Study of dirofilariasis in central Taiwan: epidemiology and pathogenesis

• Main Authors: Cheng-Hung Lai, 賴政宏
• Other Authors: Jiunn-Shiow Wang
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/56491285285472670297

博士 === 國立中興大學 === 獸醫學系 === 90 === Dirofilaria immitis is one of the most important parasites of canine, which resides primarily in the pulmonary arteries and right ventricle and may severely affect canine health. Researches about dirofilariasis in Taiwan has been conducted in the northern area and demonstrated that the incidence of dirofilariasis is progressively increasing. However, no pertinent information was available in central Taiwan. This study first investigates the prevalence of dirofilariasis in stray dogs and mosquitoes from central Taiwan, the susceptibility of naturally infected mosquitoes to D. immitis and the effect of microfilarial density on the viability of mosquitoes. A total of 218 stray dogs (115 males and 103 females) were collected from central Taiwan and examined. The numbers of positive results from blood smear and autopsies were 72 and 132, respectively. The overall prevalence was 60.55%, 62.61% in males and 58.25% in females. The number of microfilariae in 10 mL blood and adult worms in the hearts of infected dogs ranged from 0 to 635 and from 1 to 49, respectively. These results indicated that the central Taiwan has became endemic area of dirofilariasis. In five species of mosquitoes collected by using light traps, only Culex quinquefasciatus and Aedes albopictus were infected with D. immitis; the prevalence was 4.28% and 3.74%, respectively, while Cx. tritaeniorhyn- chus, Cx. annulus and Armigeres subalbatus were not infected with D. immitis. The intensity of D. immitis found in Ae. albopictus (3.43 larvae/mosquito) surpassed that found in Cx. quinquefasciatus (2.89 larvae/mosquito). After being fed with canine blood containing 7500 microfilariae (mf) per ml, Cx. quinquefasciatus ingested approximately twice as many mf as Ae. albopictus (mean of 31.73 in comparison to 16.47). However, almost triple the number of third-stage infective larvae developed in Ae. albopictus compared to Cx. quinquefasciatus (mean of 3.25 as compared with 1.10), with a vector efficiency index (VEI) of 19.73 and 3.47, respectively. Ae. albopictus and Cx. quinquefasciatus were fed canine blood with different microfilarial density of D. immitis ranging from 2500 to 25,000 microfilariae/mL. Larval development in these two mosquito species did not differ significantly. Although Cx. quinquefasciatus ingested more microfilariae, the number of larvae which developed in Ae. albopictus invariably exceeded that in Cx. quinquefasciatus. Mortality of the engorged Ae. albopictus was significantly greater than that of Cx. quinquefasciatus, and higher microfilarial density elevated the mortality in both species. Although the vector efficiency index of Ae. albopictus exceeded that of Cx. quinquefasciatus at all microfilarial densities, its survival time was markedly reduced. These results indicated that Cx. quinquefasciatus and Ae. albopictus serves as natural vectors of D. immitis in central Taiwan. Although Ae. albopictus was more efficient for heartworm transmission, the investigation found after considering the vector population and feeding habits that Cx. quinquefasciatus may play a more prominent role in transmitting dirofilariasis in central Taiwan. Finally, dogs with low microfilarial density are implicated as the main source for transmitting D. immitis from dogs to mosquitoes. Heartworm infection may cause dilatation of the heart and hypertrophy of the endocardium. Morphologically, the heart became round and the thickness of the ventricle decreased. Microscopic examination revealed cardiocytic degeneration and verrucous endocarditis. The heart consists of cardiocytes and the interstitial extracellular matrix (ECM). The ECM consists mainly of type I and type III collagens in the form of fibrils that preserve tissue architecture and chamber geometry. The present study revealed that the ECM content of heart were reduced in heartworm-infected dogs and became fragmented and dissociated by employing the silver impregnation stain. Normal and heartworm infected dogs displayed a considerable difference in quantity of collagen. The matrix metalloproteinases (MMPs), also called matrixins, are considered to play a central role in the breakdown of ECM. An obvious escalation of gelatinolytic activity of 72 (MMP-2) and 92 kDa (MMP-9) were noted in both right and left ventricle in the gelatin-substrate zymography of heartworm-infected dogs, and the enzymatic intensity of the right ventricle exceeded that of the left ventricle. These enzymes were Ca2+-dependent, the optimal pH for these enzymes ranged from 6 to 9, and the presence of 1, 10-phenanthroline completely inhibited their activity. These results indicate that heartworm infection could up-regulate the expression of MMP-2 and MMP-9, and then result in the remodeling of ECM and alter the architecture and function of the heart. Thus, regulation of myocardial MMPs expression may be an important therapeutic target to control myocardial matrix remodeling in the setting of dirofilariasis.