Enzootic Plague and the Great Gerbil of Central Asia

The Great Gerbil, Rhombomys opimus. Range from Afghanistan to China. (photo source: Wikipedia Commons)

Meet the Great Gerbil

The Great Gerbil of Central Asia is not much like the little gerbils found in American pet stores. This bad boy can get as long as 13 inches head to tail, about the size of a  prairie dog or large black rat. It holds a similar ecological niche as the prairie dog as well.

Great gerbils live in family units within large burrow systems across the Central Asian arid and semi-arid regions.Their burrows go 1.5-2 meters deep to take advantage of the warmth and moisture in deep soil. Although they live in semi-arid regions, they also survive winters under snow pack without hibernation. Great gerbils are found from the Caspian Sea to north-eastern China and have been observed in Pakistan, Afghanistan, Iran, Mongolia, and southern Russia [1, 2]. The great gerbil and its fleas are a major reservoir for Yersinia pestis in the natural foci of plague across central Asia [1]. It has long been hypothesized that coevolution of the great gerbil and Yersinia pestis has shaped both species [1]

Immunology of a Plague Reservoir

Junggar Basin, NW China (click to enlarge)

A Chinese group led by Ruifu Yang and Hanli Cao are the first to examine the immunological response of great gerbils from natural plague foci to a Yersinia pestis challenge [1]. They isolated Y. pestis and collected great gerbils from the Junggar basin of northern western China [1]. The Junggar basin (Dzungaria) area was once part of West Mongolia. Previous studies have shown that great gerbils from this region have an average plague antibody incidence of 10% with pockets reaching 30-70% [1]. They collected great gerbils from the Junggar Basin that were proven to be free of Y. pestis by a six month quarantine  in the lab and repeated negative antibody and antigen screening [1].

To mimic a flea bite, the gerbils were challenged with Y. pestis through subcutaneous injections in the groin. The infections were monitored by anal temperature, body posture, animal weight, and changes in diet, fur and respiration. Y. pestis was isolated from dead animals and all animals were assessed for liver and spleen abscesses.  They used 90 great gerbils to determine effects of Y. pestis dosage and time; guinea pigs were used as controls at a lower dosage for the liver and spleen assessment.

Anatomical assessment of the liver and spleen of the great gerbil [A] and a control guinea pig [B]. Y. pestis abscesses are clearly visible on the liver and spleen of the guinea pig [B]. Zhang et al (2012) doi:10.1371/journal.pone.0046820.g002

Infection was only observed in the great gerbil at higher infection dosages (>7.4 x 10(9) CFU), with no difference observed based on gender or age [1]. Their temperatures climbed for three days, returning to normal within seven days and their body weight returning to normal within fifteen days. Blood work showed only an elevated white blood cell count but otherwise remained stable [1]. As shown in the photos above, the great gerbil did not develop visible abscesses on the liver and spleen as the control guinea pig did or as other studies have indicated in mice.

Most of the great gerbils were used for a 21 day Y. pestis challenge. The results were remarkable, an infection rate of 81% but a mortality rate of only 12.7% [1].  Differences in susceptibility is an important factor in the great gerbil’s role in the natural plague focus. Not surprisingly the bacterial load in the liver and spleen of infected animals correlated with survival outcomes. Y. pestis was isolated from spleen and liver as early as one to two days post infection suggesting that it does become systemic early, as indicated previously in mice.  Anti-F1 antibodies (to the F1 capsule protein of Y. pestis) were detected between the second and third week producing an seroconversion rate of 85.7% [1]

What these high percentages don’t show clearly is the observed high variability between individuals in both infection rate in each time cohort and antibody response [1] . In a couple of live gerbils, a low bacterial load of Y. pestis was isolated on day 14 and 15 post infection [1].  It is possible that low-level infections can persist for longer than imagined.  Variability in reservoir host susceptibility is necessary to keep Y. pestis viable in the environment for years. This study increases the evidence that the great gerbil is the key enzootic reservoir for natural plague foci across central Asia.

References

1. Zhang Y, Dai X, Wang X, Maituohuti A, Cui Y, Rehemu A, Wang Q, Meng W, Luo T, Guo R, Li B, Abudurexiti A, Song Y, Yang R, & Cao H (2012). Dynamics of Yersinia pestis and Its Antibody Response in Great Gerbils (Rhombomys opimus) by Subcutaneous Infection. PloS one, 7 (10) PMID: 23071647
2. Nannizzi, M. 2002. “Rhombomys opimus” (On-line), Animal Diversity Web.  University of Michigan Museum of Zoology. Accessed October 20, 2012 at http://animaldiversity.ummz.umich.edu/accounts/Rhombomys_opimus/

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