Clinical recovery of Macaca fascicularis infected with Plasmodium knowlesi
Authors: Peterson Mariko S. , Joyner Chester J. , Brady Jessica A. , Wood Jennifer S. , Cabrera-Mora Monica , Saney Celia L. , Fonseca Luis L. , Cheng Wayne T. , Jiang Jianlin , Lapp Stacey A. , Soderberg Stephanie R. , Nural Mustafa V. , Humphrey Jay C. , Hankus Allison , Machiah Deepa , Karpuzoglu Ebru , DeBarry Jeremy D. ,   MaHPIC-Consortium, Anderson Dave C. , Ay Ferhat , Brito Cristiana F. A. , Barnwell John W. , DeBarry Megan , Bosinger Steven E. , Chien Jung-Ting , Choi Jinho , Gupta Anuj , Ibegbu Chris , Jiang Xuntian , Jones Dean P. , Lackman Nicolas , Lamb Tracey J. , Lee Frances E. -H. , Le Roche Karine Gaelle , Li Shuzhao , Meyer Esmeralda V. S. , Moncada-Giraldo Diego M. , Ory Dan , Pohl Jan , Safaei Saeid , Sanz Igñacio , Smith Maren , Tharp Gregory , Tran ViLinh , Trippe Elizabeth D. , Uppal Karan , Warrenfeltz Susanne , Williams Tyrone , Woods Zerotti L. , Tirouvanziam Rabindra , Kissinger Jessica C. , Moreno Alberto , Gumber Sanjeev , Voit Eberhard O. , Gutiérrez Juan B. , Cordy Regina Joice , Galinski Mary R. 
Published: December 2021

Abstract
Background
Kra monkeys (Macaca fascicularis), a natural host of Plasmodium knowlesi, control parasitaemia caused by this parasite species and escape death without treatment. Knowledge of the disease progression and resilience in kra monkeys will aid the effective use of this species to study mechanisms of resilience to malaria. This longitudinal study aimed to define clinical, physiological and pathological changes in kra monkeys infected with P. knowlesi, which could explain their resilient phenotype.

Methods
Kra monkeys (n = 15, male, young adults) were infected intravenously with cryopreserved P. knowlesi sporozoites and the resulting parasitaemias were monitored daily. Complete blood counts, reticulocyte counts, blood chemistry and physiological telemetry data (n = 7) were acquired as described prior to infection to establish baseline values and then daily after inoculation for up to 50 days. Bone marrow aspirates, plasma samples, and 22 tissue samples were collected at specific time points to evaluate longitudinal clinical, physiological and pathological effects of P. knowlesi infections during acute and chronic infections.

Results
As expected, the kra monkeys controlled acute infections and remained with low-level, persistent parasitaemias without anti-malarial intervention. Unexpectedly, early in the infection, fevers developed, which ultimately returned to baseline, as well as mild to moderate thrombocytopenia, and moderate to severe anaemia. Mathematical modelling and the reticulocyte production index indicated that the anaemia was largely due to the removal of uninfected erythrocytes and not impaired production of erythrocytes. Mild tissue damage was observed, and tissue parasite load was associated with tissue damage even though parasite accumulation in the tissues was generally low.

Conclusions
Kra monkeys experimentally infected with P. knowlesi sporozoites presented with multiple clinical signs of malaria that varied in severity among individuals. Overall, the animals shared common mechanisms of resilience characterized by controlling parasitaemia 3–5 days after patency, and controlling fever, coupled with physiological and bone marrow responses to compensate for anaemia. Together, these responses likely minimized tissue damage while supporting the establishment of chronic infections, which may be important for transmission in natural endemic settings. These results provide new foundational insights into malaria pathogenesis and resilience in kra monkeys, which may improve understanding of human infections.