ПАРАЗИТОЛОГИЯ, 2021, том 55, № 1, с. 3-11.

УДК 576.895

PROFESSOR YURI SERGEEVICH BALASHOV:

LEGACY OF AN OUTSTANDING PARASITOLOGIST

^aGamaleya National Centre for Epidemiology and Microbiology,

Moscow, 123098 Russia

^bTick-radar GmbH, 10555 Berlin, Germany

*e-mail: edkorenberg@yandex.ru

Received 16.10.2020

Received in revised form 30.10.2020

Accepted 02.11.2020

This paper is devoted to the contributions of the outstanding parasitologist Yuri S. Balashov.

It is not biographical. Its purpose is to highlight selected aspects of Yuri S. Balashov’s scientific

legacy that the authors consider most important for an international readership. This should direct

the attention of international researchers to his various scientific theses and concepts, some of which

have been only published in Russian language, and should also open up a broader discussion. The

scope of Balashov’s scientific interests was remarkable, although he devoted special attention to the

research of argasid and ixodid ticks (Argasidae and Ixodidae). The main results and conclusions

of his tremendous work have been described here using mainly his own words. Only some explana-

tions that may be required to increase the reader’s understanding of the concepts, terms and thoughts

of Yu. Balashov were made by the authors.

Keywords: parasitology, medical entomology, ecology, Argasidae, Ixodidae, ticks, natural foci

DOI: 10.31857/S0031184721010014

The life of Yuri Sergeevich Balashov (1931-2012), an outstanding parasitologist, has

been described in special publications (Filippova, 2013; Medvedev, 2012; Medvedev et al.,

2013). This paper is not biographical. Its purpose is to highlight the aspects of Balashov’s

scientific legacy that we consider most important. The scope of his scientific interests was

remarkably broad, although he devoted special attention to the research of argasid and ixodid

ticks (Argasidae and Ixodidae). Balashov was a prominent representative of the parasitol-

ogy school founded by the Academician Yevgeny N. Pavlovsky, who developed the theory

of natural focality of transmissible diseases (Pavlovsky, 1939; 1966) and whose ideas he

expounded and further developed (Balashov, 1967; 1984a; 2003; Balashov et al., 1968). He

greatly appreciated the biocenological and comparative parasitological concepts by Vladimir

N. Beklemishev (1970) about parasitic systems and the life schemes of species, which he

considered to be of high significance for the development of parasitology. This follows from

Balashov’s paper (1991) commemorating the centennial of Beklemishev’s birth, where he

expressed an idea that may stand for his “scientific signature” and serves as an epigraph

to all his scientific activities: “It would be advantageous for parasitology to further develop

the universal concept of parasitic systems that reflects the unity of all parasitic organisms

as an ecological category. This concept stimulates the exchange of ideas between traditional

parasitology and microbiology and creates prerequisites for developing our knowledge of

the general principles of parasitism as a form of existence of living organisms” (Balashov,

1991, p. 189). It is in the light of such a broad general parasitological concept that Balashov

analyzed crucial problems of medical entomology, the scientific branch treating medically

significant arthropods, including the parasite-host relationships of arthropods and terrestrial

vertebrates. He authored more than 200 scientific papers and several monographs (Balashov,

1967; 1972a; 1979; 1982; 1983; 1998; 2009). The publication of each of his books was

a major event in parasitology, and many of them are quite comprehensive and have been

milestones. However, it is the reading of these publications in a continuous chronological

sequence that gives a holistic impression of Balashov’s scientific legacy, demonstrating

long-term consistency in his progression from particular studies to multifaceted generaliza-

tions in evolutionary parasitology. He did not abandon a given problem after addressing

another one. They remained within the scope of his interests, and their range was growing

broader and more complex over time. Here we attempt to recapitulate the results of this

process, which in itself is fairly instructive, especially for young researchers working in an

era where we emphasize research on the molecular and genetic level and less on the func-

tion of whole organisms, populations and ecosystems. The main results and conclusions

of Balashov’s tremendous work will be described here using mainly his own words. Any

explanatory remarks or comments will be avoided where possible since they appear inap-

propriate in this context, although some of Balashov’s views and postulates are obviously

open to debate (Korenberg, 1999; 2010). Explanations that may be required to increase the

reader’s understanding of the concepts, terms and thoughts of Yu. Balashov will be made

by the authors in footnotes or in square brackets.

Biology and physiology of ticks

In the 1950s, Balashov initiated studies on elementary features of the biology and physi-

ology of ixodid ticks (poorly known at that time), which was the basis for all his future

research. The scope of the problems he addressed included the structure of tick mouthparts;

the process of bloodsucking and changes in the integument during this process; the functions

of dermal glands and adaptations to consuming large volumes of blood; the tick life cycle

and changes in tick body weight in the course of bloodsucking and daily rhythmicity of tick

detachment from the host after feeding; the structure of digestive organs as related to the

process of digestion; excretory processes; gonotrophic relationships and spermatogenesis;

and specific anatomical and physiological features of molting. The results of these studies

provided a basis for the widely known monograph (Balashov, 1967) translated into English

and published by the Entomological Society of America (Balashov, 1972a). This amazing

monograph brought Balashov international recognition and became a desk book for sev-

eral generations of medical entomologists worldwide. It is still an essential source for tick

researchers today containing the precious results of countless studies in the laboratory and

in the field. A special feature of his work is that it covered quite a large variety of argasid

and ixodid species occurring in a huge area, in different climatic and biogeographical zones,

the former USSR. He resumed, after about 20 years, in-depth studies on the structure of

tick organs at an advanced methodological level. Together with his colleagues L. Amosova,

V. Ivanov, S. Leonovich, and A. Reichel, he produced a series of publications on this

matter, including the unique Atlas of Electron-Microscopic Anatomy of Ixodid Ticks (Balas-

hov, 1979), which provided insight into specific features of different organs and mechanisms

of their functioning in these arthropods. In one way or another, the results of these studies

were also considered in Balashov’s subsequent monographs (1982; 1998; 2009) as a factual

basis for his theoretical generalizations. In particular, they allowed the conclusion that “the

main difference [of digestion in argasid and ixodid ticks] to that in insects is that the host

blood is digested mainly intracellularly, a process progressing slowly and asynchronously

in different parts of the gut” (Balashov, 1999, p. 758). [He mentioned in an earlier publica-

tion (1972a, p. 272) that intracellular blood digestion in ticks was first described by Roesler

(1934)]. This is also a highly relevant aspect when looking at ticks as vectors of pathogens.

Ixodid ticks take one blood meal in each postembryonic life stage. In contrast, argasid

ticks have 2-4 nymphal instars with each taking a blood meal and also the adults feed several

times. After feeding ticks pass through a series of different developmental phases, molting

into a new phase after every next bloodmeal, or in case of the adult female sooner or later

follower by oviposits. As a result, by feeding ticks in the laboratory Balashov was not only

able to follow their complete life cycles but also to investigate all the various aspects of

their biology in very different physiological states, an ideal basis for his comprehensive

biological approach.

Beginning in the late 1950s, Balashov’s attention was consistently attracted to problems

of the ecology of ticks including their mobility, periodization of life cycles, and the effect

of environmental factors on their abundance. The results of his first studies on the dynamics

of reserve nutrients and the physiological age of activated unfed ticks provided a stimu-

lus for research on improving methods for physiological age determination and analyzing

population age structure as a whole, which was carried out by Balashov himself and other

specialists for many years. Based on the analysis of specific ecological features and genetic

differences between natural populations of Ornithodoros tartakovskyi ticks, Balashov noted

that the results “...make one refuse to acknowledge that gene flow plays a significant role

in the maintenance of species unity”. Taking into account the low mobility of this tick and

its hosts, he concluded that “elementary populations¹ occupy a space confined to a group of

host burrows and may be largely isolated from neighboring populations located only a few

hundred meters away”. “Various landscape-geographic barriers creating unsuitable conditions

for ticks break up the species range into a number of population groups in isolation from

each other” (Balashov; 1971, p. 1800). He was the first to denote the hierarchy of units

in the spatial population structure of certain tick species and to indicate the real landscape

features (shown in maps) allowing these units to be distinguished.

Parasite hosts as ecological niches

In this regard, it is quite logical that, from the late 1970s to the early 1980s, the need

arose for Balashov to contemplate the diverse relationships between bloodsucking arthropods

and their hosts. Developing Beklemishev’s (1970) ideas on the evolution of parasite-host

relationships and types of life schemes in bloodsucking and parasitic arthropods further,

Balashov proposed his own concept of the origin and development of parasitism in different

groups of arthropods, which we refer to as the evolutionary polyphyletic concept. Its essence

¹ An elementary population (or core population) is an element of the spatial structure of tick popu-

lations. This is an area with a higher number of ticks than the area that surrounds it. The population as

a whole may have more than one core.

is that “... a long co-existence of arthropods and terrestrial vertebrates implies the possibility

of emergence and dissolution of various forms of symbiotic relationships between them,

including parasitism. The diversity of types of parasitism in insects and mites associated with

its repeated and independent appearance. In particular therefore, blood sucking occurred in

different families independently…” (Balashov, 2006a, p. 420; 2006b, p. 929). Based on this

premise, Balashov (1982) proposed his “classification system of the types of parasitism in

arthropods”, which he continued to improve conceptually and terminologically over almost

40 years. In the final variant, this system included seven groups with different types of parasit-

ism: (1) micro predators (free-living bloodsuckers), (2) nest-burrow ectoparasites with short

feeding(s), (3) temporary ectoparasites with long feeding(s), (4) permanent ectoparasites,

(5) intracutaneous parasites, (6) caviar parasites [egg or aviary], and (7) tissue parasites

(Balashov, 2009, p. 33). He emphasized that “almost all conceivable instances of transitions

from predation and schizophagy²to facultative ectoparasitism and from ectoparasitism to

endoparasitism have occurred among arthropods parasitizing terrestrial vertebrates.” Thus,

there are no distinct boundaries between the different types of parasitism.

The term “type of parasitism” is regarded synonymous to the terms “type of life scheme”

(Balashov, 1991, p. 190) and “life form” or “ecological group” (Balashov, 2006b, p. 930).

At the same time, “it should be remembered that as ecological categories, these terms can

combine species from phylogenetically unrelated taxa. Similar life patterns in many cases

arose in the process of parallel evolution, although in some taxa comparative parasitological

series may coincide with phylogenetics” (Balashov, 1991, p. 190). According to his ideas

“... the concept of the type of parasitism is in no way identical to the concept of ecological

niche³, but much wider” (Balashov, 2002, p. 935), since each part of the host organism can

be an ecological niche, which is mastered by certain parasites (Balashov, 2005, p. 444),

and “the host organism is a set of many ecological niches for different species of parasites”

(Balashov, 2009, pp. 227-228).

Evolution of hematophagy

Analyzing the relationships of bloodsucking arthropods with vertebrates, Balashov ar-

rived at the conclusion that “hematophagy has evolved independently and asynchronously in

several arthropod orders, families, and even genera” (Balashov, 1999, p. 759), “...but once

emerged, it has largely determined subsequent directions in the evolution of bloodsuckers

and particularly their co-evolution with vertebrate hosts” (pp. 751-752). “For the occurrence

of hematophagy [among arthropods], the most important representatives had morphophysi-

ological preadaptations to blood nutrition” (p. 758). “The necessity of piercing the skin of

vertebrates and sucking the blood caused the convergent development of the oral apparatus

of the piercing-sucking or cutting-sucking type” (p. 753). “Hematophagy in ixodid ticks

could have arisen as early as 150-200 million years ago during the Triassic; in some groups

of insects (lice, biting midges, fleas), during the Jurassic (145-200 million years ago); and

in mosquitoes and flies (horseflies in particular), only during the Paleogene” (23-66 million

² Schizophagy: the ability to eat decaying animal or vegetable residues or suck out the contents of

recently died arthropods, etc.

³ In ecology, the concept of “ecological niche” is a set of all environmental factors that determine

the possibility of the existence of a given species in nature. According to Balashov (2009, p. 351), “eco-

logical niche determines the place and role of a parasite in the community. It includes parameters of the

parasite’s habitats, its interactions with the host organisms, and the external environment”.

years ago) (p. 751). According to Balashov (2009, p. 347) the concept of parasitism means

“permanent or temporary cohabitation of different species, in which one of them uses an-

other species (host) as a source of food or habitat”. Moreover, “the categories of parasitism

and hematophagy applied to Insecta and Acarina do not always coincide” (Balashov, 2009,

p. 210), because “hematophagy is not necessarily associated with parasitism. Among Diptera

it [hematophagy] is peculiar to many groups actively attacking vertebrates but not living

on their body. In turn, not all parasitic arthropods are hematophagous and many feed on

wool, feathers, skin particles, secretions of skin glands and other parts of the host organ-

ism” (Balashov, 1999, p. 761). “Parasitism apparently evolved repeatedly and independently

in different groups of the Acarina, the initial stages of transition to parasitism progressing

in the dwellings of prospective hosts” (Balashov, 2000, p. 937). In Balashov’s opinion,

free-living bloodsuckers are not “fully functional components of parasite communities”⁴

(Balashov, 2002, p. 936). He particularized the vague notion of “parasite specificity” and,

in addition to conventional host specificity, introduced new concepts of phylogenetic and

ecological specificity, substantiating them as follows: “The specificity of parasites in the

choice of hosts may be conditioned both by affiliation of the latter with certain taxonomic

groups (phylogenetic specificity) and by ecological factors, in cases where a parasite can

live on unrelated host species inhabiting the same biotopes or occupying similar ecological

niches⁵(ecological specificity)” (Balashov, 2001, p. 475). These considerations led Balashov

to analyze the origins of parasitic and bloodsucking arthropods and their coevolution with

host species, which explained almost any of their morphological and biological features as

well as the underlying speciation processes. This problem was analyzed in most detail for

the group of ixodid ticks. In Balashov’s opinion, “proixodids might have initially fed on

various arthropods or their dead bodies and then shifted to hematophagy on vertebrates”

(Balashov, 2006a, p. 416). “Paleontological and zoogeographic data provide evidence for the

Mesozoic origin of ixodid ticks” (Balashov, 1989, p. 457). “The common ancestral group of

the Argasidae and the Ixodidae must have existed long before the Cretaceous”, in the Early

Mesozoic, and “...became segregated into an individual evolutionary branch in the Triassic”

and “...transition to parasitism in the common ancestors of the recent Ixodidae and Argasidae

could have occurred during the Late Paleozoic or Early Mesozoic (about 200-250 million

years ago), under tropical climate conditions” (Balashov, 2004, pp. 913, 915, 919). “The

parasitic relationships of the ancestors of the Ixodidae with mammals formed by the Middle

Mesozoic or even earlier” (Balashov, 1989, p. 457). “At an early stage of its evolutionary

development, after transition to hematophagy, the proixodoid lineage divided into two main

branches: the ancestors of the Argasidae and the Ixodidae” (Balashov, 1999, p. 750). “Two

recent genera, Carios and Aponomma⁶,already existed as early as in the Cretaceous (65-100

million years ago), and species of the genera Amblyomma, Ixodes, Hyalomma, and Orni-

thodoros appeared during the Eocene (30-40 million years ago)” (Balashov, 2004, p. 913).

Thus, Balashov considered that the group of ixodid ticks had formed prior to the breakup

of Pangaea into Gondwana and Laurasia. Judging from specimens found in Paleogene

ambers, the formation of the genera Amblyomma and Ixodes also occurred before this

⁴ Balashov (2009, p. 347) called the term “community of parasites” (parasitocenosis) as a set of

parasitic individuals of various species living on one host individual.

⁵ See note 3.

⁶ Carios is an argasid genus or subgenus (depending on the taxonomic concept). Some species of

the former ixodid genus Aponomma have been placed into the genus Amblyomma, and the remaining

Australian Aponomma species, into the genus Bothriocroton (Klompen et al., 2002).

breakup, no less than 180-190 million years ago. “The genera Haemaphysalis and Derma-

centor apparently evolved after the breakup of the single landmass into the northern and

southern halves, with the former genus being formed under conditions of moist subtropical

climate and forests of Southeastern Asia, and the latter in a temperate climate of steppe or

mountain landscapes. The genera Rhipicephalus and Hyalomma appear to be younger, with

the former having evolved in Africa during the period of its isolation in the Paleogene and

the latter in the deserts of Western Asia prior to the formation of its land connections with

Africa and the Arabian Peninsula” (Balashov, 1993, p. 935). Polyphagy⁷ and oligophagy of

ixodids have caused limited or no phylogenetic parallelism with hosts in their evolution.

“Restrictions in the distribution of some species are usually explained by the direct effect

of unfavorable environmental factors on the non-parasitic phases rather than by the absence

of suitable hosts” (Balashov, 1989, p. 457). The analysis of parasite-host relations in other

arthropod groups eventually led Balashov to an extremely important conclusion, which

is formulated as follows: “Сoevolution in its pure form is peculiar only in a few taxa of

parasitic arthropods” and, therefore, “... it has been an important, but not the only way of

their speciation” (Balashov, 2009, p. 230).

Reproductive isolation of ticks

A few of Balashov’s publications are devoted to the problem of interspecific hybridization

and reproductive isolation in some ixodid tick species (Balashov et al., 1998), still a very

relevant topic. It has been established experimentally that hybridization can happen between

closely related species of ticks. In particular, hybrid female ticks of the ricinus/persulcatus

group of the first generation (F1) oviposited after engorgement, but the next generation

(F2) did not produce fertile offspring. According to Balashov (1998), there is reproductive

isolation, which is due to the genetic incompatibility of ticks of different species.

Natural focality of diseases

The implications drawn from all the above accounted for Balashov’s broad general para-

sitological and biocenological approach to the problem of the natural focality of diseases,

which always remained in the focus of his attention. Balashov’s particular contribution to

the study of the epizootiology of various arthropod-borne pathogens and their invasions is

described in his last monograph (Balashov, 2009). Here we limit ourselves to considering

only some of his general conclusions, definitions, and concepts that we regard as fundamen-

tally important. Unfortunately, they are often disregarded in papers today. Balashov’s initial

premise, formulated more than 45 years ago, was that “a focus of infection is a fine self-

regulating system whose individual components are connected by feedback loops” (Balashov,

1972b, p. 175). Having analyzed feedback connections in an ixodid tick-vertebrate parasitic

system, he arrived at the conclusion that “the high stability of such systems is attributable

to moderate intensity of immune reactions in the hosts and relatively low numbers of ticks

feeding on them” (Balashov, 1992, p. 185). He further explains that this stability “is provided

by complex parasite-host interactions at the stage of feeding. As a result of such interactions,

the vertebrate host is sensitized to antigens of vector saliva and develops anti-tick resistance.

In nature, however, the main hosts do not have absolute resistance, despite regular contact

with ticks” (p. 194). Such balanced relationships also prevail between ixodid ticks and

⁷ In this context, polyphagy means that there are many different hosts, which is characteristic of at

least some parasitic stages in the life cycle of all members of given genus or a particular species.

agents of transmissible infections, so that “pathogens cause minimum harm to tick vectors

and can persist in their bodies almost lifelong, retaining the ability to be transmitted both

to vertebrate hosts and within the tick population” (Balashov, 1995, p. 337). Furthermore,

“the eventual location of microorganisms in certain organs or cells of the vector depends

on the possibility for them to penetrate the system of organ envelopes and membranes as

well as on the suitability of certain cells for the existence of those pathogens” (Balashov,

1984b, p. 30). “Actually, ixodid ticks can be part of several different natural foci within one

ecosystem” (Balashov, 1972a, p. 338), but the circle of tick hosts is wider than the circle

of animals that are susceptible⁸ [without the influence of any other factors] to a particular

pathogen (Balashov, 1972b). Vectors of infection as well as reservoir hosts of pathogens

may be of different levels: principal (main), accessory (secondary), or incidental. It was

very timely that Balashov (2009) once again made a point of these elementary notions of

epizootiology, which had almost fallen into oblivion.

“Several conditions must be met to join a new parasitic community: First, the estab-

lishment of contact with a host; second, morphophysiological suitability of the host for the

parasite; and third, the ability of the parasite to occupy its ecological niche and coexist with

other arthropod species living on the same host. The results of these interactions determine

the specific occurrence in nature of certain parasite species. Violation of one of these condi-

tions prevents the colonization of a new host” (Balashov, 2002, p. 938). Therefore, “even

in cases of frequent transcontinental transfer of ticks by birds migrating between Europe,

Africa, and Asia, migrants are usually incapable of establishing themselves in new regions”

(Balashov, 2004, p. 917). Here, these words concern arthropod vectors, but they are equally

applicable to agents of infections and invasions with natural focality.

Today, despite the distinct progress in our knowledge of ticks and tick-borne pathogens

we still cannot but agree with Balashov’s statement made more than 35 years ago: “Unfor-

tunately, we are still very far from understanding the holistic picture of the complex interac-

tions of pathogens of vector-borne infections with the arthropod vectors, which ultimately

determine the possibility of further circulation of the pathogenic agent in nature or its death

in the deadlock of infection” (Balashov, 1984a, p. 334).

Obviously, Balashov’s scientific legacy includes ideas that can serve as starting points

for further research on the problems of medical entomology and therefore deserve in-depth

and comprehensive ongoing analysis.

ACKNOWLEDGEMENTS

The authors express their deep gratitude to Professor Pat Nuttall (Department of Zoology,

University of Oxford, UK) for constructive advice on the structure of the article.

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Профессор Юрий Сергеевич Балашов:

наследие выдающегося паразитолога

Э. И. Коренберг, О. Кал

Ключевые слова: паразитология, медицинская энтомология, экология, Argasidae,

Ixodidae, клещи, природный очаг инфекции

РЕЗЮМЕ

Статья посвящена научным достижениям выдающегося паразитолога Ю.С. Балашова.

Это не биография. Цель данной статьи - осветить некоторые аспекты научного наследия

Ю.С. Балашова, особенно те, которые, по мнению авторов, наиболее важны для интернацио-

нального читателя. Это должно привлечь внимание исследователей разных стран к различным

научным трудам и концепциям, часть которых была опубликована только на русском языке, и

способствовать их более широкому обсуждению. Масштаб научных интересов Балашова был

поразителен, хотя основное внимание он уделял исследованию аргасовых и иксодовых клещей

(Argasidae и Ixodidae). Главные результаты и заключения его обширных работ описаны в основ-

ном с использованием его собственных слов и выражений. Авторам принадлежат только не-

которые объяснения, предназначенные для лучшего понимания читателем концепций, терминов

и мыслей Ю.С. Балашова.