation of circulating platelets, since platelets are an abundant source of this mediator and their presence in the circulation enables a rapid response to IV Ag challenge. A possible mechanism for this degranulation is via the activation of the complement cascade and the release of platelet activating factor. Serotonin is known to cause the Bezold-Jarisch depressor reflex, a combination of profound hypotension, bradycardia, apnoea and pulmonary hypertension similar to that demonstrated in this experimental model. Mouse models of Ag-induced anaphylaxis have demonstrated the accumulation of platelets in the lung and liver and it is hypothesised that platelets rather than mast cells may be important in Ag challenge in sensitised animals.
Further experiments are needed to fully describe the involvement of vasoactive mediators in this model. They could
include the blockade of serotonin, platelet activating factor, IgE and histamine in sensitised cats when challenged with IV D. immitis Ag. Once the Ag-induced shock reaction is more fully understood, these experiments could be followed by studies using cats naturally infected with D. immitis. Successful use of the proposed mediator blockers would then bring us closer to completely understanding the pathogenesis of the acute death syndrome in D. immitis-infected cats. It is most likely that this acute clinical reaction is not Type 1 hypersensitivity and that therapy to avoid the effects of elevating serotonin levels is indicated.
It is clear from this study that both parasite (eg. release of internal filarial antigens) and host factors (vasoactive mediator release) are important in the pathogenesis of the acute death syndrome and that alteration of the host response depends on manipulation of these