Rapid Increase in Pertactin-deficient Bordetella pertussisIsolates, Australia

Connie Lam, Sophie Octavia, Lawrence Ricafort, Vitali Sintchenko, Gwendolyn L. Gilbert, Nicholas Wood, Peter McIntyre, Helen Marshall, Nicole Guiso, Anthony D. Keil, Andrew Lawrence, Jenny Robson, Geoff Hogg, and Ruiting Lan

Author affiliations: University of New South Wales, Sydney, New South Wales, Australia (C. Lam. S. Octavia, L. Ricafort, R. Lan);University of Sydney, Sydney (V. Sintchenko, G.L. Gilbert);Westmead Hospital, Sydney, (V. Sintchenko, N. Wood, P. McIntyre); University of Adelaide, Adelaide, South Australia, Australia (H. Marshall); Institut Pasteur, Paris, France (N. Guiso);Princess Margaret Hospital for Children, Perth, Western Australia, Australia (A.D. Keil); Women’s and Children’s Hospital, Adelaide (A. Lawrence); Sullivan Nicolaides Pathology, Brisbane, Queensland, Australia (J. Robson); University of Melbourne, Parkville, Victoria, Australia (G. Hogg)

Abstract

Acellular vaccines against Bordetella pertussis were introduced in Australia in 1997. By 2000, these vaccines had replaced whole-cell vaccines. During 2008–2012, a large outbreak of pertussis occurred. During this period, 30% (96/320) of B. pertussis isolates did not express the vaccine antigen pertactin (prn). Multiple mechanisms of prn inactivation were documented, including IS481 and IS1002 disruptions, a variation within a homopolymeric tract, and deletion of the prn gene. The mechanism of lack of expression of prn in 16 (17%) isolates could not be determined at the sequence level. These findings suggest that B. pertussis not expressing prn arose independently multiple times since 2008, rather than by expansion of a single prn-negative clone. All but 1 isolate had ptxA1, prn2, and ptxP3, the alleles representative of currently circulating strains in Australia. This pattern is consistent with continuing evolution of B. pertussis in response to vaccine selection pressure.

Bordetella pertussis is the gram-negative coccobacillus that causes the respiratory disease pertussis, also known as whooping cough. The incidence of pertussis infection and related deaths decreased dramatically after implementation of immunization with a whole-cell vaccine (WCV) during the 1950s. Because of side effects of WCV, such as high rates of fever and local reactions, and variable efficacy of WCVs, a less reactogenic acellular vaccine (ACV) was developed in the 1980s. ACVs have now replaced WCVs in many industrialized countries for primary and booster vaccinations against pertussis.

Figure 1

Thumbnail of Pertussis cases/100,000 population in Australia, 2008–20012, since mandatory reporting was instituted in 1991 and changes to pertussis vaccination schedule, including introduction of whole-cell vaccine (WCV) booster vaccinations for 4–5-year-old children in 1994–1995 and introduction of acellular vaccine (ACV) booster vaccinations in 1997. By 1999–2000, ACVs were used for all pertussis vaccinations. In 2003, the booster vaccinations for children 18 months of age was removed and repl

Figure 1. . Pertussis cases/100,000 population in Australia, 2008–20012, since mandatory reporting was instituted in 1991 and changes to pertussis vaccination schedule, including introduction of whole-cell vaccine (WCV) booster vaccinations for 4–5-year-old children...

Although ACV formulations differ in the number of component pertussis antigens, the vaccine used in Australia contains pertussis toxin (ptx), pertactin (prn), and filamentous hemagglutinin (fha). A 5-component (ptx, prn, fha, fimbrial antigen [fim]2, and fim3) ACV is used for short periods in some regions (1). ACVs were introduced for the fourth and fifth doses in most states in Australia during 1997 and for all doses during 1999 (Figure 1). South Australia introduced ACVs for all doses in 1997. The current vaccination schedule for pertussis comprise 3 primary doses of ACV at 2, 4, and 6 months of age, and a booster vaccination at 4 years of age. A booster vaccination with ACV at 18 months of age, which was introduced in 1985, was removed from the National Immunization Program in Australia in 2003, and an adult-formulated ACV was introduced for children at 12–17 years of age in school-based programs in 2004 (2,3).

Since 1991, data on reported pertussis cases show that outbreaks occurred in Australia in 1996–1997, 2001, and 2004, and a series of outbreaks occurred in different regions starting in 2008 (Figure 1) (2,3). Multiple factors probably contributed to the resurgence of pertussis in high-income countries that had long-standing pertussis immunization programs. These factors include waning immunity (exacerbated by the change from WCVs to ACVs and, in Australia, cessation of the booster vaccination at 18 months of age) and increased use of more sensitive diagnostic tests, such PCR (4).

An additional possible contributing factor is evolution of B. pertussis through vaccine-driven adaptation (5). The most prominent recent changes in circulating B. pertussis strains are polymorphisms within genes encoding 2 of the 3 main virulence factors (ptx and prn) contained in the vaccine. Variations have also been reported in ptxP, the promoter of the ptx operon (6). In Australia, we have shown by single nucleotide polymorphism (SNP) typing that among B. pertussis isolates, ptxP3–containing strains predominate (7), and these strains belong to SNP cluster I (8,9).

Surveillance of recent B. pertussis isolates in several countries has identified prn deletions and gene disruptions, which lead to lack of expression of mature prn (10–13). This protein is a 69-kDa adhesin that aids B. pertussis attachment to epithelial cells and is one of the most polymorphic virulence genes within B. pertussis (it has 13 documented alleles) (5). SNPs and differences in the number of amino acid (GGFGP and PQP) repeats contribute to variation within the prn gene; variations are usually limited to 2 regions known as region 1 and region 2.

In this study, we identified B. pertussis isolates that do not express prn (prn negative) from a set of isolates collected in Australia during 1997–2012. We also characterized the causes of their lack of expression and evaluated trends in the proportion of prn-negative isolates over this period.