Heterogeneity in hosts of Avian Malaria

Hello and welcome back to another installment of DisEco Blog!

Since reading about Avian Malaria and all the interesting things that surround it, I decided to focus on the Plasmodium pathogen for today’s blog.

So, continuing on the last blog post dealing with SIR models, we learned about the dynamics of this pathogen and briefly explained some ecological factors affecting its transmission. Today I will refresh your memory on those ecological factors and also add in some interesting physiological factors to overall explain the topic of heterogeneity in host competence.

Since avian malaria has had a massive impact on the avian species, especially honeycreepers, of the Hawaiian islands, I will use that as an example. Essentially, transmission of malaria, susceptibility and its impact on the population depends on the species and the elevation of Hawaii. As it was previously mentioned in the last blog post, malaria infection is lowest at higher elevations of the islands where the climate is less favored by the mosquitoes. Thus, higher elevations provide a refuge for susceptible honeycreepers. However, elevation is not entirely the solution to the decline of native Hawaiian bird species as the movement of birds to lower elevations in search of food and upward movement by the mosquito has resulted in malaria infection in higher elevation populations (Ahumada et al. 2004).

Picture of native birds in Hawaii obtained from https://kauaiforestbirds.org/meet-the-birds-2/

Ahumada et al. 2004 also compared three species of birds from the Hawaiian islands, Apapane, Amakihi, and Iiwi, to compare avian malaria infections between them. They found that malaria mortality was greatest in the Iiwi, lower in the low-elevation Amakihi, and intermediate in mid and high elevation Amakihi and Apapane. They also found that the mosquitoes seemed to prefer feeding on the Iiwi over Apapane in high elevation forests. Overall, it seems that Amakihi and Apapane are least susceptible to avian malaria and are transmitter of the disease to the more susceptible Iiwi species. Poor Iiwi’s 😦

This same study explained that they noticed a pattern of high malaria infection or fatality among young birds, which shows how avian malaria has a substantial impact on young birds than on adults. More research is needed to explain why different species, and different aged birds in the Hawaiian islands have different prevalence of avian malaria.

However, other studies seem to have the answer to the physiological factor that affects the prevalence birds to avian malaria. The Sepil et al. study investigated avian malaria infection and analysis of the Mhc class I genes in the wild great tit population (pictured below).

Great Tit picture obtained from eBird.org

They wanted to understand the role that these genes might play in determining host susceptibility and resistance to the Plasmodium infections. Findings of the study included the presence of two Mhc supertypes which had a significant association with the chance of host infection with two species of Plasmodium. The results strongly supported that supertype 6 led to a quantitative resistance to P. circumflexum, and great tits carrying supertype 17 had greater resistance to P. relictum.

Important information: alleles that limit deleterious effects of infection, but do not prevent the infection confer quantitative resistance and this is essential with virulent parasites. On the contrary, qualitative resistance alleles prevent infection and are more beneficial to hosts that are exposed to benign parasites. This suggests that susceptibility to avian malaria by the host is dependent on the virulence of the Plasmodium species.

The presence of this mhc gene has been seen in many other passerines. Since it seems to increase susceptibility or resistance to avian malaria, it is believed that the mhc gene diversity in birds will continue since avian malaria is a common disease, and prevalence varies with space and time. With this, it is up to survival of the fittest, as in if birds have the gene that makes them resistant to the deadly pathogen, then they will survive and also reproduce, introducing the gene in to the population.

Genes and geographical location seem to be big factors in avian malaria spread. Birds with the proper genes (ex: mhc supertypes) , and birds living in areas where mosquitoes do not thrive, will have the greatest competency. The more birds that are infected with plasmodium in an area the greater the chance of mosquitoes spreading the disease. Thus, this disease tends to show aggregated distribution patterns geographically especially in areas like Africa, South and Central America, South Asia and the Caribbean. However, its become such a widespread disease that it found globally except Antarctica.

That is all for this week’s blog! Hope you enjoyed the information and remember to stay curious! 🙂

SOURCES:

Sepil I, Lachish S, Hinks AE, Sheldon BC. 2013 Mhc supertypes confer both qualitative and quantitative resistance to avian malaria infections in a wild bird population. Proc R Soc B 280: 20130134. http://dx.doi.org/10.1098/rspb.2013.0134.

Ahumada, J. A., D. LaPointe,and M. D. Samuel. 2004. Modeling the population dynamics of Culex quinquefasciatus (Diptera: Culicidae), along an elevational gradient in Hawaii. Journal of Medical Entomology 41: 1157–1170.