Seasonal changes in density and tissue distribution of Onchocerca cervicalis microfilariae in ponies and related changes in Culicoides variipennis populations in Louisiana.

This research investigates how the population density and tissue distribution of the parasite ‘Onchocerca cervicalis‘ vary seasonally in ponies. It further examines the correlation of these changes with populations of ‘Culicoides variipennis’, a known insect vector for this parasite.

Research Methods

• Onchocerca cervicalis microfilariae, the larval form of a type of parasitic roundworm, were studied in the skin of 15 infected pony mares over a period of 13 months in south Louisiana.
• The researchers used a statistical tool known as the triple running mean analysis to ascertain patterns in microfilariae population density and their occurrence in various levels of the dermis, the thick layer of living tissue below the skin’s surface.
• They also monitored the population density of Culicoides variipennis, an insect that has been proven to transmit O. cervicalis, to detect any possible correlation with seasonal variations in the microfilariae population.

Findings

• The study found a distinct variation in microfilariae density and distribution in the dermis across different seasons. Peak levels of density were noted in the spring, followed by a significant drop of 58% in the summer, a slight recovery of 19% in the fall, and then another drop to the lowest levels in winter.
• Microfilariae were found to occupy all layers of the skin (dermis) during the spring, summer, and fall, but were absent from the superficial (top) layers during winter.
• The study also observed that the density of Culicoides variipennis, the vector for O. cervicalis, fluctuated seasonally, mirroring the changes in the microfilariae density.
• Nevertheless, the density of microfilariae in individual ponies did not always align with the overall population trend, as revealed by the harmonic wave analysis of the data.

Implications

• These findings help illuminate the dynamic nature of O. cervicalis microfilariae population in infected ponies. The results could be useful in predicting the periods of heightened risk of infection and planning effective parasite control strategies accordingly.
• The seasonal mirroring of population density between the parasites and their insect vector suggests a potential relationship, however the causality and the nature of this possible interaction warrant further investigation.
• Finally, the variance in individual subject responses underscores the need for careful consideration of individual health and resistance factors when diagnosing or treating O. cervicalis infections in ponies.